Estimating the population prevalence of injection drug use and infection with human immunodeficiency virus among injection drug users in Glasgow, Scotland

Using dual capture/recapture studies to estimate the population size of persons who inject drugs (PWID) in the city of Hai Phong, Vietnam

BACKGROUND

Good estimates of key population sizes are critical for appropriating resources to prevent HIV infection. We conducted two capture/recapture studies to estimate the number of PWID currently in Hai Phong, Vietnam.

METHODS

A 2014 respondent-driven sampling (RDS) survey served as one capture, and distribution of cigarette lighters at drug use "hotspots" in 2016 served as another "capture." A 2016 survey using RDS, conducted 1 week after lighter distribution, served as "recapture" for both captures. Recaptured participants in the two surveys were identified with a computerized fingerprint reader. Recaptured participants from the lighter distribution were asked to show their lighters.

RESULTS

1385 participants were included in the "recapture" survey. They were 94% male and had a median age of 39. All (100%) injected heroin, and HIV prevalence was 30%. 144 of the 603 participants in the 2014 survey and 152 of the 600 PWID who had received lighters were "recaptured" in the 2016 survey. After adjusting for police suppression of drug use hotspots and conducting sensitivity analyses, our best estimate of the population size from the lighter recapture was 4617 (95% CI: 4090-5143), and our best estimate from the 2014 survey recapture was 5220 (95% CI: 4568-5872). A combined best estimate of the PWID population in Hai Phong is 5000, range 4000-6000.

CONCLUSIONS

The capture/recapture studies produced consistent estimates. Adding a lighter/token distribution to planned RDS surveys may provide an inexpensive method for estimating PWID population size. Analyses of the estimates should include contextual information about the local drug scene.

Recapture or precapture? Fallibility of standard capture-recapture methods in the presence of referrals between sources

Capture-recapture methods, largely developed in ecology, are now commonly used in epidemiology to adjust for incomplete registries and to estimate the size of difficult-to-reach populations such as problem drug users. Overlapping lists of individuals in the target population, taken from administrative data sources, are considered analogous to overlapping “captures” of animals. Log-linear models, incorporating interaction terms to account for dependencies between sources, are used to predict the number of unobserved individuals and, hence, the total population size. A standard assumption to ensure parameter identifiability is that the highest-order interaction term is 0. We demonstrate that, when individuals are referred directly between sources, this assumption will often be violated, and the standard modeling approach may lead to seriously biased estimates. We refer to such individuals as having been “precaptured,” rather than truly recaptured. Although sometimes an alternative identifiable log-linear model could accommodate the referral structure, this will not always be the case. Further, multiple plausible models may fit the data equally well but provide widely varying estimates of the population size. We demonstrate an alternative modeling approach, based on an interpretable parameterization and driven by careful consideration of the relationships between the sources, and we make recommendations for capture-recapture in practice.

Estimating the size of hidden populations from register data

Background

Prevalence estimates of drug use, or of its consequences, are considered important in many contexts and may have substantial influence over public policy. However, it is rarely possible to simply count the relevant individuals, in particular when the defining characteristics might be illegal, as in the drug use case. Consequently methods are needed to estimate the size of such partly ‘hidden’ populations, and many such methods have been developed and used within epidemiology including studies of alcohol and drug use. Here we introduce a method appropriate for estimating the size of human populations given a single source of data, for example entries in a health-care registry.

Methods

The setup is the following: during a fixed time-period, e.g. a year, individuals belonging to the target population have a non-zero probability of being “registered”. Each individual might be registered multiple times and the time-points of the registrations are recorded. Assuming that the population is closed and that the probability of being registered at least once is constant, we derive a family of maximum likelihood (ML) estimators of total population size. We study the ML estimator using Monte Carlo simulations and delimit the range of cases where it is useful. In particular we investigate the effect of making the population heterogeneous with respect to probability of being registered.

Results

The new estimator is asymptotically unbiased and we show that high precision estimates can be obtained for samples covering as little as 25% of the total population size. However, if the total population size is small (say in the order of 500) a larger fraction needs to be sampled to achieve reliable estimates. Further we show that the estimator give reliable estimates even when individuals differ in the probability of being registered. We also compare the ML estimator to an estimator known as Chao’s estimator and show that the latter can have a substantial bias when applied to epidemiological data.

Conclusions

The population size estimator suggested herein complements existing methods and is less sensitive to certain types of dependencies typical in epidemiological data.

Injecting drug users in Scotland, 2006: Listing, number, demography, and opiate-related death-rates

Using Bayesian capture-recapture analysis, we estimated the number of current injecting drug users (IDUs) in Scotland in 2006 from the cross-counts of 5670 IDUs listed on four data-sources: social enquiry reports (901 IDUs listed), hospital records (953), drug treatment agencies (3504), and recent Hepatitis C virus (HCV) diagnoses (827 listed as IDU-risk). Further, we accessed exact numbers of opiate-related drugs-related deaths (DRDs) in 2006 and 2007 to improve estimation of Scotland's DRD rates per 100 current IDUs. Using all four data-sources, and model-averaging of standard hierarchical log-linear models to allow for pairwise interactions between data-sources and/or demographic classifications, Scotland had an estimated 31700 IDUs in 2006 (95% credible interval: 24900-38700); but 25000 IDUs (95% CI: 20700-35000) by excluding recent HCV diagnoses whose IDU-risk can refer to past injecting. Only in the younger age-group (15-34 years) were Scotland's opiate-related DRD rates significantly lower for females than males. Older males' opiate-related DRD rate was 1.9 (1.24-2.40) per 100 current IDUs without or 1.3 (0.94-1.64) with inclusion of recent HCV diagnoses. If, indeed, Scotland had only 25000 current IDUs in 2006, with only 8200 of them aged 35+ years, the opiate-related DRD rate is higher among this older age group than has been appreciated hitherto. There is counter-balancing good news for the public health: the hitherto sharp increase in older current IDUs had stalled by 2006.

