Distribution-free estimation of age-related centiles

Normal Percentiles for Respiratory Rate in Children-Reference Ranges Determined from an Optical Sensor

(1) Background: Increased respiratory rates (RRs) are described in several medical conditions, including pneumonia, bronchiolitis and asthma. There is variable methodology on how centiles for RR are derived in healthy children. Available age percentiles for RR have been generated using methods that have the potential themselves to alter the rate. (2) Methods: An optical respiratory sensor was used to measure RR. This technique enabled recording in awake children without the artefact of the observer's presence on the subject's RR. A cross-sectional sample of healthy children was obtained from maternity wards, childcare centres and schools in Brisbane, Queensland, Australia. (3) Results: RRs were observed in 560 awake and 103 sleeping children of which data from 320 awake and 94 sleeping children were used to develop centile charts for children from birth to 13 years of age. RR is higher when children are awake compared to asleep. There were significant differences between awake and sleeping RR in young children. The awake median RR was 59.3 at birth and 25.4 at 3 years of age. In comparison, the median sleeping RR was 41.4 at birth and 22.0 at 3 years. (4) Conclusions: The centile charts will assist in determining abnormal RRs in children and will contribute to further systematic reviews related to this important vital sign. This is particularly in relation to the data on children aged from 0 to 3 years, where data are presented on both the awake and sleeping state. Many studies in the literature fail to acknowledge the impact of sleep state in young children on RR.

Effects of a preoperative neuromobilization program offered to individuals with carpal tunnel syndrome awaiting carpal tunnel decompression surgery: A pilot randomized controlled study

STUDY DESIGN

Pilot randomized controlled trial with parallel groups.

INTRODUCTION

Engaging individuals with carpal tunnel syndrome (CTS) awaiting carpal tunnel decompression surgery in a preoperative rehabilitation program may mitigate pain and sensorimotor impairments, enhance functional abilities before surgery, and improve postoperative outcomes.

PURPOSE OF THE STUDY

To assess the feasibility and the efficacy of a novel preoperative neuromobilization exercise program (NEP).

METHODS

Thirty individuals with CTS were randomly allocated into a four-week home-based neuromobilization exercise group or a standard care group while awaiting surgery. Outcome measures included feasibility (ie, recruitment, attrition, adherence, satisfaction, and safety) and efficacy metrics (ie, median nerve integrity and neurodynamics, tip pinch grip, pain, and upper limb functional abilities) collected before (ie, at the baseline and about four weeks later) and four weeks after surgery.

RESULTS

Thirty individuals with CTS were recruited (recruitment rate = 11.8%) and 25 completed the study (attrition rate = 16.7%). Adherence (94%) and satisfaction with the program (eg, enjoy the exercises and likeliness to repeat the NEP (≥4.2/5) were high and no serious adverse event was reported. NEP-related immediate pre- and post-surgery beneficial effects on pain interference were documented (P = .05, η² = .10), whereas an overall increased neurodynamics (P = .04, η² = .11) and decreased pain severity (P = .01, η² = .21) were observed.

DISCUSSION

Engaging in the proposed NEP has limited beneficial effect as a stand-alone intervention on pre- and post-surgery outcomes for individuals with CTS. Expanding the program's content and attribute by adding other components including desensitization maneuvers and novel therapies promoting corticospinal plasticity is recommended.

CONCLUSION

A preoperative NEP completed by individuals with CTS awaiting surgery is feasible, acceptable, and safe. However, given the limited beneficial effectsof the program, revision of its content and attributes is recommended before proceeding to large-scale trials.

Body fat accrual trajectories for a sample of Asian-Canadian and Caucasian-Canadian children and youth: A longitudinal DXA-based study

BACKGROUND

Body fat accrual trajectories can be used to monitor trends in body fat mass and identify potential health risks. Currently, no body fat percent (BF%) centile distance and velocity curves exist for Canadian youth.

OBJECTIVES

To develop sex-specific and ethnic-specific BF% centile distance and velocity curves for White and Asian-Canadian youth.

METHODS

We utilized 4734 observations from 944 participants (female = 487; Asian = 532) to create sex-specific BF% velocity curves for age 10 to 18 years and sex-specific and ethnic-specific BF% percentile distance curves for ages 9 to 18 years for White and Asian children. BF% was derived from whole body DXA scans.

RESULTS

BF% centile distance curves for Asian and White girls were similar. BF% at most centiles plateaued around age 16 for Asian but not for White boys. Velocity curves for boys declined from age 11 to 13 years and then increased until age 18 years. For girls from 10 to 15 years, velocity curves converged towards the 50th centile then remained flat from 16 to 18 years.

CONCLUSIONS

BF% distance and velocity centiles can be used to identify when an individual veers from an average BF% accrual trajectory. In future, these curves may be used to investigate differences in fat mass and accrual across Canada.

Design and other methodological considerations for the construction of human fetal and neonatal size and growth charts

This paper discusses the features of study design and methodological considerations for constructing reference centile charts for attained size, growth, and velocity charts with a focus on human growth charts used during pregnancy. Recent systematic reviews of pregnancy dating, fetal size, and newborn size charts showed that many studies aimed at constructing charts are still conducted poorly. Important design features such as inclusion and exclusion criteria, ultrasound quality control measures, sample size determination, anthropometric evaluation, gestational age estimation, assessment of outliers, and chart presentation are seldom well addressed, considered, or reported. Many of these charts are in clinical use today and directly affect the identification of at‐risk newborns that require treatment and nutritional strategies.

