Critical lessons from a pragmatic randomized trial of home-based COVID-19 testing in rural Native American and Latino communities

PURPOSE

Native Americans and Latinos have higher COVID-19 infection and mortality rates and may have limited access to diagnostic testing. Home-based testing may improve access to care in rural and underserved populations. This study tests the effect of community health worker (CHW) support on accessibility, feasibility, and completion of COVID-19 home testing among Native American and Latino adults living on the Flathead Reservation in Montana and in Yakima Valley, Washington.

METHODS

A two-arm, multisite, pragmatic randomized controlled trial was conducted using block randomization stratified by site and participant age. Active arm participants received CHW assistance with online COVID-19 test kit registration and virtual swabbing support. The passive arm participants received standard-of-care support from the kit vendor. Logistic regression modeled the association between study arm and test completion (primary outcome) and between study arm and test completion with return of valid test results (secondary outcome). Responses to posttest surveys and interviews were summarized using deductive thematic analysis.

FINDINGS

Overall, 63% of participants (n = 268) completed COVID-19 tests, and 50% completed tests yielding a valid result. Active arm participants had higher odds of test completion (odds ratio: 1.66, 95% confidence interval [1.01, 2.75]). Differences were most pronounced among adults ≥60 years. Participants cited ease of use and not having to leave home as positive aspects, and transportation and mailing issues as negative aspects of home-based testing.

CONCLUSIONS

CHW support led to higher COVID-19 test completion rates, particularly among older adults. Significant testing barriers included language, educational level, rurality, and test kit issues.

A pragmatic randomized trial of home-based testing for COVID-19 in rural Native American and Latino communities: Protocol for the "Protecting our Communities" study

BACKGROUND

Home-based testing for COVID-19 has potential to reduce existing health care disparities among underserved populations in the United States. However, implementation of home-based tests in these communities may face significant barriers. This study evaluates the acceptability, feasibility, and success of home-based testing and the potential added benefit of active support from trusted community health workers for Native Americans and Hispanic/Latino adults living in rural Montana and Washington states.

METHODS/DESIGN

The academic-community research team designed the trial to be responsive to community needs for understanding barriers and supports to home-based COVID-19 testing. The "Protecting Our Community" study is a two-arm pragmatic randomized controlled trial in which a total of 400 participants are randomized to active or passive arms. Participants of both study arms receive a commercially available home collection COVID-19 test kit, which is completed by mailing a self-collected nasal swab to a central laboratory. The primary study outcome is return of the kit to the central lab within 14 days. The cultural, social, behavioral, and economic barriers to home-based COVID-19 testing are also assessed by qualitative research methods. A survey and semi-structured interviews are conducted after the trial to evaluate perceptions and experience of home-based testing.

DISCUSSION

Implementing home-based testing in underserved populations, including among Native American and Hispanic/Latino communities, may require additional support to be successful. The Protecting Our Community trial examines the effect of trusted community health workers on use of home-based testing, which may be adaptable for community-driven models of home-based testing in other underserved populations.

Health Consumer Engagement, Enablement, and Empowerment in Smartphone-Enabled Home-Based Diagnostic Testing for Viral Infections: Mixed Methods Study

Background

Health consumers are increasingly taking a more substantial role in decision-making and self-care regarding their health. A range of digital technologies is available for laypeople to find, share, and generate health-related information that supports their health care processes. There is also innovation and interest in home testing enabled by smartphone technology (smartphone-supported home testing [smart HT]). However, few studies have focused on the process from initial engagement to acting on the test results, which involves multiple decisions.

Objective

This study aimed to identify and model the key factors leading to health consumers’ engagement and enablement associated with smart HT. We also explored multiple levels of health care choices resulting from health consumer empowerment and activation from smart HT use. Understanding the factors and choices associated with engagement, enablement, empowerment, and activation helps both research and practice to support the intended and optimal use of smart HT.

Methods

This study reports the findings from 2 phases of a more extensive pilot study of smart HT for viral infection. In these 2 phases, we used mixed methods (semistructured interviews and surveys) to shed light on the situated complexities of health consumers making autonomous decisions to engage with, perform, and act on smart HT, supporting the diagnostic aspects of their health care. Interview (n=31) and survey (n=282) participants underwent smart HT testing for influenza in earlier pilot phases. The survey also extended the viral infection context to include questions related to potential smart HT use for SARS-CoV-2 diagnosis.