Estimating the variability in the risk of infection for hepatitis C in the Glasgow injecting drug user population

Glasgow (Scotland's largest city) has a high prevalence of injecting drug use and has one of the highest prevalences of hepatitis C virus (HCV) infection in injecting drug users (IDUs) in Western Europe. HCV prevalence data from surveys of Glasgow's IDUs from 1990 to 2007 were utilized and a model was applied that described the prevalence of HCV as a function of the rate (force) of infection. Force-of-infection estimates for HCV that may vary over time and injecting career length over a range of variables were investigated. New initiates to injecting were found to be at increased risk of HCV infection, with being recruited from a street location and reporting injecting in prison leading to a significant increase in the risk of infection in new initiates. These results indicate areas of importance for the planning of public health measures that target the IDU population.

Estimating prevalence of injecting drug use: a comparison of multiplier and capture-recapture methods in cities in England and Russia

We consider the question of what method should be recommended to estimate the prevalence of injecting drug use (IDU); and compare multiplier and capture-recapture (CRC) methods of estimating prevalence of injecting drug use (IDU). The prevalence of injecting drug use in four cities (Brighton, Liverpool, London and Togliatti) was estimated using similar methods: covariate capture-recapture (CRC) and multipliers. The multipliers, generated either from a community recruited survey or historical/literature-based, were applied to a range of 'benchmarks': specialist drug treatment, arrests, accident and emergency department (A&E), syringe exchange, HIV tests and opiate overdose deaths. The CRC estimates were assumed to be 'preferred/gold standard' [2,304 (95% confidence interval 1,514 - 3,737) in Brighton, 2,910 (2,546 - 4,977) in Liverpool, 16,782 (13,793 - 21,620) in 12 London boroughs and 15,039 (12,696 - 18,515) male IDU in Togliatti]. The ranges given by the multiplier estimates obtained through the community survey varied from 200 to 770 in Brighton, 530 to 1,300 in Liverpool, 2,900 to 10,600 in London and 12,400 to 91,000 in Togliatti. Several multipliers gave implausible results, lower than the observed data collected for another benchmark, and in the three English cities all these multiplier estimates were below the lower 95% confidence interval of the CRC estimate. In Togliatti, only one multiplier estimate was close to the preferred CRC estimates, with the rest implausibly high. The multiplier estimates based on historical/literature multipliers also ranged widely from 390 to 4,800 for Brighton, from 1,645 to 2,800 in Liverpool, from 4,650 to 12,600 in the 12 London boroughs and 12,800 to 32,000 in Togliatti. In the three UK cities the mortality multiplier estimates were closest to the capture-recapture estimates. The study was a practical demonstration comparing a range of multiplier estimates with a single CRC study. In almost all the individual comparisons the multiplier estimates performed poorly. CRC methods should be preferred as the means of estimating numbers of drug users with multiplier methods being used with caution and only where CRC is not possible.

Viral hepatitis in incarcerated adults: a medical and public health concern

Viral hepatitis is a common problem in the incarcerated population. It causes significant morbidity and mortality, and incarcerated inmates receive their health care almost exclusively from corrections-based health systems. The seroprevalence of hepatitis B and C infections is increased in this population, and a number of risk factors for viral hepatitis are particularly common and infer higher risk among inmates, including injection drug use (IDU), high-risk sexual activity, and tattoos. IDU, in particular, has been identified as an important and common risk factor for viral hepatitis in inmates, and variable rates of IDU among inmates have been found to be the most important cause of the marked variability of seroprevalence rates for exposure to hepatitis C virus. A number of risk reduction and management strategies have been identified that can decrease transmission to other inmates. Prison-based hepatitis A and hepatitis B vaccination programs, needle exchange programs, methadone maintenance programs, risk education programs, and hepatitis C virus antiviral programs, for example, have been shown to be safe and effective risk reduction and management strategies. Preliminary studies have shown that these strategies are underutilized in the United States. Reasons for this phenomenon are multifactorial, involving financial as well as ethical and political considerations. Additional funding, research, and formal consideration / discussion of the complex issues involving viral hepatitis in the US incarcerated population are clearly important for the sake of inmates and the community at large. In this article, the published medical literature regarding this important topic is reviewed.

Capture—recapture and anchored prevalence estimation of injecting drug users in England: national and regional estimates

Capture—recapture (C—RC) using four data sources, one of which accounted for 81% of captured injectors, and multiple indicator methods (MIM) were used to obtain national, regional and local estimates of the prevalence of injecting drug use among opiate and/or crack cocaine users in England. Persons aged 15 to 64 years, in contact with health and/or criminal justice services during 2005/2006, and known to be using opiates and/or crack cocaine and injecting drugs were included in the C—RC analysis. The MIM analysis included indicators relating to drug treatment, drug-related deaths, population density and drug offences.

There were an estimated 130,000 opiate and/or crack cocaine users who injected drugs in 2005/06 (95% confidence interval 125,800 to 137,000), corresponding to 3.9 per thousand of the population aged 15 to 64 years (95% confidence interval 3.8—4.1). Regional variation in the prevalence of injecting was evident, ranging from 6.1 per thousand of the population aged 15 to 64 years in Yorkshire and the Humber (95% confidence interval 5.6 to 6.6) to 2.3 per thousand in the East of England (95% confidence interval 1.8 to 2.9). Application of gender and age-group distributions for treated injecting drug users (IDUs) to the prevalence estimates suggested that there were 97,200 male injectors (95% confidence interval 94,000 to 102,500) and 63,600 female injectors aged 25 to 34 years (95% confidence interval 61,500 to 67,000).

The prevalence estimates provide a basis from which numbers of current IDUs infected with hepatitis C virus (HCV) can be approximated.

Estimating current injectors in Scotland and their drug-related death rate by sex, region and age-group via Bayesian capture--recapture methods

Using Bayesian capture-recapture methods, we estimate current injectors in Scotland in 2003, and, thereby, injectors' drug-related death rates for the period 2003-2005. Four different data sources are considered [Hepatitis C Virus (HCV) database, hospital admissions, social enquiry reports, and drug misuse database reports by General Practices or Drug Treatment Agencies] which provide covariate information on sex, region (Greater Glasgow versus elsewhere in Scotland) and age group (15-34 years and 35+ years).We quantified Scotland's current injectors in 2003 at 27,400 (95% highest probability density interval: 20,700-32,100) by incorporating underlying model uncertainty in terms of the possible interactions present between data sources and/or covariates. The posterior probability was 72% that Scotland had more current injectors in 2003 than in 2000. Detailed comparison with 2000 gave evidence of importantly changed numbers of current injectors for different covariate classes.In addition, and of particular social interest, is the estimation of injectors' drug-related death rates. Expert information was used to construct upper and lower bounds on the number of drug-related deaths pertaining to injectors, which were then used to provide bounds on injectors' drug-related death rates. Failure to incorporate expert information could result in over-estimation of drug-related death rates for subclasses of injectors.