This paper therefore reiterates some of the concepts previously identified as important for growth studies, focusing on considerations and concepts related to study design, sample size, and methodological considerations with an aim of obtaining valid reference or standard centile charts. We discuss some of the key issues and provide more details and practical examples based on our experiences from the INTERGROWTH‐21^st Project. We discuss the statistical methodology and analyses for cross‐sectional studies and longitudinal studies in a separate article in this issue.

Current Issues in the Development of Foetal Growth References and Standards

PURPOSE OF REVIEW

This paper discusses the current issues in the development of foetal charts and is informed by a scoping review of studies constructing charts between 2012 and 2018.

RECENT FINDINGS

The scoping review of 20 articles revealed that there is still a lack of consensus on how foetal charts should be constructed and whether an international chart that can be applied across populations is feasible. Many of these charts are in clinical use today and directly affect the identification of at risk newborns that require treatment and nutritional strategies. However, there is no agreement on important design features such as inclusion and exclusion criteria; sample size and agreement on definitions such as what constitutes a healthy population of pregnant women that can be used for constructing foetal standards.

SUMMARY

This paper therefore reiterates some of these current issues and the scoping review showcases the heterogeneity in the studies developing foetal charts between 2012 and 2018. There is no consensus on these pertinent issues and hence if not resolved will lead to continued surge of foetal reference and standard charts which will only exacerbate the current problem of not being able to make direct comparisons of foetal size and growth across populations.

Serum variations of anti-mullerian hormone and total testosterone with aging in healthy adult Iranian men: A population-based study

Background

Literature proves anti-mullerian hormone (AMH) and total testosterone (TT) as two important reproductive hormones in male development, however evidence regarding age variations of these hormones is lacking.

Aims

To estimate the normal serum AMH values and to assess the age-specific TT levels in men aged 30–70, we conducted the present population-based study.

Methods

A total of 831 healthy eligible men, aged 30–70 years, were recruited from Tehran Lipid and Glucose study cohort. Centiles for AMH were estimated according to the exponential normal 3-parameter model. The parametric method of Royston available in general software was applied for the first time to estimate the age-specific AMH and TT percentiles of 5^th, 10^th, 25^th, 50^th, 75^th, 90^th and 95^th.

Results

Mean AMH level was 6.93, ranging from 0.1 to 40.1 ng/ml. Serum AMH concentrations followed a steady reduction with increasing age. Mean TT level was 4.8, ranging from 0.44 to 11.4 ng/ml.

Discussion

A measurable serum concentrations of AMH in healthy males throughout lifespan with variations, based on age, confirming a slight age-related AMH decline. Fractional polynomial (FP) regression models revealed that the mean and standard deviation (SD) of the TT were not associated with age, so the percentiles estimated were not age-specific.

Conclusion

We presented a nomogram of age-specific AMH values in a healthy cohort of Iranian men. This finding might have clinical importance in dealing hormonal disorders in men.

Nutritional status and height, weight and BMI centiles of school-aged children and adolescents of 6-18-years from Kinshasa (DRC)

BACKGROUND

The last study to establish centiles of main anthropometric measurements in Kinshasa was conducted over 60 years ago, which questions its current adequacy to describe or monitor growth in this population.

AIM

To assess the nutritional status of school-aged children and adolescents and to estimate centile curves of height, weight and body mass index (BMI).

SUBJECTS AND METHODS

A representative sample of 7541 school-aged children and adolescents (48% boys) aged 6-18 years was measured between 2010-2013. Smooth centiles of height, weight and BMI-for-age were estimated with the LMS method and compared with the WHO 2007 reference. Nutritional status was assessed by comparing measurements of height and BMI against the appropriate WHO cut-offs.

RESULTS

Compared to the WHO reference, percentiles of height and BMI were generally lower. This difference was larger in boys than in girls and increased as they approached adolescence. The prevalence of short stature (< -2 SD) and thinness (< -2 SD) was higher in boys (9.8% and 12%) than in girls (3.4% and 6.1%), but the prevalence of overweight (> 1 SD) was higher in girls (8.6%) than in boys (4.5%).

CONCLUSION

Children from Kinshasa fall below WHO centile references. This study established up-to-date centile curves for height, weight and BMI by age in children and adolescents. These reference curves describe the current status of these anthropometric markers and can be used as a basis for comparison in future studies.

Growth references for Tsimane forager-horticulturalists of the Bolivian Amazon

OBJECTIVES

Growth standards and references currently used to assess population and individual health are derived primarily from urban populations, including few individuals from indigenous or subsistence groups. Given environmental and genetic differences, growth may vary in these populations. Thus, there is a need to assess whether international standards are appropriate for all populations, and to produce population specific references if growth differs. Here we present and assess growth references for the Tsimane, an indigenous population of Bolivian forager-horticulturalists.

METHODS

Mixed cross-sectional/longitudinal anthropometrics (9,614 individuals; 30,118 observations; ages 0-29 years) were used to generate centile curves and Lambda-Mu-Sigma (LMS) tables for height-for-age, weight-for-age, body mass index (BMI)-for-age, and weight-for-height (WFH) using Generalized Additive Models for Location Shape and Scale (GAMLSS). Velocity curves were generated using SuperImposition by Translation and Rotation (SITAR). Tsimane ≤5 years were compared to World Health Organization (WHO) standards while those >5 years were compared to WHO school age references. All ages were compared to published references for Shuar forager-horticulturalists of the Ecuadorian Amazon.

