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

Dependence-Robust Confidence Intervals for Capture-Recapture Surveys

Capture-recapture (CRC) surveys are used to estimate the size of a population whose members cannot be enumerated directly. CRC surveys have been used to estimate the number of Coronavirus Disease 2019 (COVID-19) infections, people who use drugs, sex workers, conflict casualties, and trafficking victims. When k-capture samples are obtained, counts of unit captures in subsets of samples are represented naturally by a 2 k contingency table in which one element-the number of individuals appearing in none of the samples-remains unobserved. In the absence of additional assumptions, the population size is not identifiable (i.e., point identified). Stringent assumptions about the dependence between samples are often used to achieve point identification. However, real-world CRC surveys often use convenience samples in which the assumed dependence cannot be guaranteed, and population size estimates under these assumptions may lack empirical credibility. In this work, we apply the theory of partial identification to show that weak assumptions or qualitative knowledge about the nature of dependence between samples can be used to characterize a nontrivial confidence set for the true population size. We construct confidence sets under bounds on pairwise capture probabilities using two methods: test inversion bootstrap confidence intervals and profile likelihood confidence intervals. Simulation results demonstrate well-calibrated confidence sets for each method. In an extensive real-world study, we apply the new methodology to the problem of using heterogeneous survey data to estimate the number of people who inject drugs in Brussels, Belgium.

Use of Population-Based Surveys for Estimating the Population Size of Persons Who Inject Drugs in the United States

BACKGROUND

In the United States, injection is an increasingly common route of administration for opioids and other substances. Estimates of the number of persons who inject drugs (PWID) are needed for monitoring risk-specific infectious disease rates and health services coverage.

METHODS

We reviewed design and instruments for 4 national household surveys, 2012-2016, for their ability to produce unbiased injection drug use (IDU) prevalence estimates. We explored potential analytic adjustments for reducing biases through use of external data on (1) arrest, (2) narcotic overdose mortality, and (3) biomarker-based sensitivity of self-reported illicit drug use.

RESULTS

Estimated national past 12 months IDU prevalence ranged from 0.24% to 0.59% across surveys. All surveys excluded unstably housed and incarcerated persons, and estimates were based on <60 respondents reporting IDU behavior in 3 surveys. No surveys asked participants about nonmedical injection of prescription drugs. Analytic adjustments did not appreciably change IDU prevalence estimates due to suboptimal specificity of data points.

CONCLUSIONS

PWID population size estimates in the United States are based on small numbers and are likely biased by undercoverage of key populations and self-report. Novel methods as discussed in this article may improve our understanding of PWID population size and their health needs.

Estimates of people who injected drugs within the last 12 months in Belgium based on a capture-recapture and multiplier method

BACKGROUND

For Belgium, available estimates of the number of people who inject drugs (PWID) are based on data from more than fifteen years ago and apply only to those who report ever injecting drugs. As a result, no reliable baseline data exist to determine the scale of services for PWID.

METHODS

We obtained pseudo-anonymized identifier information from treatment and harm reduction service providers and a fieldwork study between February and April 2019 in Brussels. We estimated the number of PWID, defined as people who injected within the last 12 months, in Brussels using capture-recapture (CRC) methodology. To obtain national estimates, we scaled the proportion of PWID in Brussels to the total number of this population in Belgium based on two existing drug treatment registers, which were then multiplied with the result of the CRC.

RESULTS

The total population of PWID is estimated to be 703 (95 %CI 538-935) for Brussels and between 6620 (95 %CI 4711 - 8576) and 7018 (95 %CI 4794 - 9527) for Belgium.

CONCLUSIONS

These estimates provide crucial information to ensure that services to PWID are adequately maintained. They clearly indicate the need to maximize efforts to achieve the targets set by WHO for 2030 on the provision of 300 sterile needles and syringes per PWID per year, a 90 % reduction of new HCV infections, and a 65 % reduction of liver-related mortality.