Estimating the size and dynamics of an injecting drug user population and implications for health service coverage: comparison of indirect prevalence estimation methods

AIMS

(i) To compare indirect estimation methods to obtain mean injecting drug use (IDU) prevalence for a confined geographic location; and (ii) to use these estimates to calculate IDU and injection coverage of a medically supervised injecting facility.

DESIGN

Multiple indirect prevalence estimation methods.

SETTING

Kings Cross, Sydney, Australia.

PARTICIPANTS

IDUs residing in Kings Cross area postcodes recorded in surveillance data of the Sydney Medically Supervised Injecting Centre (MSIC) between November 2001 and October 2002.

MEASUREMENTS

Two closed and one open capture-recapture (CRC) models (Poisson regression, truncated Poisson and Jolly-Seber, respectively) were fitted to the observed data. Multiplier estimates were derived from opioid overdose mortality data and a cross-sectional survey of needle and syringe programme attendees. MSIC client injection frequency and the number of needles and syringes distributed in the study area were used to estimate injection prevalence and injection coverage.

FINDINGS

From three convergent estimates, the mean estimated size of the IDU population aged 15-54 years was 1103 (range 877-1288), yielding a population prevalence of 3.6% (2.9-4.3%). Mean IDU coverage was 70.7% (range 59.1-86.7%) and the mean adjusted injection coverage was 8.8% (range 7.3-10.8%). Approximately 11.3% of the total IDU population were estimated to be new entrants to the population per month.

CONCLUSIONS

Credible local area IDU prevalence estimates using MSIC surveillance data were obtained. MSIC appears to achieve high coverage of the local IDU population, although only an estimated one in 10 injections occurs at MSIC. Future prevalence estimation efforts should incorporate open models to capture the dynamic nature of IDU populations.

Prevalence and determinants of hepatitis C virus infection among female drug injecting sex workers in Glasgow

Background

Few studies of the prevalence of hepatitis C virus (HCV) infection have focussed on women who work as street sex workers to finance their drug use.

Methods

The investigators report the survey findings of such a population in Glasgow. All women attending the health and social care drop-in centre, situated in Glasgow's "Red Light Area", during a four-week period in 1999 were invited to participate in a survey involving the provision of a saliva sample for anonymous HCV testing and the self-completion of a questionnaire seeking demographic, sexual and injecting practice data.

Results

Of the 223 women who attended, 51% agreed to participate. Of the 98 women who provided a sufficient saliva sample, 64% (95% CI: 54%–74%) tested HCV antibody positive; 98% of those who tested positive had ever injected drugs. Adjusting for the 85% sensitivity of the saliva test, the HCV antibody prevalence among IDU sex workers sampled was 81%; a rate which is considerably higher than those recorded, contemporaneously, among Glasgow IDUs generally. Two factors were independently associated with HCV antibody positivity in saliva: ever shared needles and syringes (adjusted OR 5.7, 95% CI 2–16) and number of times imprisoned (adjusted OR 7.3, 95% CI 1.4–39, for more than five times compared to zero times).

Conclusion

Women who engage in street sex work to finance their drug habit are a particularly desperate, chaotic and vulnerable population. This study demonstrates that their HCV infection risk may be greater than that for other IDUs. Those responsible for designing interventions to prevent HCV infection among IDUs should consider the special needs of this group.

Validating estimates of problematic drug use in England

Background

UK Government expenditure on combatting drug abuse is based on estimates of illicit drug users, yet the validity of these estimates is unknown. This study aims to assess the face validity of problematic drug use (PDU) and injecting drug use (IDU) estimates for all English Drug Action Teams (DATs) in 2001. The estimates were derived from a statistical model using the Multiple Indicator Method (MIM).

Methods

Questionnaire study, in which the 149 English Drug Action Teams were asked to evaluate the MIM estimates for their DAT.

Results

The response rate was 60% and there were no indications of selection bias. Of responding DATs, 64% thought the PDU estimates were about right or did not dispute them, while 27% had estimates that were too low and 9% were too high. The figures for the IDU estimates were 52% (about right), 44% (too low) and 3% (too high).

Conclusion

This is the first UK study to determine the validity estimates of problematic and injecting drug misuse. The results of this paper highlight the need to consider criterion and face validity when evaluating estimates of the number of drug users.

Hepatitis C risk assessment, testing and referral for treatment in urban primary care: Role of race and ethnicity

AIM: To determine rates of hepatitis C (HCV) risk factor ascertainment, testing, and referral in urban primary care practices, with particular attention to the effect of race and ethnicity.

METHODS: Retrospective chart review from four primary care sites in Philadelphia; two academic primary care practices and two community clinics was performed. Demographics, HCV risk factors, and other risk exposure information were collected.

RESULTS: Four thousand four hundred and seven charts were reviewed. Providers documented histories of injection drug use (IDU) and transfusion for less than 20% and 5% of patients, respectively. Only 55% of patients who admitted IDU were tested for HCV. Overall, minorities were more likely to have information regarding a risk factor documented than their white counterparts (79% vs 68%, P < 0.0001). Hispanics were less likely to have a risk factor history documented, compared to blacks and whites (P < 0.0001). Overall, minorities were less likely to be tested for HCV than whites in the presence of a known risk factor (23% vs 35%, P = 0.004). Among patients without documentation of risk factors, blacks and Hispanics were more likely to be tested than whites (20% and 24%, vs 13%, P < 0.005, respectively).

CONCLUSION: (1) Documentation of an HCV risk factor history in urban primary care is uncommon, (2) Racial differences exist with respect to HCV risk factor ascertainment and testing, (3) Minority patients, positive for HCV, are less likely to be referred for subspecialty care and treatment. Overall, minorities are less likely to be tested for HCV than whites in the presence of a known risk factor.