RESULTS

Tsimane growth differs from WHO values in height and weight, but is similar for BMI and WFH. Tsimane growth is characterized by slow height velocity in childhood and early adolescent peak height velocity at 11.3 and 13.2 years for girls and boys. Tsimane growth patterns are similar to Shuar, suggesting shared features of growth among indigenous South Americans.

CONCLUSIONS

International references for BMI-for-age and WFH are likely appropriate for Tsimane, but differences in height-for-age and weight-for-age suggest Tsimane specific references may be useful for these measures.

Worm plot to diagnose fit in quantile regression

The worm plot is a series of detrended Q-Q plots, split by covariate levels. The worm plot is a diagnostic tool for visualizing how well a statistical model fits the data, for finding locations at which the fit can be improved, and for comparing the fit of different models. This paper shows how the worm plot can be used in conjunction with quantile regression. No parametric distributional assumptions are needed to create the worm plot. We fitted both an LMS and a quantile regression model on Dutch height data. The worm plot shows that the quantile regression model is superior to the LMS model in terms of fit. At the same time, it also contains a warning that the particular quantile model used may actually overfit the data. The resulting quantile curves are wiggly at the extremes, and appear less well suited for drawing growth diagrams. The paper concludes that the worm plot is a natural diagnostic tool for quantile regression.

Physical growth of the shuar: Height, Weight, and BMI references for an indigenous amazonian population

OBJECTIVES

Information concerning physical growth among small-scale populations remains limited, yet such data are critical to local health efforts and to foster basic understandings of human life history and variation in childhood development. Using a large dataset and robust modeling methods, this study aims to describe growth from birth to adulthood among the indigenous Shuar of Amazonian Ecuador.

METHODS

Mixed-longitudinal measures of height, weight, and body mass index (BMI) were collected from Shuar participants (n = 2,463; age: 0-29 years). Centile growth curves and tables were created for each anthropometric variable of interest using Generalized Additive Models for Location, Scale, and Shape (GAMLSS). Pseudo-velocity and Lambda-Mu-Sigma curves were generated to further investigate Shuar patterns of growth and to facilitate comparison with United States Center for Disease Control and Prevention and multinational World Health Organization growth references.

RESULTS

The Shuar are small throughout life and exhibit complex patterns of growth that differ substantially from those of international references. Similar to other Amazonians, Shuar growth in weight compares more favorably to references than growth in height, resulting in BMI curves that approximate international medians. Several additional characteristics of Shuar development are noteworthy, including large observed variation in body size early in life, significant infant growth faltering, extended male growth into adulthood, and a markedly early female pubertal growth spurt in height. Phenotypic plasticity and genetic selection in response to local environmental factors may explain many of these patterns.

CONCLUSIONS

Providing a detailed reference of growth for the Shuar and other Amazonian populations, this study possesses direct clinical application and affords valuable insight into childhood health and the ecology of human growth.

Growth Velocity of Infants From Birth to 5 Years Born in Maku, Iran

BACKGROUND

Growth velocity standards are essential for proper evaluation of child growth. The goal of this study was to construct weight, height and head circumference growth velocity charts for infants.

METHODS

This study includes 256 infants (124 boys and 132 girls) born in Maku, Northwest of Iran, and monitored from birth until they were 5 years. The weights and heights of the subjects were recorded at birth, one, two, four, six months and 1, 1.5, 2, 3, 4 and 5 years of age, while the head circumferences were measured until they were 1.5 years old. In this study, the LMS method using LMS chart maker software, was utilized to obtain growth velocity centiles.

RESULTS

Growth velocity charts for weight, height and head circumference (5th, 50th, 95th percentiles) were obtained. The velocity growth charts decreased rapidly from birth to 2 years and then remained relatively constant up to 5 years for both sexes. The growth velocity of boys was higher than girls through the first year of age but became equal at 12 months of age and no significant difference was seen up to 5 years.

CONCLUSION

Growth velocity studies are really sparse in Iran. In this study, longitudinal data were used to obtain growth velocity centiles. Furthermore, the weight and height velocities of infants from Shiraz, southern Iran, and U.K were compared.

Uterine artery impedance during the first eight postpartum weeks

The aim of this study was to construct reference ranges for the uterine artery (UtA) mean pulsatility (PI) and resistance (RI) indices from 1–8 weeks postpartum. A prospective, cross-sectional, and observational study was performed with 320 healthy women from week 1 through week 8 postpartum. UtAs were examined transvaginally using colour and pulsed Doppler imaging, and the means of the right and left values of the PI and RI, as well as the presence or absence of a bilateral protodiastolic notch, were recorded. The 5^th, 50^th and 95^th reference percentile curves for the UtA-PI and UtA-RI were derived using regression models. The adjusted reference intervals uncovered a convergence trend at the week 8 time-point, although impedance was lower at the week 1 time-point in multiparous women compared with primiparous women. The notching prevalence was 22.5% (9/40) at week 1 and 95.0% (38/40) at week 8. The study revealed consistent evidence of a progressive increase of postpartum uterine impedance and provided new average UtA-PI and UtA-RI reference charts for weeks 1 through 8. Multiparity does not change the trend but does impart a lower rate of increase, likely as a consequence of previous vascular structural and functional differences.