Prevalence of Injecting Drug Use and Coverage of Interventions to Prevent HIV and Hepatitis C Virus Infection Among People Who Inject Drugs in Canada

Objectives. To determine the number of people who inject drugs (PWID) in Canada and the annual coverage of opioid agonist treatment (OAT) and needle-and-syringe provision for PWID.Methods. We estimated the number of PWID in 11 of 13 Canadian provinces and territories in 2011 by using indirect multiplier methods based on provincial and territorial methadone recipient totals and proportion of surveyed PWID receiving methadone. We modeled annual increases for 2011 to 2016 on Quebec and British Columbia longitudinal data. We calculated needle-and-syringe coverage (World Health Organization [WHO] recommendation: ≥ 200 per PWID) and OAT coverage (WHO recommendation: ≥ 40 per 100 PWID) per province and territory annually.Results. An estimated 130 000 individuals in Canada (0.55%) injected drugs in 2011, increasing to 171 900 individuals (0.70%) in 2016. Needle-and-syringe coverage increased from 193 to 291 per PWID, and OAT coverage increased from 55 to 66 per 100 PWID over the study period.Conclusions. While the number of PWID increased between 2011 and 2016, OAT coverage remained high, and needle-and-syringe coverage generally improved over time.Public Health Implications. These data will inform public health surveillance, service planning, and resource allocation, and assist monitoring of treatment and harm-reduction coverage outcomes.

Drug consumption rooms: A systematic review of evaluation methodologies

ISSUES

Drug consumptions rooms (DCR) and supervised injecting facilities (SIF) are expanding internationally. Previous reviews have not systematically addressed evaluation methodologies.

APPROACH

Results from systematic searches of scientific databases in English until June 2017 were coded for paper type, country and year of publication. For evaluation papers, study outcome, methodology/study design and main indicators of DCR/SIF 'exposure' were recorded.

KEY FINDINGS

Two hundred and nineteen eligible peer-reviewed papers were published since 1999: the majority from Canada (n = 117 papers), Europe (n = 36) and Australia (n = 32). Fifty-six papers reported evaluation outcomes. Ecological study designs (n = 10) were used to assess the impact on overdose, public nuisance and crime; modelling techniques (n = 6) estimated impact on blood-borne diseases, overdose deaths and costs. Papers using individual-level data included four prospective cohorts (n = 28), cross-sectional surveys (n = 7) and service records (n = 5). Individual-level data were used to assess safer injecting practice, uptake into health and social services and all the other above outcomes except for impact on crime and costs. Four different indicators of DCR/SIF attendance were used to measure service 'exposure'.

IMPLICATIONS

Research around DCRs/SIFs has used ecological, modelling, cross-sectional and cohort study designs. Further research could involve systematic inclusion of a control group of people who are eligible but do not access SIFs, validation of self-reported proportion of injections at SIFs or a stepped-wedge or a cluster trial comparing localities.

CONCLUSIONS

Methodologies appropriate for DCR/SIF evaluation have been established and can be readily replicated from the existing literature. Research on operational aspects, implementation and transferability is also warranted.

Patterns of Peer Distribution of Injecting Equipment at an Authorized Distribution Site in Sydney, Australia

BACKGROUND

Extended distribution refers to the practice whereby people who inject drugs pass on sterile injecting equipment to their networks and can be a means to access people who inject drugs who do not attend state-sanctioned needle and syringe programs. While it is legal, to possess a sterile syringe for the purpose of injecting drugs in New South Wales, Australia, it is a criminal offence to pass this equipment on for others to use. In 2013 a pilot project was established to trial the authorization of "extended" peer distribution. This research describes patterns of distribution among attendees participating in this trial.

METHODS

A cross-sectional survey was conducted during one week in October 2014 of the trial with 200 clients. The survey focused on the extent, characteristics, and perceived risks and benefits of extended distribution practices within peer groups.

RESULTS

Extended distribution is widespread, not in an organized or intentional manner but as a consequence of day-to-day drug using activities. The profiles of those who do and do not distribute were similar. Willingness to distribute small quantities of equipment to others was higher than willingness to distribute larger quantities, and willingness to distribute was related to perceived benefits of extended distribution. Police scrutiny was a key reason for not wanting to distribute.

CONCLUSION

Extended peer distribution is widespread though mostly not organized. This study supports the evidence that drug users act responsibly to prevent harm and promote the use of sterile equipment among their peers.

Programmatic mapping to determine the size and dynamics of sex work and injecting drug use in Mauritius

HIV in Mauritius is in a concentrated phase. Most HIV infection is among key populations (KPs) including female sex workers (FSW), people who inject drugs (PWID), men who have sex with men (MSM), and transgender sex workers (TGSW). The objective of this research was to use geographical mapping to determine the precise locations, typologies and population estimates of each KP in Mauritius. From May to July 2014 a programmatic mapping approach determined national estimates and information on the specific locations, "hotspots", frequented by KPs in Mauritius. Data were collected through multiple levels of activity. Key informants provided information about hotspots where KPs congregated and were actively engaged in risky behaviours. Validation of hotspots was done by engaging KPs. A total of 17 248 KP members were estimated spread over 1 964 hotspots in Mauritius. The largest KP was PWID with an estimate of 7 598 (range: 4 091-6 223), followed by 6 223 (range: 5 090-7 456) FSWs, spread over 694 and 731 hotspots respectively. A total of 2 020 (range: 1 595-2 446) MSM and 294 geographic MSM hotspots, and 1 407 TGSWs (range: 1 165-1 649) distributed over 245 hotspots. This research provided Mauritius with validated KPs size estimations and provided the first national data on KP hotspots and operational dynamics. The use of these data will strengthen HIV prevention, intervention, and programme planning in the country through focusing efforts to systematically target high-risk areas.