Estimating crashes involving heavy vehicles in Western Australia, 1999-2000: a capture-recapture method

A two-sample exploratory study of police and hospital records was undertaken to estimate the number of fatalities and serious injuries for heavy vehicle drivers involved in a crash in Western Australia. The capture-recapture method was used to assess differences and similarities in characteristics of heavy vehicle drivers from both sources. Each heavy vehicle driver involved in a crash from the police report was matched against the heavy vehicle driver's hospitalisation record from the Hospital Morbidity Data System, with surname, initials, date of birth, gender, date of crash, road user type and vehicle type as matching fields. The estimated number of fatalities and serious injuries to heavy vehicle drivers from 1st July 1999 to 31st December 2000 was 5 and 59, respectively, which was 25 and 31% higher based on the capture-recapture methodology than the aggregated (non-overlapping) total officially reported to the police and hospitals. No significant age difference (p>0.05) was found for drivers involved in a heavy vehicle crash between the two sources (37 years versus 40 year of age). However, female heavy vehicle drivers were over-represented in the hospital records (11%) compared to the police records (1%). The capture-recapture approach is useful for evaluating the completeness of data sources and identifying biases within datasets. The underestimation of heavy vehicle drivers seriously injured and killed has important implications for heavy vehicle safety management and resource allocation in Western Australia.

Modeling the current and future disease burden of hepatitis C among injection drug users in Scotland

Quantitative estimates of the current and future burden of hepatitis C virus (HCV) disease are required to plan a public health response to the HCV epidemic with regard to both prevention and treatment. A forward projection model was used to estimate the numbers of both current and former injecting drug users (IDUs) who acquired HCV and progressed to moderate and severe disease in Glasgow and Scotland during 1960-2030. The model was designed to synthesize information on the incidence and cessation of injecting drug use, the incidence of HCV infection among IDUs, the rate of HCV disease progression, and the annual number of IDUs developing HCV-related decompensated cirrhosis. During 2003, a total of 17,400 and 42,900 HCV-infected IDUs were estimated in Glasgow and Scotland, respectively; this compares with approximately 5,000 and 13,900 diagnosed, respectively, and 13,200 and 32,200 with chronic HCV, respectively. The number of IDUs developing HCV-related decompensated cirrhosis in Scotland is estimated to double between 2000 and 2020. As many as 16% and 27% of former IDUs in 2005 aged 30-39 and 40-49 years, respectively, were estimated to have moderate disease, which highlights the potential benefit of targeting HCV testing at former IDUs who belong to these age groups. In conclusion, the identification and treatment of a larger proportion of former IDUs with HCV disease and education about the importance of minimal alcohol consumption are needed to help achieve a greater impact on the future morbidity and mortality of this disease.

Is human immunodeficiency virus/acquired immunodeficiency syndrome decreasing among Brazilian injection drug users? Recent findings and how to interpret them

We briefly review findings from Brazilian settings where the human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS) epidemic among injection drug users (IDUs) seems to be decreasing, highlighting recent findings from Rio de Janeiro and discussing methodological alternatives. Former analyses using serologic testing algorithm for recent HIV seroconversion have shown that HIV incidence has been low in IDUs recruited by two different surveys carried out in Rio, where low injection frequencies and infection rates have been found among new injectors. The proportion of AIDS cases among IDUs in Rio has been fairly modest, compared to São Paulo and especially to the southernmost states. Notwithstanding, the interpretation of findings from serial surveys constitutes a challenge, magnified in the assessment of HIV spread among IDUs due to the dynamic nature of the drug scenes and limitations of sampling strategies targeting hard-to-reach populations. Assessment of epidemic trends may profit from the triangulation of data, but cannot avert biases associated with sampling errors. Efforts should be made to triangulate data from different sources, besides exploring specific studies from different perspectives. In an attempt to further assess the observed trends, we carried out original analyses using data from Brazilian AIDS databank.

Bayesian inference on the patient population size given list mismatches

In applying capture-recapture methods for closed populations to epidemiology, one needs to estimate the total number of people with a certain disease in a certain research area by using several lists with information of patients. Problems of lists error often arise due to mistyping or misinformation. Adopting the concept of tag-loss methodology in animal populations, Seber et al. (Biometrics 2000; 56:1227-1232) proposed solutions to a two-list problem. This article reports an interesting simulation study, where Bayesian point estimates based on improper constant and Jeffreys prior for unknown population size N could have smaller frequentist standard errors and MSEs compared to the estimates proposed in Seber et al. (2000). The Bayesian credible intervals based on the same priors also have super frequentist coverage probabilities while some of the frequentist confidence intervals procedures have drastically poor coverage. Seber's real data set on gestational diabetics is analysed with the proposed new methods.

Capture-recapture estimates of problem drug use and the use of simulation based confidence intervals in a stratified analysis

STUDY OBJECTIVE

To establish the prevalence of problem drug use in the 10 local authorities within the Metropolitan County of Greater Manchester between April 2000 and March 2001.

SETTING AND PARTICIPANTS

Problem drug users aged 16-54 resident within Greater Manchester who attended community based statutory drug treatment agencies, were in contact with general practitioners, were assessed by arrest referral workers, were in contact with the probation service, or arrested under the Misuse of Drugs Act for offences involving possession of opioids, cocaine, or benzodiazepines.

DESIGN

Multi-sample stratified capture-recapture analysis. Patterns of overlaps between data sources were modelled in a log-linear regression to estimate the hidden number of drug users within each of 60 area, age group, and gender strata. Simulation methods were used to generate 95% confidence intervals for the sums of the stratified estimates.

MAIN RESULTS

The total number of problem drug users in Greater Manchester was estimated to be 19 255 giving a prevalence of problem drug use of 13.7 (95% CI 13.4 to 15.7) per 1000 population aged 16-54. The ratio of men to women was 3.5:1. The distribution of problem drug users varied across three age groups (16-24, 25-34, and 35-54) and varied between the 10 areas.

CONCLUSIONS

Areas in close geographical proximity display different patterns of drug use in terms of prevalence rates and age and gender patterns. This has important implications, both for future planning of service provision and for the way in which the impact of drug misuse interventions are evaluated.

Injecting drug use in Brighton, Liverpool, and London: best estimates of prevalence and coverage of public health indicators

STUDY OBJECTIVE

To estimate the prevalence of injecting drug use (IDU) in three cities in England and to measure the coverage of key public health indicators.