Results

Our resulting model revealed the smart HT engagement and enablement factors, as well as choices resulting from empowerment and activation. The model included factors leading to engagement, specifically various intrinsic and extrinsic influences. Moreover, the model included various enablement factors, including the quality of smart HT and the personal capacity to perform smart HT. The model also explores various choices resulting from empowerment and activation from the perspectives of various stakeholders (public vs private) and concerning different levels of impact (personal vs distant).

Conclusions

The findings provide insight into the nuanced and complex ways health consumers make decisions to engage with and perform smart HT and how they may react to positive results in terms of public-private and personal-distant dimensions. Moreover, the study illuminates the role that providers and smart HT sources can play to better support digitally engaged health consumers in the smart HT decision process.

Diagnostic Accuracy of an At-Home, Rapid Self-test for Influenza: Prospective Comparative Accuracy Study

Background

Rapid diagnostic tests (RDTs) for influenza used by individuals at home could potentially expand access to testing and reduce the impact of influenza on health systems. Improving access to testing could lead to earlier diagnosis following symptom onset, allowing more rapid interventions for those who test positive, including behavioral changes to minimize spread. However, the accuracy of RDTs for influenza has not been determined in self-testing populations.

Objective

This study aims to assess the accuracy of an influenza RDT conducted at home by lay users with acute respiratory illness compared with that of a self-collected sample by the same individual mailed to a laboratory for reference testing.

Methods

We conducted a comparative accuracy study of an at-home influenza RDT (Ellume) in a convenience sample of individuals experiencing acute respiratory illness symptoms. Participants were enrolled in February and March 2020 from the Greater Seattle region in Washington, United States. Participants were mailed the influenza RDT and reference sample collection materials, which they completed and returned for quantitative reverse-transcription polymerase chain reaction influenza testing in a central laboratory. We explored the impact of age, influenza type, duration, and severity of symptoms on RDT accuracy and on cycle threshold for influenza virus and ribonuclease P, a marker of human DNA.

Results

A total of 605 participants completed all study steps and were included in our analysis, of whom 87 (14.4%) tested positive for influenza by quantitative reverse-transcription polymerase chain reaction (70/87, 80% for influenza A and 17/87, 20% for influenza B). The overall sensitivity and specificity of the RDT compared with the reference test were 61% (95% CI 50%-71%) and 95% (95% CI 93%-97%), respectively. Among individuals with symptom onset ≤72 hours, sensitivity was 63% (95% CI 48%-76%) and specificity was 94% (95% CI 91%-97%), whereas, for those with duration >72 hours, sensitivity and specificity were 58% (95% CI 41%-74%) and 96% (95% CI 93%-98%), respectively. Viral load on reference swabs was negatively correlated with symptom onset, and quantities of the endogenous marker gene ribonuclease P did not differ among reference standard positive and negative groups, age groups, or influenza subtypes. The RDT did not have higher sensitivity or specificity among those who reported more severe illnesses.

Conclusions

The sensitivity and specificity of the self-test were comparable with those of influenza RDTs used in clinical settings. False-negative self-test results were more common when the test was used after 72 hours of symptom onset but were not related to inadequate swab collection or severity of illness. Therefore, the deployment of home tests may provide a valuable tool to support the management of influenza and other respiratory infections.

Harmony COVID-19: A ready-to-use kit, low-cost detector, and smartphone app for point-of-care SARS-CoV-2 RNA detection

RNA amplification tests sensitively detect severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, but their complexity and cost are prohibitive for expanding coronavirus disease 2019 (COVID-19) testing. We developed “Harmony COVID-19,” a point-of-care test using inexpensive consumables, ready-to-use reagents, and a simple device. Our ready-to-use, multiplexed reverse transcription, loop-mediated isothermal amplification (RT-LAMP) can detect down to 0.38 SARS-CoV-2 RNA copies/μl and can report in 17 min for high–viral load samples (5000 copies/μl). Harmony detected 97 or 83% of contrived samples with ≥0.5 viral particles/μl in nasal matrix or saliva, respectively. Evaluation in clinical nasal specimens (n = 101) showed 100% detection of RNA extracted from specimens with ≥0.5 SARS-CoV-2 RNA copies/μl, with 100% specificity in specimens positive for other respiratory pathogens. Extraction-free analysis (n = 29) had 95% success in specimens with ≥1 RNA copies/μl. Usability testing performed first time by health care workers showed 95% accuracy.