Bayesian calibration for forensic age estimation

Forensic medicine is increasingly called upon to assess the age of individuals. Forensic age estimation is mostly required in relation to illegal immigration and identification of bodies or skeletal remains. A variety of age estimation methods are based on dental samples and use of regression models, where the age of an individual is predicted by morphological tooth changes that take place over time. From the medico-legal point of view, regression models, with age as the dependent random variable entail that age tends to be overestimated in the young and underestimated in the old. To overcome this bias, we describe a new full Bayesian calibration method (asymmetric Laplace Bayesian calibration) for forensic age estimation that uses asymmetric Laplace distribution as the probability model. The method was compared with three existing approaches (two Bayesian and a classical method) using simulated data. Although its accuracy was comparable with that of the other methods, the asymmetric Laplace Bayesian calibration appears to be significantly more reliable and robust in case of misspecification of the probability model. The proposed method was also applied to a real dataset of values of the pulp chamber of the right lower premolar measured on x-ray scans of individuals of known age.

Production of infant scale evaluation (PRISE) in Italian normal hearing children: a validation study

OBJECTIVES

Parental questionnaires are important tools in the evaluation of auditory and language skills of very young children affected by sensorineural hearing loss. One of these instruments is the Production on Infant Scale Evaluation (PRISE). The purposes of this study were to adapt and validate the PRISE on Italian children with normal hearing; and to obtain normative data.

METHODS

A back translation technique was used to adapt the Italian version of PRISE. The PRISE was submitted to parents of 234 normal children aged between 3 and 18 months of life. All of them passed local universal newborn hearing screenings and they presented no audiological risk factors.

RESULTS

The PRISE internal reliability (Cronbach's alpha) was 0.87. Split-half reliability indexes were λ(4) = 0.89 and λ(6) = 0.89. Corrected item-total correlation coefficients were significant for all items. The correlation of PRISE with a modified Infant Toddler Meaningful Auditory Integration Scale (IT-MAIS), collected for convergent validity measurement purposes, was good (r = 0.743). A positive correlation of PRISE scores with age was found, reflecting on the age-dependence of pre-verbal skills.

CONCLUSION

These findings demonstrate high reliability and convergent validity of the Italian PRISE version. This questionnaire constitutes a robust tool for assessing early language development in infants and toddlers with normal hearing. It seems particularly sensitive to the normal language development in the first years of life, which can be very useful for early rehabilitation of hearing loss.

Growth references for 0-19 year-old Norwegian children for length/height, weight, body mass index and head circumference

BACKGROUND

Previous growth references for Norwegian children were based on measurements from the 1970s and 1980s. New reference data, collected through the Bergen Growth Study and the Medical Birth Registry of Norway, are presented as LMS values.

MATERIALS AND METHODS

A cross-sectional sample of children aged 0-19 years in stratified randomized design measured in 2003-2006 as a part of the Bergen Growth Study (n = 7291) and birth data of children born in 1999-2003 from the Medical Birth Registry of Norway (n = 12 576) was used to estimate the new references by the means of the LMS method. Measurement reliability was assessed by test-rest studies.

RESULTS

New references were constructed for length/height, weight, body mass index (BMI) and head circumference. Length/height and weight for children aged 0-4 years were similar to previous Norwegian references, but mean height increased up to a maximum of 3.4 cm in boys and 2.5 cm in girls during the pubertal years. Mean height was similar to (or slightly higher) in comparison with other recent European references. Reliability of the measurements compared well with published estimates.

CONCLUSION

Because of the observed secular trends in growth, it is advised to use the new references, which have been endorsed by the Norwegian Department of Health.

Comparing the Nidek MP-1 and Humphrey field analyzer in normal subjects

PURPOSE

To compare visual fields on the Nidek MP-1 to those obtained on the Humphrey field analyzer (HFA) in healthy volunteers and assess the effects of differences in stimulus parameters and testing strategies that may influence the interpretation of results in patients. A secondary aim was to establish MP-1 normative data to calculate the total deviation analyses and global indices analogous to those used by the HFA.

METHODS

Fifty healthy volunteers (age 43.5 ± 13.9 years, range, 18 to 68 years) underwent repeat MP-1 and HFA visual field testing, using the 10-2 pattern. MP-1 data were converted to HFA equivalent dB units. Between instrument comparisons of HFA and MP-1 sensitivities, regression of sensitivity with age and examination duration were assessed. Test-retest variability was examined between visits.

RESULTS

MP-1 (mean = 32.82 dB, SD = 1.92 dB) and HFA sensitivities (mean = 32.84 dB, SD = 1.83 dB) were not significantly different (p = 0.759). SD values for the HFA (range, 1.11 to 3.30 dB) were similar to the MP-1 (range, 0.14 to 2.75 dB). However, asymmetry comparisons between instruments showed significantly decreased superior rather than inferior retinal values for the MP-1. There was a small but significant difference (p = 0.004) in mean test duration between the MP-1 (mean = 6:11 min, SD = 1:49 min) and the HFA (mean = 5:14 min, SD = 0:42 min). There was also a difference in the decline of mean sensitivity with age, a decline of 0.1 and 0.4 dB per decade was noted in MP-1 and HFA sensitivity, respectively. Test-retest variability was similar between instruments. A small but non-significant increase in mean sensitivity at the second visit for both the MP-1 (p = 0.060) and HFA (p = 0.570) was found.