DESIGN

Capture-recapture techniques with covariate effects.

SETTING

Liverpool, Brighton, and 12 London boroughs, 2000/01.

PARTICIPANTS

IDU collated and matched across five data sources-community recruited survey, specialist drug treatment, arrest referral, syringe exchange, and accident and emergency-896 in Brighton, 1224 in Liverpool, and 6111 in London.

MAIN RESULTS

It is estimated that in 2000/01 the number and prevalence of IDU aged 15-44 was 2300 (95%CI 1500 to 3700) and 2.0% (95%CI% 1.3% to 3.2%) in Brighton; 2900 (95%CI 2500 to 5000) and 1.5% (95%CI 1.3% to 2.6%) in Liverpool; 16 700 (95%CI 13 800 to 21 600) and 1.2% (95%CI 1.0% to 1.6%) in 12 London boroughs; with a prevalence of 1.7% (95%CI 1.2% to 3.3%) in inner London. It is estimated that: less than one in four IDU are in treatment in the three areas; syringe exchange programmes covered about 25% of injections in Brighton and Liverpool and 20% in London; and that the annual opioid mortality rate among IDU was 2% in Brighton compared with less than 1% in Liverpool and London.

CONCLUSIONS

Credible estimates of the prevalence of injecting drug use (and key public health indicators) can be determined using covariate capture-recapture techniques. These suggest that: targets to double the number in treatment are possible: syringe distribution should be increased; and further attention, especially in Brighton, given to reducing overdose mortality.

Prevalence of problematic cocaine consumption in a city of southern Europe, using capture-recapture with a single list

This study aims to determine the magnitude and characteristics of problematic cocaine consumption in the city of Barcelona, Spain. Capture-recapture with a single source was used to estimate prevalence. Log-linear regression models with interaction terms were fitted to the total sample and to subgroups according to other drugs consumed. Emergency room indicator data were obtained from the Barcelona Drug Information System. Drug-related emergencies of Barcelona residents for 1999 were analyzed. During 1999, a total of 4,035 drug-related emergencies were seen in Barcelona hospitals. Of these, 1,656 (41%) involved cocaine consumption; 41% of these patients had consumed cocaine with an opiate; 29% used cocaine with other substances; and 30% used cocaine alone. It was estimated that there was a total of 25,988 problematic cocaine users (95% confidence interval 11,782-58,064), yielding a rate of 31.27 per 1,000 inhabitants aged 15 to 54 years (95% confidence interval 14.2-69.9). The number of cocaine-related emergencies was high enough to allow capture-recapture to be applied, thus obtaining an estimate of the prevalence of problematic cocaine consumption, and high enough to characterize users according to different profiles. The use of capture-recapture with a single source can be interesting for problems related to the urban context.

An HIV prevalence-based model for estimating urban risk populations of injection drug users and men who have sex with men

Issues of cost and complexity have limited the study of the population sizes of men who have sex with men (MSM) and injection drug users (IDUs), two groups at clearly increased risk for human immunodeficiency virus (HIV) and other acute and chronic diseases. We developed a prototypical, easily applied estimation model for these populations and applied it to Miami, Florida. This model combined HIV prevalence estimates, HIV seroprevalence rates, and census data to make plausible estimates of the number and proportion of MSM and IDUs under a number of assumptions. Sensitivity analyses were conducted to test the robustness of the model. The model suggests that approximately 9.5% (plausible range 7.7%-11.3%) of Miami males aged 18 years or older are MSM (point estimate, N = 76,500), and 1.4% (plausible range 0.9%-1.9%) of the total population aged 18 years or older are IDUs (point estimate, N = 23,700). Males may be about 2.5 times more likely than females to be IDUs. The estimates were reasonably robust to biases. The model was used to develop MSM and IDU population estimates in selected urban areas across Florida and should be replicable in other medium-to-large urban areas. Such estimates could be useful for behavioral surveillance and resource allocation, including enhanced targeting of community-based interventions for primary and secondary HIV prevention.

Estimating prevalence of problem drug use at national level in countries of the European Union and Norway

OBJECTIVES

This paper will present the most recent estimates for problematic drug use in European Member States and explore the problems of comparability.

METHODS

Estimates of problematic drug use, derived according to agreed guidelines, were collected from all EU countries and Norway. Methods included multipliers based on treatment, police, mortality or AIDS/HIV data, the capture-recapture method and the multivariate indicator method. Prevalence estimates were transformed into rates per 1000 population aged 15-64 years.

RESULTS

Target populations varied according to data selection. Estimates for six partially overlapping types of drug use could be identified: 'problem opiate use', 'problem opiate or cocaine use', 'problem amphetamine or opiate use', 'problem drug use', '(current) injecting' and 'life-time injecting'. Rates of injectors ranged from 2.6 in Germany to 4.8 in Luxembourg; rates in Austria, Denmark, Finland, Portugal and the United Kingdom fell within this range. For problem opiate use, a group of high prevalence countries were found with average rates exceeding six cases (Italy, Luxembourg, Spain and the United Kingdom) and a group with lower prevalence with average rates close to three cases (Austria, Germany, Ireland, the Netherlands). Rates obtained for France (problem opiate or cocaine use), Finland and Sweden (problem amphetamine or opiate use) are not directly comparable and fall between these rates.

CONCLUSIONS

Cross-national comparisons should be made with care and estimated target populations may vary greatly between countries. For estimating various forms of problem drug use at national level, a multi-method approach is recommended.

Estimation of the number of injecting drug users attending an outreach syringe-exchange program and infection with human immunodeficiency virus (HIV) and hepatitis C virus: the AjUDE-Brasil project

This study estimated the number of street injecting drug user (IDU) clients of a syringe-exchange program (SEP) who were infected with human immunodeficiency virus (HIV) and hepatitis C virus (HCV). A two-sample capture-recapture method was carried out. The first capture listed all interviewed IDUs outreached for syringe exchange from April 1 to May 1, 1998; the second capture involved those outreached from May 2 to June 6, 1998. Blood spots were collected for HIV and HCV serologies. Analysis used captured probability model varying with time. We interviewed 55 IDUs in the first capture and 99 in the second; 17 participated in both samples. An estimate of 317 IDUs attending the SEP was obtained (95% confidence interval [CI] 235-467). Based on the overall seroprevalence rates for HIV (47.7%) and HCV (53.1%), it was estimated that 151 IDUs were HIV infected (95% CI 112-223) and 168 (95% CI 125-248) were HCV infected. Enumeration of IDUs associated with estimates of the total number of HIV and HCV seropositives provide a powerful tool for SEPs to help monitor the number of IDUs, to plan for provisions, and to organize the new demands on existing health facilities for HIV and HCV care.