Identifying Enablers of Participant Engagement in Clinical Trials of Consumer Health Technologies: Qualitative Study of Influenza Home Testing

Background

A rise in the recent trend of self-managing health using consumer health technologies highlights the importance of efficient and successful consumer health technology trials. Trials are particularly essential to support large-scale implementations of consumer health technologies, such as smartphone-supported home tests. However, trials are generally fraught with challenges, such as inadequate enrollment, lack of fidelity to interventions, and high dropout rates. Understanding the reasons underlying individuals’ participation in trials can inform the design and execution of future trials of smartphone-supported home tests.

Objective

This study aims to identify the enablers of potential participants’ trial engagement for clinical trials of smartphone-supported home tests. We use influenza home testing as our instantiation of a consumer health technology subject to trial to investigate the dispositional and situational enablers that influenced trial engagement.

Methods

We conducted semistructured interviews with 31 trial participants using purposive sampling to facilitate demographic diversity. The interviews included a discussion of participants’ personal characteristics and external factors that enabled their trial engagement with a smartphone-supported home test for influenza. We performed both deductive and inductive thematic analyses to analyze the interview transcripts and identify enabler themes.

Results

Our thematic analyses revealed a structure of dispositional and situational enablers that enhanced trial engagement. Situationally, clinical affiliation, personal advice, promotional recruitment strategies, financial incentives, and insurance status influenced trial engagement. In addition, digital health literacy, motivation to advance medical research, personal innovativeness, altruism, curiosity, positive attitude, and potential to minimize doctors’ visits were identified as the dispositional enablers for trial engagement in our study.

Conclusions

We organized the identified themes for dispositional and situational enablers of trial engagement with a smartphone-supported home test into a research framework that can guide future research as well as the trial design and execution of smartphone-supported home tests. We suggest several trial design and engagement strategies to enhance the financial and scientific viability of these trials that pave the way for advancements in patient care. Furthermore, our study also offers practical strategies to trial organizers to enhance participants’ enrollment and engagement in clinical trials of these home tests.

Home testing for COVID-19 and other virus outbreaks: The complex system of translating to communities

Home testing is an emerging innovation that can enable nations and health care systems to safely and efficiently test large numbers of patients to manage COVID-19 and other viral outbreaks.  In this position paper, we explore the process of moving home testing across the translational continuum from labs to households, and ultimately into practice and communities for optimal public health impact. We focus on the four translational science drivers to accelerate the implementation of systems-wide home testing programmes 1) collaboration and team science, 2) technology, 3) multilevel interventions, and 4) knowledge integration. We use the Socio Ecological Model (SEM) as a framework to illustrate our vision for the ideal future state of a comprehensive system of stakeholders utilising tech-enabled home testing for COVID-19 and other virus outbreaks, and we suggest SEM as a tool to address key translational readiness and response questions.

Evaluating an app-guided self-test for influenza: lessons learned for improving the feasibility of study designs to evaluate self-tests for respiratory viruses

Background

Seasonal influenza leads to significant morbidity and mortality. Rapid self-tests could improve access to influenza testing in community settings. We aimed to evaluate the diagnostic accuracy of a mobile app-guided influenza rapid self-test for adults with influenza like illness (ILI), and identify optimal methods for conducting accuracy studies for home-based assays for influenza and other respiratory viruses.

Methods

This cross-sectional study recruited adults who self-reported ILI online. Participants downloaded a mobile app, which guided them through two low nasal swab self-samples. Participants tested the index swab using a lateral flow assay. Test accuracy results were compared to the reference swab tested in a research laboratory for influenza A/B using a molecular assay.

Results

Analysis included 739 participants, 80% were 25–64 years of age, 79% female, and 73% white. Influenza positivity was 5.9% based on the laboratory reference test. Of those who started their test, 92% reported a self-test result. The sensitivity and specificity of participants’ interpretation of the test result compared to the laboratory reference standard were 14% (95%CI 5–28%) and 90% (95%CI 87–92%), respectively.

Conclusions

A mobile app facilitated study procedures to determine the accuracy of a home based test for influenza, however, test sensitivity was low. Recruiting individuals outside clinical settings who self-report ILI symptoms may lead to lower rates of influenza and/or less severe disease. Earlier identification of study subjects within 48 h of symptom onset through inclusion criteria and rapid shipping of tests or pre-positioning tests is needed to allow self-testing earlier in the course of illness, when viral load is higher.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12879-021-06314-1.