CONCLUSIONS

Both instruments showed similar variability and test-retest variability when results were compared using equivalent units. However, there are important differences in sensitivity values, stimulus parameters, and testing strategies that have to be taken into account when comparisons are made.

Linear time-dependent reference intervals where there is measurement error in the time variable-a parametric approach

This article re-examines parametric methods for the calculation of time specific reference intervals where there is measurement error present in the time covariate. Previous published work has commonly been based on the standard ordinary least squares approach, weighted where appropriate. In fact, this is an incorrect method when there are measurement errors present, and in this article, we show that the use of this approach may, in certain cases, lead to referral patterns that may vary with different values of the covariate. Thus, it would not be the case that all patients are treated equally; some subjects would be more likely to be referred than others, hence violating the principle of equal treatment required by the International Federation for Clinical Chemistry. We show, by using measurement error models, that reference intervals are produced that satisfy the requirement for equal treatment for all subjects.

Lung function testing in health and disease: issues pertaining to Asia-Pacific populations

The Asia-Pacific region comprise of diverse ethnic populations and healthcare systems of highly variable developmental stages. We face challenges in lung function testing, many of which are different from the issues encountered in the west. In some places, access to proper lung function testing facilities is very limited due to resource constraints or geographic inconvenience. It is important to note the potential differences of normative values in Asia-Pacific populations, compared with established values derived from populations of Caucasian ethnicity, hence the need to choose from available prediction formulae the ones that most closely approximate the subjects being tested. The criteria for defining abnormality is a hot topic globally. In the Asia-Pacific region, the trade-off between simplicity and thus ease of application and the pitfall of under- or over-diagnosis based on fixed cut-off values has to be recognized. The promotion of wider availability and proper utilization of lung function testing will go a long way in the enhancement of respiratory health and disease management in the region.

A two-part model for reference curve estimation subject to a limit of detection

Reference curves are commonly used to identify individuals with extreme values of clinically relevant variables or stages of progression which depend naturally on age or maturation. Estimation of reference curves can be complicated by a technical limit of detection (LOD) that censors the measurement from the left, as is the case in our study of reproductive hormone levels in boys around the time of the onset of puberty. We discuss issues with common approaches to the LOD problem in the context of our pubertal hormone study, and propose a two-part model that addresses these issues. One part of the proposed model specifies the probability of a measurement exceeding the LOD as a function of age. The other part of the model specifies the conditional distribution of a measurement given that it exceeds the LOD, again as a function of age. Information from the two parts can be combined to estimate the identifiable portion (i.e. above the LOD) of a reference curve and to calculate the relative standing of a given measurement above the LOD. Unlike some common approaches to LOD problems, the two-part model is free of untestable assumptions involving unobservable quantities, flexible for modeling the observable data, and easy to implement with existing software. The method is illustrated with hormone data from the Third National Health and Nutrition Examination Survey.

Neonatal anthropometric charts: the Italian neonatal study compared with other European studies

BACKGROUND AND OBJECTIVE

This was a nationwide prospective study carried out in Italy between 2005 and 2007, involving 34 centers with a neonatal intensive care unit. The study reports the Italian Neonatal Study charts for weight, length, and head circumference of singletons born between 23 and 42 gestational weeks, comparing them with previous Italian data and with the most recent data from European countries.

PATIENTS AND METHODS

Single live born babies with ultrasound assessment of gestational age within the first trimester, and with both parents of Italian origin. Only fetal hydrops and major congenital anomalies diagnosed at birth were excluded. The reference set consists of 22,087 girls and 23,375 boys.

RESULTS

At each gestational age, boys are heavier than girls by about 4%. Later-born neonates are heavier than firstborn neonates by about 3%. The effects of sex and birth order on length and head circumference are milder. No differences were observed between babies born in central-north Italy and southern Italy. A large variability emerged among European neonatal charts, resulting in huge differences in the percentage of Italian Neonatal Study neonates below the 10th centile, which is traditionally used to define small-for-gestational-age babies. In the last 2 decades prominent changes in the distribution of birth weight emerged in Italy and in the rest of Europe, in both term and preterm neonates.

CONCLUSIONS

The existing European neonatal charts, based on more or less recent data, were found to be inappropriate for Italy. Until an international standard is developed, the use of national updated reference charts is recommended.

Normal values for fundus perimetry with the microperimeter MP1

PURPOSE

To identify age-stratified normal light sensitivity values for microperimetry (fundus perimetry) and to evaluate the short-term repeatability of the MP1 microperimeter in normal volunteers.

DESIGN

Multicenter, prospective, observational study.

PARTICIPANTS

One hundred ninety subjects.

METHODS

One hundred ninety eyes of 190 healthy volunteers (age range, 20-75 years) underwent automatic, full-threshold microperimetry of the central field (20 x 20 degrees, 77 stimulated points) with the MP1 microperimeter (Nidek Technologies, Gamagori, Japan). Fixation was documented simultaneously. A subgroup of 10 subjects was retested after 1 hour and 1 week to determine the repeatability of this technique.

MAIN OUTCOME MEASURES

By linear regression analyses, light sensitivity values were obtained from 4 fundus areas and were analyzed for differences related to region, age, and, in a subset of subjects, repeat testing over time and right and left eye variability. Short-term repeatability for each area was evaluated by calculating intraclass correlation coefficients.