Lethal outbreak of infection with Clostridium novyi type A and other spore-forming organisms in Scottish injecting drug users

This report describes the investigation and management of an unprecedented outbreak of severe illness among injecting drug users (IDUs) in Scotland during April to August 2000. IDUs with severe soft tissue inflammation were prospectively sought among acute hospitals and a mortuary in Scotland. Cases were categorised as definite or probable: probable cases had severe injection site inflammation or multi-system failure; definite cases had both. Information about clinical course, mortality, post-mortem findings and laboratory data was gathered by standardised case-note review and interview. Sixty cases were identified--23 definite and 37 probable. Most had familial or social links with each other and 50 were from Glasgow. Median age was 30 years; 31 were female. The majority, especially definite cases, injected heroin/citric acid extravascularly. Of definite cases, 20 died (87% case-fatality rate; 13 after intensive care), 15 had necrotising fasciitis, 22 had injection site oedema and 13 had pleural effusion. Median white cell count was 60 x 10(9)/L. Of 37 probable cases, three died (8% case-fatality rate). Overall, the most frequently isolated pathogen was Clostridium novyi type A (13 cases: 8 in definite cases). The findings are consistent with an infection resulting from injection into soft tissue of acidified heroin contaminated with spore-forming bacteria. Toxin production led to a severe local reaction and, in many, multi-system failure.

Estimating the number of opiate users in amsterdam by capture-recapture: the importance of case definition

One of the objectives of Amsterdam's methadone maintenance treatment is maximising its coverage among problematic opiate users. In order to evaluate what proportion is reached, the capture-recapture method is conducted to estimate the prevalence of problematic opiate use. Samples of opiate users in contact with police, hospital or treatment are used. The treatment sample is limited to the low threshold treatment sample (treatment with minimal requirements to the clients). Based on differences of log likelihood ratio, Akaike's and Bayesian information criteria, log linear models are selected. The size of the population of problematic opiate users in 1997 is estimated to be 4130 (95% confidence interval: 3753-4566). Within 3 months 50% was registered: 16% at the police, 2.5% at the hospital and 40% at treatment. This study shows that the Amsterdam methadone treatment programs succeed in reaching a high proportion of problematic opiate users. The estimation of the prevalence of problematic opiate users is considered to be valid. However, if, instead of the low threshold treatment, the total treatment sample had been used, the population of interest and the sampled population would not match correctly, and prevalence would have been overestimated.

Trends in HCV prevalence among injecting drug users in Glasgow and Edinburgh during the era of needle/syringe exchange

In 1998, we reported that anti-HCV prevalence among injectors from Glasgow had declined between 1990 and 1995. We set out to ascertain if the anti-HCV prevalence among injectors from Edinburgh had declined similarly during this period and if there had been any trend in prevalence among injectors from both cities since 1995. Residual sera from both cities' injecting drug users who had undergone named HIV testing were identified, linked to age band and gender information and tested anonymously for anti-HCV. Among Edinburgh's injectors, significant (p < 0.0001) decreases in anti-HCV prevalence from 69% (1989/90) to 13% (1997) and from 80% (1989/90) to 54% (1997) were seen in those aged < 25 y and > or = 25 y, respectively. Among Glasgow's injectors, a significant (p < 0.0001) decrease in prevalence from 91% (1990) to 43% (1997) was seen only among those aged < 25 y. Of both cities' 15-19 y olds, sampled during 1995-97, 17% (24/139) were anti-HCV-positive. The findings suggest that the incidence of HCV among young injectors continued to decrease during the 1990s--the era of needle/syringe exchange and other interventions--but is still too high. Further investigative and preventive work is required.

[Estimation with the capture-recapture method of the number of economic immigrants in Mallorca]

AIM

estimate the number of irregular economic immigrants in Mallorca.

DESIGN

We used the capture-recapture method, an indirect method based on contrasts of data from two or more sources. Data were obtained from the Delegación de Gobierno (police and immigration authority), Comisiones Obreras (labor union), and institutions that provide health-related services to immigrants. Individuals were identified by birth date and country of origin.

RESULTS

The total number of economic immigrants estimated with this method was 39 392. According to the Delegación de Gobierno data, the number of regular immigrants on the date of our inquiry was 9000. With the capture-recapture method, the number of irregular immigrants in Mallorca was therefore estimated at 30 000.

CONCLUSIONS

The capture-recapture method can be useful to estimate the population of irregular immigrants in a given area at a given time, if sufficiently precise information on the identity of each individual can be obtained.

The epidemiology of hepatitis C in a UK health regional population of 5.12 million

BACKGROUND

The epidemiology and natural history of hepatitis C virus (HCV) infection in the UK are uncertain. Previous reports are from small or selected populations such as blood donors or tertiary referral centres.

AIMS

To study the epidemiology and natural history of HCV infection.

METHODS

Prospective study incorporating five centres within the Trent region. Patients were managed and followed up according to a commonly agreed protocol.

SUBJECTS

A total of 1128 HCV positive patients. Patients with haemophilia, human immunodeficiency virus, and chronic renal failure were excluded.

RESULTS

Between September 1991 and December 1998, 2546 anti-HCV positive patients were identified of whom 1128 (44%) were enrolled in the cohort. A risk factor(s) for infection was identified in 93.4% of patients who completed the questionnaire; 81% of patients were HCV RNA positive. A total of 397 initial liver biopsies were scored by a single pathologist. These showed a correlation between high alcohol intake and fibrosis score. Multivariate analysis showed fibrosis to be associated with age over 40, past evidence of hepatitis B virus infection, and higher necroinflammatory grade but not with sex, viral genotype, maximum known alcohol intake, estimated duration of infection, or mode of transmission. Twelve (7.8%) of 153 patients who received interferon therapy had sustained serum virus clearance. Sixty six patients have died during the follow up period, 31 with a liver related cause of death. This represents a considerable excess over the expected death rate for a cohort of this age and sex distribution.