Diagnostic accuracy of an app-guided, self-administered test for influenza among individuals presenting to general practice with influenza-like illness: study protocol

INTRODUCTION

Diagnostic tests for influenza in Australia are currently only authorised for use in clinical settings. At-home diagnostic testing for influenza could reduce the need for patient contact with healthcare services, which potentially could contribute to symptomatic improvement and reduced spread of influenza. We aim to determine the accuracy of an app-guided nasal self-swab combined with a lateral flow immunoassay for influenza conducted by individuals with influenza-like illness (ILI).

METHODS AND ANALYSIS

Adults (≥18 years) presenting with ILI will be recruited by general practitioners (GP) participating in Australian Sentinel Practices Research Network. Eligible participants will have a nasal swab obtained by their GP for verification of influenza A/B status using reverse transcription polymerase chain reaction (RT-PCR) test at an accredited laboratory. Participants will receive an influenza test kit and will download an app that collects self-reported symptoms and influenza risk factors, then instructs them in obtaining a low-nasal self-swab, running a QuickVue influenza A+B lateral flow immunoassay (Quidel Corporation) and interpreting the results. Participants will also interpret an enhanced image of the test strip in the app. The primary outcome will be the accuracy of participants' test interpretation compared with the laboratory RT-PCR reference standard. Secondary analyses will include accuracy of the enhanced test strip image, accuracy of an automatic test strip reader algorithm and validation of prediction rules for influenza based on self-reported symptoms. A post-test survey will be used to obtain participant feedback on self-test procedures.

ETHICS AND DISSEMINATION

The study was approved by the Human Research and Ethic Committee (HREC) at the University of Adelaide (H-2019-116). Protocol details and any amendments will be reported to https://www.tga.gov.au/. Results will be published in the peer-reviewed literature, and shared with stakeholders in the primary care and diagnostics communities.

TRIAL REGISTRATION NUMBER

Australia New Zealand Clinical Trial Registry (U1111-1237-0688).

LB21. The Seattle Flu Study: A Community-Based Study of Influenza

Background

Influenza epidemics and pandemics cause significant morbidity and mortality. An effective response to a potential pandemic requires the infrastructure to rapidly detect and contain new and emerging flu strains at a population level. The objective of this study was to use data gathered simultaneously from community and hospital sites to develop a model of how flu enters and spreads in a population.

Methods

In the 2018–2019 season, we enrolled individuals with respiratory illness from community sites throughout the Seattle area, including homeless shelters, childcare facilities, Seattle-Tacoma International Airport, workplaces, college campuses, clinics, and at home (Figure 1). We collected data and nasal swabs from individuals with at least two respiratory symptoms. Additionally, we collected residual nasal swabs and data from individuals who sought care at four regional hospitals. Home-based self-testing for influenza and prediction models for influenza were piloted. Swabs were tested with a multiplex molecular assay, and influenza whole-genome sequencing was performed. Geospatial mapping and computational modeling platforms were developed to characterize regional spread of respiratory pathogens.

Results

A total of 18,847 samples were collected in the 2018–2019 season. Of those tested to date, 291/3,653 (8%) community and 2,393/11,273 (21%) hospital samples have influenza detected. Of the community enrollments, 39% had influenza-like illness. Community enrollees were in age groups not well-represented from hospitals. Influenza A/H3N2 activity peaked on college campuses and homeless shelters 2 weeks before the peak in hospitals. We observed multiple independent introductions of influenza strains into the city and evidence of sustained transmission chains within the city (Figures 2 and 3).

Conclusion

Utilizing the city-wide infrastructure we developed, we observed the introduction of influenza A/H3N2 into the community before the hospital and evidence of transmissions of unique strains into and within the Seattle area. These data provide the blueprint for implementing city-wide, community-based surveillance systems for rapid detection, real-time assessment of transmission patterns, and interruption of spread of seasonal or pandemic strains.