RESULTS

Linear regression analysis showed a significantly greater (P<0.0001) mean macular sensitivity of 19.6+/-0.5 dB in the 20 to 29 years of age group compared with 18.6+/-1.5 dB in the oldest age group of 70 to 75 years. These results were confirmed by the fifth percentile of light sensitivity threshold distribution. Normal and 95% confidence interval age-stratified values were calculated. When results for all 190 subjects were analyzed by region, the superior retinal sector showed significantly lower mean sensitivity values than other sectors (P<0.01, Bonferroni test). In a subset of 10 subjects, repeatability of the test performed at 3 separate visits showed consistent values over time in all areas (P<0.01, intraclass correlation coefficients).

CONCLUSIONS

Automatic fundus perimetry with the MP1 microperimeter allows for an accurate, repeatable, topographically specific examination of the retinal threshold in selected retinal areas. These findings are the first extensive database of age-related, normal MP1 microperimetry results available to clinicians.

Physiologically interpretable prediction equations for spirometric indexes

The need for ethnic-specific reference values of lung function variables (LFs) is acknowledged. Their estimation requires expensive and laborious examinations, and therefore additional use of results in physiology and epidemiology would be profitable. To this end, we proposed a form of prediction equations with physiologically interpretable coefficients: a baseline, the onset age (A0) and rate (S) of LF decline, and a height coefficient. The form was tested with data from healthy, nonsmoking Poles aged 18–85 yr (1,120 men, 1,625 women) who performed spirometry maneuvers according to American Thoracic Society criteria. The values of all the coefficients (also A0) for several LFs were determined with regression of LF on patient's age and deviation of patient's height from the mean height in the year group of this patient. S values for forced vital capacity (FVC), forced expiratory volume in 1 s (FEV1), peak expiratory flow, and maximal expiratory flow at 75% of FVC (MEF75) were very similar in both sexes (1.03 ± 0.07%/yr). FEV1/FVC declines four to five times slower. S for MEF25 appeared age dependent. A0 was smallest (28–32 yr) for MEF25 and FEV1. About 50% of each age subgroup (18–40, 41–60, 61–85 yr) exhibited LFs below the mean, and 4–6% were below the 5th percentile lower limits of normal, and thus the form of equations proposed in the paper appeared appropriate for spirometry. Additionally, if this form is accepted, epidemiological and physiological comparison of different LFs and populations will be possible by means of direct comparison of the equation coefficients.

Computation of profile likelihood-based confidence intervals for reference limits with covariates

Many biochemical quantities depend on age or some other covariate. Reference limits that allow for these dependencies help physicians to interpret the results of biochemical tests. Because reference limits must be estimated, it is important to assess their precision with, for example, confidence intervals. This paper relies on the assumption that data can be modeled by a generalized linear model and presents a method for calculating approximate profile likelihood-based confidence intervals for reference limits. The calculation of confidence intervals is based on a new method that draws on profile likelihood-based confidence intervals in general statistical models. The asset of this new method is that only two constrained optimization problems have to be solved instead of several in the standard method. We motivate our confidence interval calculation method with two applications. The first is for data on immunoglobulin concentration in the context of a generalized linear model with gamma distribution. This model is compared with the often used lognormal model. The second application handles data on serum alpha-fetoprotein and is presented in a linear regression situation. In the latter application the widths of the calculated profile confidence intervals are compared with exact and approximate regression-based intervals and the actual confidence levels are determined by simulation.

Variability of the prevalence of undiagnosed airflow obstruction in smokers using different diagnostic criteria

PURPOSES

To estimate the prevalence of undiagnosed airflow obstruction (AFO) in Hong Kong smokers with no previous diagnosis of respiratory disease, and to assess its variability when applying different prediction equations and diagnostic criteria.

METHODS

A multicenter, population-based, cross-sectional prevalence study was performed in smokers aged 20 to 80 years. Three different criteria (fixed 70% [Global Initiative for Chronic Obstructive Lung Disease and British Thoracic Society], fixed 75%, and European Respiratory Society [ERS]) were applied to define a lower limit of normal (LLN) of the FEV(1)/FVC ratio to compare with the Hong Kong Chinese reference equation (criterion 1), which had used a distribution-free method to obtain the lower fifth percentile of FEV(1)/FVC ratio as the LLN.

RESULTS

In 525 male patients, using criterion 1 (local internal prediction equation) and defining AFO as FEV(1)/FVC less than LLN, the overall prevalence of AFO was 13.7%: 8.3% in age > or = 20 to 40 years, 14.0% in age > or = 40 to 60 years, and 17.8% in age > or = 60 to 80 years. When the local internal prediction equation was used as the comparison reference, the fixed-ratio methods tended to miss AFO in younger age groups and overdiagnose AFO in old age, while the ERS criteria, which uses an almost lower fifth percentile-equivalent method, showed less of such a trend but still only showed moderate agreement with criterion 1.

CONCLUSIONS

Undiagnosed AFO was prevalent in Hong Kong smokers. Estimated prevalence rates were highly affected by the criteria used to define AFO. The predicted lower fifth percentile values calculated from a local reference equation as the LLN of FEV(1)/FVC ratio should be used for the diagnosis of AFO.

Reference values of diffusing capacity of non-smoking Chinese in Hong Kong

BACKGROUND AND OBJECTIVE

This study was conducted to define normal reference values and lower limits of normal (LLN) for single-breath carbon monoxide diffusing capacity (DLco) and DLco per unit of alveolar volume (Kco) for Chinese adults in Hong Kong.