Theoretical and Empirical Comparisons of Methods to Estimate the Size of Hard-to-Reach Populations: A Systematic Review

Worldwide, the HIV epidemic is concentrated among hidden populations (i.e., female sex workers, men who have sex with men, and people who inject drugs). To understand the true scope and scale of the HIV epidemic, estimates of the sizes of these populations are needed. Various methods are available to enumerate hidden populations, but the degree of agreement between these methods has not been formally evaluated. We systematically reviewed the peer-reviewed literature to assess the extent to which different population size estimation methods provide the same estimate of a target population. Of the 341 studies identified from our search, 25 met our eligibility criteria. Twenty-one unique methods were documented. The service multiplier method was the most common in the review. Eighty target populations were estimated, covering 16 countries. We observed variable population size estimates, with little agreement between methods. We note trends in the relative performance of individual methods.

Estimating the number of regular and dependent methamphetamine users in Australia, 2002-2014

OBJECTIVE

To estimate the number of regular and dependent methamphetamine users in Australia.

DESIGN

Indirect prevalence estimates were made for each year from 2002-03 to 2013-14. We applied multiplier methods to data on treatment episodes for amphetamines (eg, counselling, rehabilitation, detoxification) and amphetamine-related hospitalisations to estimate the numbers of regular (at least monthly) and dependent methamphetamine users for each year. Dependent users comprised a subgroup of those who used the drug regularly, so that estimates of the sizes of these two populations were not additive.

RESULTS

We estimated that during 2013-14 there were 268 000 regular methamphetamine users (95% CI, 187 000-385 000) and 160 000 dependent users (95% CI, 110 000-232 000) aged 15-54 years in Australia. This equated to population rates of 2.09% (95% CI, 1.45-3.00%) for regular and 1.24% (95% CI, 0.85-1.81%) for dependent use. The rate of dependent use had increased since 2009-10 (when the rate was estimated to be 0.74%), and was higher than the previous peak (1.22% in 2006-07). The highest rates were consistently among those aged 25-34 years, in whom the rate of dependent use during 2012-2013 was estimated to be 1.50% (95% CI, 1.05-2.22%). There had also been an increase in the rate of dependent use among those aged 15-24 years (in 2012-13 reaching 1.14%; 95% CI, 0.80-1.69%).

CONCLUSIONS

There have been increases over the past 12 years in the numbers of regular and dependent methamphetamine users in Australia. Our estimates suggest that the most recent numbers are the highest for this period, and that the increase has been most marked among young adults (those aged 15-34 years).

IMPLICATIONS

There is an increasing need for health services to engage with people who have developed problems related to their methamphetamine use.

Supervised injection services: what has been demonstrated? A systematic literature review

BACKGROUND

Supervised injection services (SISs) have been developed to promote safer drug injection practices, enhance health-related behaviors among people who inject drugs (PWID), and connect PWID with external health and social services. Nevertheless, SISs have also been accused of fostering drug use and drug trafficking.

AIMS

To systematically collect and synthesize the currently available evidence regarding SIS-induced benefits and harm.

METHODS

A systematic review was performed via the PubMed, Web of Science, and ScienceDirect databases using the keyword algorithm [("supervised" or "safer") and ("injection" or "injecting" or "shooting" or "consumption") and ("facility" or "facilities" or "room" or "gallery" or "centre" or "site")].

RESULTS

Seventy-five relevant articles were found. All studies converged to find that SISs were efficacious in attracting the most marginalized PWID, promoting safer injection conditions, enhancing access to primary health care, and reducing the overdose frequency. SISs were not found to increase drug injecting, drug trafficking or crime in the surrounding environments. SISs were found to be associated with reduced levels of public drug injections and dropped syringes. Of the articles, 85% originated from Vancouver or Sydney.

CONCLUSION

SISs have largely fulfilled their initial objectives without enhancing drug use or drug trafficking. Almost all of the studies found in this review were performed in Canada or Australia, whereas the majority of SISs are located in Europe. The implementation of new SISs in places with high rates of injection drug use and associated harms appears to be supported by evidence.