Disclosures

Helen Y. Chu, MD MPH, Merck (Advisor or Review Panel member), Michael Boeckh, MD PhD, Ablynx (Consultant, Grant/Research Support), Ansun Biopharma (Consultant, Grant/Research Support), Bavarian Nordic (Consultant), Gilead (Consultant, Grant/Research Support), GlaxoSmithKline (Consultant), Vir Bio (Consultant, Grant/Research Support), Janet A. Englund, MD, Chimerix (Grant/Research Support), GlaxoSmithKline (Grant/Research Support), MedImmune/Astrazeneca (Grant/Research Support), Meissa Vaccines (Consultant), Merck (Grant/Research Support),Novavax (Grant/Research Support), Sanofi Pastuer (Consultant), Matthew Thompson, MD, Alere Inc. (Research Grant or Support), Roche Molecular Diagnostics (Consultant, Research Grant or Support, Speaker’s Bureau), . Other Authors: No reported disclosures.

1775. A Community-wide Study to Evaluate the Accuracy of Self-testing for Influenza: Works in Progress

Background

Seasonal influenza (flu) occurs annually, causing disease with substantial morbidity and mortality. Currently, flu is suspected from clinical features, but requires a laboratory test to confirm infection. No influenza tests in the United States are approved for use outside of clinical settings. We aimed to determine the accuracy of influenza self-testing using an at-home, app-guided, lateral flow assay compared with a molecular reference standard conducted at a laboratory among adults self-reporting influenza-like illness (ILI).

Methods

This is an observational study of individuals with self-reported ILI throughout the continental 48 United States recruited from the Flu Near You platform, online marketing, and clinics in the Seattle area. Recruitment took place from March 4 to April 26, 2019. Participants were directed to an iPhone App that determined eligibility, consent, and responses to symptom questions and risk factors. Individuals were mailed a commercially available CLIA-waived influenza lateral flow test to conduct at home, guided by the app, and returned the used test along with a second nasal swab collected in viral transport media to the research team. Influenza testing was performed by RT–PCR on the second nasal swab, as well as the residual fluid from the RDT. Accuracy of home test result (read by the participant), as well as image capture of the lateral flow test strip, were compared with the lab-based reference standard.

Results

To date, 1127 at-home flu tests were mailed to participants and 711 (63.1%) samples returned to the lab. There were 17 flu-positive results from the rapid diagnostic test for a flu positivity rate of 2.4%. Testing using the reference standard is currently in progress. We will share diagnostic accuracy results once testing of the reference standard is completed. Of the kits returned, 353 (49.7%)had an error recorded, which included errors in return packaging, reference standard, rapid test tube sample, or rapid test strip errors.

Conclusion

Overall, findings from this study will determine the accuracy of an at-home rapid diagnostic test, and inform more widely research design for evaluating smartphone-enhanced home tests for pathogens. Many samples returned to the lab had a recorded error, suggesting at-home testing requires additional feasibility testing and refinement of the current methods used.

Disclosures

All authors: No reported disclosures.

Cigarette smoking, tar yields, and non-fatal myocardial infarction: 14,000 cases and 32,000 controls in the United Kingdom. The International Studies of Infarct Survival (ISIS) Collaborators

OBJECTIVES

To assess the effects of cigarette smoking on the incidence of non-fatal myocardial infarction, and to compare tar in different types of manufactured cigarettes.

METHODS

In the early 1990s responses to a postal questionnaire were obtained from 13,926 survivors of myocardial infarction (cases) recently discharged from hospitals in the United Kingdom and 32,389 of their relatives (controls). Blood had been obtained from cases soon after admission for the index myocardial infarction and was also sought from the controls. 4923 cases and 6880 controls were current smokers of manufactured cigarettes with known tar yields. Almost all tar yields were 7-9 or 12-15 mg/cigarette (mean 7.5 mg for low tar (< 10 mg) and 13.3 for medium tar (> or = 10 mg). The cited risk ratios were standardised for age and sex and compared myocardial infarction rates in current cigarette smokers with those in non-smokers who had not smoked cigarettes regularly in the past 10 years.

RESULTS

At ages 30-49 the rates of myocardial infarction in smokers were about five times those in non-smokers (as defined); at ages 50-59 they were three times those in non-smokers, and even at ages 60-79 they were twice as great as in non-smokers (risk ratio 6.3, 4.7, 3.1, 2.5, and 1.9 at 30-39, 40-49, 50-59, 60-69, 70-79 respectively; each 2P < 0.00001). After standardisation for age, sex, and amount smoked, the rate of non-fatal myocardial infarction was 10.4% (SD 5.4) higher in medium tar than in low tar cigarette smokers (2P = 0.06). This percentage was not significantly greater at ages 30-59 (16.6% (7.1)) than at 60-79 (1.0% (8.5)). In both age ranges the difference in risk between cigarette smokers and non-smokers was much larger than the difference between one type of cigarette and another (risk ratio 3.39 and 3.95 at ages 30-59 for smokers of similar numbers of low and of medium tar cigarettes, and risk ratio 2.35 and 2.37 at ages 60-79). Most possible confounding factors that could be tested for were similar in low and medium tar users, with no significant differences in blood lipid or albumin concentrations.