METHODS

Healthy non-smoking men and women aged 18-80 years were recruited by random digit dialing. DLco and Kco were measured according to American Thoracic Society standards. Reference equations were obtained by multiple linear regression; LLN were derived by distribution-free method for estimation of age-related centiles.

RESULTS

Tests from 568 subjects (259 men, 309 women) were analysed. DLco declined with age in both genders, and increased with height and the interaction term of height and age in men and women, respectively. Considering Hb values did not improve the reference equations. Kco declined with age and increased with weight in both genders, while height and its interaction term with age were additional determinants in women. The reference DLco was lower than some Caucasian values, and was only explained partially by a smaller body size and alveolar volume in Chinese. The distribution-free method yielded better overall approximation to the fifth percentile compared with the traditional method of determining LLN.

CONCLUSIONS

The equations for reference values and LLN of diffusing capacity derived in this study are of clinical relevance to Chinese subjects.

A method is presented to plan the required sample size when estimating regression-based reference limits

BACKGROUND AND OBJECTIVE

Reference limits are widely used in anthropometry, the behavioral sciences, medicine, and clinical chemistry. They describe the distribution of a quantitative variable in a healthy population, and are often a smooth function of age or another determinant. Thus, instead of estimating reference limits separately for several age groups, it is more economical and parsimonious to use regression methods to estimate reference limits as a function of age. Although the variability of regression-based reference limits has been addressed previously, the available methods to determine the sample sizes needed to estimate them are neither transparent nor user-friendly.

METHODS

We propose a simple and intuitive formula using margins of error, to project the sample sizes required to achieve a given degree of precision, for different sampling strategies.

RESULTS

We present two examples for the calculation of the sample size required to estimate a specific reference limit using various age distributions.

CONCLUSION

We provide a simple formula to calculate the sample size needed to estimate a specific reference limit to a specified degree of precision. The structure of the formula can easily accommodate different age-sampling strategies.

Italian cross-sectional growth charts for height, weight and BMI (2 to 20 yr)

The aim of this study is to extend to pre-school ages the Italian Society for Pediatric Endocrinology and Diabetes (SIEDP)-2002 growth charts for height, weight and body mass index (BMI), to obtain charts (SIEDP-2006) that apply to the Italian population from 2 to 20 yr of age, taken as a whole, or separately in two geographical areas (Central-North Italy and South Italy). The charts are based on a sample of about 70,000 subjects attending infant, primary and secondary schools, between 1994 and 2004. The distribution of the sample by gender, age and geographic area was roughly similar to that of Italian school population in the last decade of the 20th century. Height and weight were measured using portable Harpenden stadiometers and properly calibrated scales, respectively. SIEDP-2006 references are presented both as centiles and as LMS curves for the calculation of SD scores, and include the extra-centiles for overweight and obesity. Large differences in BMI growth pattern emerged between the SIEDP-2006, 2000 CDC and UK90 references: in Italy, BMI is higher and its distribution is more skewed during childhood and adolescence. At the end of growth, median values of the three references are similar, but the 97th centile of 2000 CDC charts is much higher and increases more steeply than that of SIEDP-2006 charts, which on the contrary reach a plateau. SIEDP-2006 references intend to supply pediatricians with a tool that avoids the use of charts that are outdated or that refer to other populations, and thus should be suitable for adequately monitoring the growth of their patients.

Construction of the World Health Organization child growth standards: selection of methods for attained growth curves

The World Health Organization (WHO), in collaboration with a number of research institutions worldwide, is developing new child growth standards. As part of a broad consultative process for selecting the best statistical methods, WHO convened a group of statisticians and child growth experts to review available methods, develop a strategy for assessing their strengths and weaknesses, and discuss methodological issues likely to be faced in the process of constructing the new growth curves. To select the method(s) to be used, the group proposed a two-stage decision-making process. First, to select a few relevant methods based on a list of set criteria and, second, to compare the methods using available tests or other established procedures. The group reviewed 30 methods for attained growth curves. Using the pre-defined criteria, a few were selected combining five distributions and two smoothing techniques. Because the number of selected methods was considered too large to be fully tested, a preliminary study was recommended to evaluate goodness of fit of the five distributions. Methods based on distributions with poor performance will be eliminated and the remaining methods fully tested and compared.

Updated spirometric reference values for adult Chinese in Hong Kong and implications on clinical utilization

STUDY OBJECTIVES

The accuracy of reference values of lung function is important for assessment of severity and functional impairment of respiratory diseases. The aim of the study was to establish updated prediction formulae of spirometric parameters for Hong Kong Chinese and to compare the reference values with those derived from other studies in white and Chinese subjects.

DESIGN

Cross-sectional multicenter study.

SETTING

Lung function laboratories of eight regional hospitals in Hong Kong.

PARTICIPANTS

Subjects were recruited by random-digit dialing. One thousand one hundred seventy-six subjects who fulfilled recruitment criteria underwent spirometry.

MEASUREMENTS

Spirometry was performed according to American Thoracic Society recommendations, and the technique was standardized among the eight participating lung function laboratories.