Illicit and pharmaceutical drug consumption estimated via wastewater analysis. Part B: placing back-calculations in a formal statistical framework

Concentrations of metabolites of illicit drugs in sewage water can be measured with great accuracy and precision, thanks to the development of sensitive and robust analytical methods. Based on assumptions about factors including the excretion profile of the parent drug, routes of administration and the number of individuals using the wastewater system, the level of consumption of a drug can be estimated from such measured concentrations. When presenting results from these 'back-calculations', the multiple sources of uncertainty are often discussed, but are not usually explicitly taken into account in the estimation process. In this paper we demonstrate how these calculations can be placed in a more formal statistical framework by assuming a distribution for each parameter involved, based on a review of the evidence underpinning it. Using a Monte Carlo simulations approach, it is then straightforward to propagate uncertainty in each parameter through the back-calculations, producing a distribution for instead of a single estimate of daily or average consumption. This can be summarised for example by a median and credible interval. To demonstrate this approach, we estimate cocaine consumption in a large urban UK population, using measured concentrations of two of its metabolites, benzoylecgonine and norbenzoylecgonine. We also demonstrate a more sophisticated analysis, implemented within a Bayesian statistical framework using Markov chain Monte Carlo simulation. Our model allows the two metabolites to simultaneously inform estimates of daily cocaine consumption and explicitly allows for variability between days. After accounting for this variability, the resulting credible interval for average daily consumption is appropriately wider, representing additional uncertainty. We discuss possibilities for extensions to the model, and whether analysis of wastewater samples has potential to contribute to a prevalence model for illicit drug use.

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.

The hidden population: Some methodological issues about estimation of problematic drug use

Aim

This article analyses and discusses the estimation of serious or problematic drug use through an empirical example based on a local Swedish study in Gothenburg.

Methods and Data

This was a case-finding study with questionnaires directed at organisations which have contact with the target group. The material was supplemented with information from the two documentation systems DOK and ASI. A total of 2,148 reports were collected. Health care data of 1,096 individuals was also collected for analysis with the truncated Poisson method. Analyses with capture-recapture or truncated Poisson were conducted to calculate the size of the hidden population.

Results

The statistical analyses resulted in variable numbers for the hidden population, and the total prevalence of serious drug abuse in Gothenburg is estimated to be between 2,200 and 4,400 people.

Conclusion

The study shows that estimation of the presence and prevalence of problematic drug abuse involves many methodological difficulties and challenges. The significant variation of the size of the hidden population presented in the study raises doubts about the reliability and validity of the different methods. The methods are clearly sensitive to the importance of fulfilling the different basic assumptions.

Improved benchmark-multiplier method to estimate the prevalence of ever-injecting drug use in Belgium, 2000-10

Background

Accurate estimates of the size of the drug-using populations are essential for evidence-based policy making. However, drug users form a ‘hidden’ population, necessitating the use of indirect methods to estimate population sizes.

Methods

The benchmark-multiplier method was applied to estimate the population size of ever injecting drug users (ever-IDUs), aged 18–64 years, in Belgium using data from the national HIV/AIDS register and from a sero-behavioral study among injecting drug users. However, missing risk factor information and absence of follow-up of the HIV⁺/AIDS^– cases, limits the usefulness of the Belgian HIV/AIDS register as benchmark. To overcome these limitations, statistical corrections were required. In particular, Imputation by Chained Equations was used to correct for the missing risk factor information whereas stochastic mortality modelling was applied to account for the mortality among the HIV⁺/AIDS^– cases. Monte Carlo simulation was used to obtain confidence intervals, properly reflecting the uncertainty due to random error as well as the uncertainty associated with the two statistical corrections mentioned above.

Results

In 2010, the prevalence (/1000) of ever-IDUs was estimated to be 3.5 with 95% confidence interval [2.5;4.8]. No significant time trends were observed for the period 2000–2010.

Conclusions

To be able to estimate the ever-IDU population size using the Belgian HIV/AIDS register as benchmark, statistical corrections were required without which seriously biased estimates would result. By developing the improved methodology, Belgium is again able to provide ever-IDU population estimates, which are essential to assess the coverage of treatment and to forecast health care needs and costs.

Prevalence of problem drug use and injecting drug use in Luxembourg: a longitudinal and methodological perspective

BACKGROUND

To estimate the prevalence of problem drug use (PDU) and injecting drug use (IDU) in Luxembourg and analyze trends between 1997 and 2009. To assess the feasibility of prevalence estimations based on drug use surveillance systems.