CONCLUSIONS

HCV infection is an emerging health problem in the Trent region. Identifying risk factors for infection and disease severity will enhance understanding and facilitate improved intervention. An excess mortality in infected individuals is already evident in this unselected cohort.

Estimating the size of hard-to-reach populations: a novel method using HIV testing data compared to other methods

OBJECTIVE

To estimate population size of hard-to-reach groups such as injecting drug users and men who have sex with men.

DESIGN

Several different methods were used to estimate the size of these populations in Canada's three largest cities (Toronto, Montreal and Vancouver).

METHODS

A novel method (referred to as the indirect method) was developed for use in Toronto and Vancouver that combines HIV serodiagnostic information with data on HIV testing behavior. Population size estimates were obtained by dividing the number of injecting drug users or men who have sex with men recorded in HIV serodiagnostic databases in a given year by the proportion of the corresponding group that reported being tested in a 1-year period. Results of this method were compared with four other methods: (1) population surveys; (2) capture-recapture (for injecting drug users only); (3) a modified Delphi technique; and (4) a method based on the proportion of never-married men aged 45 and over (for men who have sex with men only). Only these other methods were used in Montreal.

RESULTS

The survey method gave the lowest estimates which are best viewed as minimum estimates given the relative inability of surveys to access these populations and the reluctance of participants to admit to sensitive behaviors. The indirect method produced results more closely comparable with those obtained by other methods, but they are probably slight overestimates, at least for injecting drug users, due to possible underestimation of the proportion tested for HIV. Point estimates using the indirect method were 17,700 and 17,500 for injecting drug users in Toronto and Vancouver, respectively, and 39,100 and 15,900 for men who have sex with men. In Toronto, results for the other methods ranged from 12,300-13,360 for injecting drug users and 18,800-35,000 for men who have sex with men. For Vancouver, these ranges were 6400-11,670 and 7000-26,500, respectively. In Montreal, ranges were 4300-12,500 for injecting drug users and 18,500-40,000 for men who have sex with men.

CONCLUSIONS

This novel method provides estimates of population size of hard-to-reach groups such as injecting drug users and men who have sex with men that are comparable with results derived by other methods. These estimates may be useful for the purposes of planning, implementing and evaluating prevention and care services, especially when they are combined with the results of other estimation methods to improve the degree of confidence in the resulting estimates.

Surveillance for incident HIV infection: new technology and new opportunities

Although surveillance for HIV infection has traditionally focused on the incidence of AIDS and the prevalence of HIV, new diagnostic technologies that allow the estimation of incident HIV infection have become available. Number and distribution of new cases of HIV infection, rather than old cases, are the data most relevant to guide rational application of HIV prevention programs. Historically, incident HIV infection has been measured in longitudinal cohort studies, diagnosed clinically or since 1993 by detection of seroconverting patients (during the window period before appearance of HIV antibody) who are viremic as measured by p24 antigen or RNA-PCR. The sensitive-less sensitive EIA test (or serologic testing algorithm for recent HIV seroconversion [STAHRS]) has now made the serologic diagnosis of incident HIV infection in individual patients as well as the estimation of HIV incidence in populations possible. Examples of the public health application of this are studies of HIV incidence in anonymous test site attendees, sexually transmitted disease clinic patients, and in-treatment injection drug users in San Francisco. These sorts of studies allow us not only to measure incidence cross-sectionally but also facilitate surveillance for HIV subtypes and primary antiretroviral resistance, targeting early antiretroviral therapy and partner notification, and understanding who is "failing" prevention. Having an HIV surveillance system that focuses on incident rather than prevalent infection should be our long-term goal.

Capture-recapture: a useful methodological tool for counting traffic related injuries?

INTRODUCTION

Although the capture-recapture technique is increasingly employed in studies of human populations to correct for under-ascertainment in traditional epidemiological surveillance, it has rarely been used in injury research.

OBJECTIVES

To estimate the completeness of official data sources on traffic related injuries (TRIs) by using the capture-recapture technique and to calculate an ascertainment corrected number of fatal and serious TRIs among Scottish young people aged 15-24 years. The appropriateness of the approach in this context is also assessed.

METHOD

A two sample capture-recapture technique was applied to two official sources of TRI data. Data on TRIs were obtained from the Scottish Health Service and the STATS19 dataset at the University of Essex Data Archive for 1995. Four standards (A-D) of matching were applied to fatalities and serious TRIs to allow plausible relaxation of matching standards within the context of the data collection setting. The completeness of each data source was assessed, and an ascertainment corrected number of fatalities and serious TRIs calculated.

RESULTS

The ascertainment corrected number of TRI fatalities among 15-24 year olds using standard D was 104. This represents only a small increase in the number of fatalities using capture-recapture than when using each individual dataset. The completeness of the Scottish Health Service database for TRI fatalities was 93%. The STATS19 database was 95% complete. The ascertainment corrected number of TRI hospital admissions was 1969. The STATS19 and the Scottish Health Service databases were approximately two thirds and three quarters complete respectively for non-fatal TRIs requiring hospitalisation.

CONCLUSIONS

Injury researchers have advocated the linkage of major datasets to supplement and improve the quality of injury data. Using capture-recapture we found that routine databases enumerate TRI fatalities accurately, in contrast to injury morbidity databases that do not. Capture-recapture is a potentially useful method of evaluating the completeness of data sources and identifying biases within datasets. However, ascertainment corrected rates should be viewed with caution. A number of requirements of the capture-recapture technique are unachieved in this study of injury in the human population.

Use of mark-recapture techniques to estimate the size of hard-to-reach populations

OBJECTIVES

To review the main problems associated with mark-recapture methods of population estimation, and to indicate some practical strategies for addressing these problems, with illustrations from a study of drug-use prevalence across Wales.

METHODS

Unnamed identifier data were collected in 1994 on 2610 drug users who were in contact with various agencies across Wales: the police, drug treatment agencies, needle exchanges, probation services, and agencies reporting to the Welsh Drugs Misuse Database.

RESULTS

Based on the dependency relationships between different agencies' datasets, different estimates of the 'hidden' populations (not in contact with agencies) were modelled for each county, for males and females, for injecting drug users and serious drug users, and for those under 25 and those over 25 years of age. Different models were also constructed for the same subpopulations, using different agency datasets and different criteria of overlap between them, yielding a total of 230 different models.