RESULTS

Evaluable data of 1,089 (494 men and 595 women) healthy nonsmokers aged 18 to 80 years were analyzed. Age and height were found to be the major determinants of FEV1 and FVC, with a linear decline of height-adjusted values with age in both sexes. Spirometric values of this population have increased compared to Chinese populations of similar sex, age, and height two decades ago. Reference values derived from white populations were higher than our values by 5 to 19%, and the degree of overestimation varied with age, sex, and lung function parameter. We also demonstrated that the blanket application of correction factors for Asian populations may not be appropriate. In this study cohort, the distribution-free estimation of age-related centiles was more appropriate for the determination of lower limits of normal.

CONCLUSIONS

Our findings underscore the need to use reference values based on updated data derived from local populations or those matched for ethnicity and other sociodemographic characteristics.

Height and weight of primary schoolchildren in Shiraz city, southern Iran, 2002

This paper aims at presenting updated growth reference data for height and weight of 2,397 healthy schoolchildren (1,268 boys and 1,129 girls) aged 6.5-11.5 years in Shiraz (Iran), using a multistage sampling scheme. Our schoolchildren are now significantly taller and heavier for their age than their peers born 15 years earlier. However, the statistical models for estimating age-related centiles were consistent. A comparison of our current data with Centers for Disease Control growth charts for 2000 show that our height and weight medians correspond almost to the 40th centile of the latter, indicating less of a deficit than the previous growth study in Shiraz. We conclude that it is more realistic to use local growth standards for clinical work in Iran, which should be updated periodically.

Growth velocity standards from longitudinally measured infants of age 0-2 years born in Shiraz, southern Iran

This paper presents growth velocity data for a cohort of 317 infants (164 girls and 153 boys) born in Shiraz, southern Iran, in 1996 and followed longitudinally for 2 years. The growth velocity of five anthropometric measurements (height, weight, arm, head, and chest circumferences) declined rapidly from 2 weeks to 7 months of age and less steeply thereafter for both sexes. Growth velocities for boys were higher than girls in the first months but did not differ significantly later on. However, growth velocity centiles for girls lie below those for boys throughout the age range studied. Weight velocities in the Iranian infants were significantly lower than for corresponding infants in the U.K., the Middle East, and Africa, but height velocities were generally higher.

A comparison of goodness of fit tests for age-related reference ranges

Age-related reference ranges based on cubic spline or kernel-based smoothing methods lack a parametric framework with which to assess goodness of fit, as the distribution of the log-likelihood ratio comparing models with different degrees of smoothing is not known in general. Several methods have been proposed to assess goodness of fit in this case: Healy's grid test, Royston's Q tests and van Buuren's worm plot. Here we compare the performance of the various methods, including an extension of the Q tests, and show how they can be used, with other information including the appearance of the fitted curves, to inform the choice of model for age-related reference ranges of height and weight in a large Dutch data set.

Centile Charts II: Alternative Nonparametric Approach for Establishing Time-Specific Reference Centiles and Assessment of the Sample Size Required

Background: Reference intervals, and more generally centile estimates, are used to characterize a reference population for the purposes of interpreting an individual patient’s clinical measurement. We describe methods of calculating reference intervals where these centiles vary with a covariate, usually age or time.

Methods: The US Food and Drug Administration and the IFCC have made recommendations on two approaches: the parametric approach, which models the structural characteristics of the data set with a theoretical distribution, and the nonparametric approach, which makes no particular assumption about this structure. In this report we propose a nonparametric procedure that relies on the principles of regression and show how sample size determination can be assessed. We also show how the sample size calculation is influenced by the distribution of the times measured.

Results: We illustrated our method on three data sets and compared the results for our proposed nonparametric method with parametric estimates. We showed that the bias is reduced and that the nonparametric method is less likely to produce fluctuating profiles.

Conclusions: To achieve adequate precision the sample size needs to be larger than 120, as has often been recommended. If there is doubt about the parametric model, then threshold sample sizes may need to be as high as 500.

Parametric methods for estimating covariate-dependent reference limits

Age-specific reference limits are required for many clinical laboratory measurements. Statistical assessment of calculated intervals must be performed to obtain reliable reference limits. When parametric, covariate-dependent limits are derived, normal distribution theory usually is applied due to its mathematical simplicity and relative ease of fitting. However, it is not always possible to transform data and achieve a normal distribution. Therefore, models other than those based on normal distribution theory are needed. Generalized linear model theory offers one such alternative. Regardless of the statistical model used, the assumptions behind the model should always be examined.

Diagnostics for the exponential normal growth curve model

The exponential normal regression model provides a flexible parametric approach to constructing growth charts or centile curves. We develop diagnostics for this model, with a special emphasis towards assessing the impact of individual cases on estimated centile curves. We also propose a standardized score residual that assesses the fit of individual cases to the location, scale, and skewness functions, respectively. An application of the proposed diagnostics is presented.

Likelihood-based modelling of age-related normal ranges for ordinal measurements: changes in visual acuity through early childhood

This paper investigates the construction of age-related standards for ordinal outcome data. Asymmetric logistic models are used to describe the age-related changes in the cumulative probabilities for each ordinal level. Maximum likelihood estimation of model parameters allows the use of likelihood ratio tests to ascertain the appropriate model complexity. In contrast to methodologies for constructing age-related standards where the outcome is continuous, we show how the methodology leads directly to centile estimation for individuals. The method is illustrated using visual acuity measurements collected from 2968 children between 2 and 9 years of age made on a 30-point ordinal scale. We show how, in this instance, smoothing of parameters across ordinal categories leads to reduced validity of the centiles, justifying the need for specialized methodology for non-continuous outcomes.