CONCLUSION

The present study indicates that the imminent change of tar yields in the European Union to comply with an upper limit of 12 mg/cigarette will not increase (and may somewhat decrease) the incidence of myocardial infarction, unless they indirectly help perpetuate tobacco use. Even low tar cigarettes still greatly increase rates of myocardial infarction, however, especially among people in their 30s, 40s, and 50s, and far more risk is avoided by not smoking than by changing from one type of cigarette to another.

Three-year follow-up of the Oxford Cholesterol Study: assessment of the efficacy and safety of simvastatin in preparation for a large mortality study

We report the results of a randomized single-centre study designed to assess the effects of simvastatin on blood lipids, blood biochemistry, haematology and other measures of safety and tolerability in preparation for a large-scale multicentre mortality study. Six hundred and twenty-one individuals considered to be at increased risk of coronary heart disease were randomized, following a 2-month placebo 'run-in' period, to receive 40 mg daily simvastatin, 20 mg daily simvastatin or matching placebo. Their mean age was 63 years, 85% were male, 62% had a history of prior myocardial infarction (MI), and the mean baseline total cholesterol was 7.0 mmol.l-1. Median follow-up in the present report is 3.4 years. Eight weeks after randomization, 40 mg daily simvastatin had reduced non-fasting total cholesterol by 29.2% +/- 1.1 (2.03 +/- 0.08 mmol.l-1) and 20 mg daily simvastatin had reduced it by 26.8% +/- 1.0 (1.87 +/- 0.07 mmol.l-1). Almost all of the difference in total cholesterol at 8 weeks was due to the reduction in LDL cholesterol (40.8% +/- 1.6 and 38.2% +/- 1.4 among patients allocated 40 mg and 20 mg of simvastatin daily respectively), but simvastatin also reduced triglycerides substantially (19.0% and 17.3%) and produced a small increase in HDL cholesterol (6.4% and 4.8%). These effects were largely sustained over the next 3 years, with 40 mg daily simvastatin producing a slightly greater reduction in total cholesterol at 3 years (25.7% +/- 1.9 reduction) than did 20 mg daily simvastatin (22.2% +/- 1.8). There were no differences between the treatment groups in the numbers of reports of 'possible adverse effects' of treatment or of a range of different symptoms or conditions (including those related to sleep or mood) recorded at regular clinic follow-up. Mean levels of alanine aminotransferase, aspartate aminotransferase and creatine kinase were slightly increased by treatment, but there were no significant differences between the treatment groups in the numbers of patients with significantly elevated levels. A slightly lower platelet count in the simvastatin group was the only haematological difference from placebo, with no difference in the numbers of patients with low platelet counts. In summary, the simvastatin regimens studied produced large sustained reductions in total cholesterol, LDL cholesterol and triglyceride and small increases in HDL cholesterol. They were well tolerated, with no evidence of serious side-effects during the first 3 years of this study.(ABSTRACT TRUNCATED AT 400 WORDS)

Progesterone retention by rat uterus I. Pharmacokinetics after uterine intraluminal instillation

Tritium-labeled progesterone was administered to mature female rats in the proestrous stage by three different routes, gastric intubation, subcutaneous injection, and uterine intraluminal instillation, to study the kinetics involved in the uptake and retention of radioactivity by the uterus and various other tissues. Progesterone was retained at a much higher level and for a more prolonged period in the rat uterus after uterine intraluminal instillation. Progesterone bioavailability to the uterus was 45 times higher by uterine intraluminal instillation than by either gastric intubation or subcutaneous injection. Progesterone absorption by the rat endometrium was extremely fast. The observed biphasic decrease of radioactivity from the uterine tissue was explained adequately by a pharmacokinetic model in which progesterone is assumed to be present in two compartments within the uterine tissue. The pharmacokinetic parameters showed that the progesterone biological half-life in the uterine tissue during the alpha-phase was about 6.5 min while that in the beta-phase was about 230 min.