METHODS

Serial multi-method PDU/IDU prevalence estimations based upon capture-recapture, Poisson regression, multiplier and back-calculation methods. Comparative analysis of methods and assessment of their robustness to variations of external factors.

RESULTS

National PDU and IDU prevalence rates were estimated at 6.16/1,000 (95% CI 4.62/1,000 to 7.81/1,000) and 5.68/1,000 (95% CI 4.53/1,000 to 6.85/1,000) inhabitants aged 15-64 years, respectively. Absolute prevalence and prevalence rates of PDU increased between 1997 and 2000 and declined from 2003 onwards, whereas IDU absolute prevalence and prevalence rates witnessed an increasing trend between 1997 and 2007.

CONCLUSIONS

Drug use surveillance systems can be valuable instruments for the estimation and trend analysis of drug misuse prevalence given multiple methods are applied that rely on serial and representative data from different sources and different settings, control multiple counts and build upon standardized and sustained data collection routines. The described institutional contact indicator revealed to be a useful tool in the context of PDU/IDU prevalence estimations and thus contributes to enhancing evidence-based drug policy planning.

Conducting a respondent-driven sampling survey with the use of existing resources in Sydney, Australia

BACKGROUND

Respondent-driven sampling (RDS) is a form of chain-referral sampling that is increasingly being used for HIV behavioural surveillance. When used for surveillance purposes, a sampling method should be relatively inexpensive and simple to operate. This study examined whether an RDS survey of people who inject drugs (PWID) in Sydney, Australia, could be successfully conducted through the use of minimal and existing resources.

METHOD

The RDS survey was conducted on the premises of a local needle and syringe program (NSP) with some adjustments to take into account the constraints of existing resources. The impact of the survey on clients and on staff was examined by summarizing NSP service data and by conducting post-survey discussions with NSP staff.

RESULTS

From November 2009 till March 2010, 261 participants were recruited in 16 waves. A significant increase was found in the number of services provided by the NSP during and after data collection. Generally, staff felt that the survey had a positive impact by exposing a broader group of people to the NSP. However, conducting the survey may have led to privacy issues for NSP clients due to an increased number of people gathering around the NSP.

CONCLUSIONS

This study shows that RDS can be conducted with the use of minimal and existing resources under certain conditions (e.g., use of a self-administered questionnaire and no biological samples taken). A more detailed cost-utility analysis is needed to determine whether RDS' advantages outweigh potential challenges when compared to simpler and less costly convenience methods.

Respondent-Driven Sampling (RDS) of People Who Inject Drugs: Assessing RDS Assumptions and Requirements

Respondent-driven sampling (RDS) is a relatively new form of sampling hidden populations. RDS has the ability to produce valid population estimates, provided that certain assumptions are met. This study examined whether an RDS survey of people who inject drugs (PWID) conducted in South East Sydney was able to meet the assumptions and requirements necessary to produce valid population estimates. From November 2009 to March 2010, 261 participants were recruited in 16 waves. While the method's requirements and assumptions were largely met, an apparent lack of random recruitment and the low proportion of coupons returned were sources of concern. This study shows that RDS can be used successfully in Australia as a means of recruiting PWID. However, more work is needed to confirm the soundness of the assumptions on which the calculation of valid population estimates are based.

Assessing IDU prevalence and health consequences (HCV, overdose and drug-related mortality) in a primary care trust: implications for public health action

BACKGROUND

We report on an exercise to estimate the prevalence of injecting drug use (IDU) and associated harms in a single primary care trust.

METHODS

Covariate capture-recapture methods to estimate (i) IDU prevalence; respondent driven sampling to measure (ii) prevalence of HCV and HIV and record linkage to measure (iii) mortality risk.

RESULTS

(i) The overall estimated number of IDU was 5540 (95% confidence interval, CI: 4710-6780) for all cases and 3280 (95% CI: 1940-4610) for cases matched to primary care register, i.e. a prevalence of 2.2 and 1.3% aged 15-54, respectively. (ii) The prevalence of HCV, hepatitis B and HIV was: 53, 32 and 0.7%. Over 70% of IDU in Bristol reported having at least one vaccination for HBV; more than half of those who were HCV positive were undiagnosed. (iii) The all-cause and overdose mortality rates for IDU were 0.75 and 0.4% respectively; and the standardized mortality ratio was 7.8 (95% CI: 5.4-10.8).

CONCLUSION

Locally specific and useful intelligence on injecting and its health consequence can be generated to inform local public health action, and may contribute information to validate national prevalence estimates.