CONCLUSIONS

The issues of sample heterogeneity and population definition are particularly intractable in mark-recapture studies. Sample heterogeneity may be partly addressed by separately modelling different subpopulations to check whether they show the same dependency relationships as the main population. Population definition may be partly addressed by restricting modelling to datasets thought to share roughly congruent population definitions.

Prevalence of hepatitis C virus infection among injecting drug users in Glasgow 1990-1996: are current harm reduction strategies working?

OBJECTIVES

To determine the prevalence of HCV antibodies among injecting drug users and to gauge the effectiveness of needle/syringe exchange in preventing the transmission of HCV infection.

METHODS

Between 1990-1994 and in 1996, annual cross-sectional surveys of injecting drug users in Glasgow were conducted. In order to ensure as representative a sample as possible, the 1949 respondents were recruited from both 'in-treatment' and 'out-of treatment' settings. Injectors were interviewed about their risk behaviours for blood-borne viruses and provided a saliva sample which was initially tested, anonymously, for HIV antibodies, and subsequently tested for hepatitis C infection.

RESULTS

Among 1949 injectors, the prevalence of salivary antibodies, indicative of hepatitis C viraemia, was 61%(95%, confidence interval (CI) 59%-63%): the estimated prevalence of serum antibody positivity was 72%. Length of injecting, year of commencing drug injecting and the number of times in prison were predictive of antibody positivity. Thirty-one per cent of injectors who commenced their injecting after 1992, following the full establishment of needle/syringe exchange in the city, were salivary antibody positive, and the majority of their infections were acquired outside the prison setting. Respondents who began injecting after the introduction of needle/syringe exchange in the city were significantly less likely to test HCV antibody positive than those who commenced injecting prior to the advent of needle/syringe exchange, after adjusting for length of injecting career.

CONCLUSION

The prevalence of HCV among injectors in Glasgow has decreased during the era of needle/syringe exchange. However, there is evidence to suggest that the incidence of infection remains high. Since the prevalence of hepatitis C viraemia among the city's injecting population is extremely high, ongoing transmission is inevitable unless more effective interventions are identified and implemented urgently.

Unlinked anonymous HIV study of hospital patients and general practice attenders in Glasgow, 1991-1997

AIM

To determine whether HIV is spreading from injecting drug users and homosexual/bisexual males into lower risk heterosexual populations in Glasgow, Scotland, and to pilot a method of monitoring HIV prevalence which involves testing routine biochemistry specimens.

METHODS

An unlinked anonymous HIV testing study of hospital patients and general practice attenders was conducted during January 1992 to December 1997. Testing was performed on routine biochemistry specimens from patients aged 16-49 years attending two hospitals with catchment areas covering the north and the east of the city.

RESULTS

78,260 specimens were tested in the study period and no patient objected to their samples being tested anonymously. HIV prevalence rates among male and female subjects were 0.63% and 0.01%, respectively; the large difference in prevalence resulted, in part, from the inclusion of HIV infected haemophiliac patients who attended one of the hospitals. Prevalence among male general practice patients ranged between 0.1% and 0.2%, while that for male patients attending surgical or surgically related specialties was 0.1%.

CONCLUSIONS

The prevalence data indicate that HIV has not seeded from the high risk groups into the wider heterosexual population, and that the risk of a surgeon acquiring HIV occupationally is extremely low in a city which has an HIV prevalence similar to or greater than that seen in most other parts of the United Kingdom. Large numbers of residual specimens from busy biochemistry laboratories can be processed for unlinked anonymous testing without interfering with the laboratories' routine functions. This survey approach might be best suited to monitoring HIV trends in developing countries with relatively high prevalence rates and where transmission is principally heterosexual.

Extent and contributing factors of drug expenditure of injectors in Glasgow. Multi-site city-wide cross-sectional study

BACKGROUND

Recent concern about drug use has focused attention on the illegal income generated by users.

AIMS

To investigate factors associated with drugs expenditure and to estimate the cost of illegal acquisitions used to pay for drugs.

METHOD

We collected self-report data from 954 current injectors, interviewed at multiple street, needle/syringe exchange and drug treatment sites throughout Glasgow.

RESULTS

Injectors' mean weekly drug spending was 324 Pounds. The mean annual illegal drugs spend was estimated to be 11,000 Pounds per injector. We provide a central estimate--194 million Pounds per annum--of the retail value of goods acquired illegally by injectors in Glasgow in order to pay for drugs. Higher drug spends were associated with having been imprisoned more often and with those reporting acquisitive crime, drug dealing and prostitution. Treatment with methadone, among individuals who injected in the previous two months, was associated with a 20% reduction in a typical spend on drugs.

CONCLUSIONS

Treatment effectiveness needs to be measured both in terms of health benefit and in terms of reduction in drugs expenditure and recidivism.

A small area analysis estimating the prevalence of addiction to opioids in Barcelona, 1993

STUDY OBJECTIVE

To determine the distribution of opioid use prevalence in small areas and its relation with socioeconomic indicators.

DESIGN

Capture-recapture was applied using data from the Barcelona Drug Information System for 1993 (treatment demands, hospital emergency room visits, deaths from heroin acute adverse reaction and pre-trial prison admissions). To avoid dependence between sources, a log-linear regression model with interactions was fitted. For small neighbourhoods, where capture-recapture estimates were not obtainable, the Heroin Problem Index (HPI) was used to predict prevalence rates from a regression model. The correlation between estimated opioid use prevalence by neighbourhoods and their socioeconomic level was computed.

MAIN RESULTS

The city's estimated prevalence was 12.9 opioid addicts per 1000 inhabitants aged 15 to 44 years (95% CI: 10.1, 17.2), which represents 9176 persons. The highest rate was found in the inner city neighbourhood. Comparing rates obtained for each neighbourhood with their unemployment rates, a high correlation coefficient was obtained (r = 0.80, p < 0.001).

CONCLUSION

The main contribution of this study is that of combining capture-recapture with the HPI to produce small area prevalence estimates, which would not have been possible using only one method. Areas with higher socioeconomic status showed proportionally low addiction prevalences, but in depressed areas, prevalences varied widely.