Estimating heroin epidemics with data of patients in methadone maintenance treatment, collected during a single treatment day

Estimating incidence of problem drug use using the Horwitz-Thompson estimator - A new approach applied to people who inject drugs in Oslo 1985-2008

BACKGROUND

The trend in the number of new problem drug users per year (incidence) is the most important measure for studying the diffusion of problem drug use. Due to sparse data sources and complicated statistical models, estimation of incidence of problem drug use is challenging. The aim of this study is to widen the palette of available methods and data types for estimating incidence of problem drug use over time, and for identifying the trends.

METHODS

This study presents a new method of incidence estimation, applied to people who inject drugs (PWID) in Oslo. The method took into account the transition between different phases of drug use progression - active use, temporary cessation, and permanent cessation. The Horwitz-Thompson estimator was applied. Data included 16 cross-sectional samples of problem drug users who reported their onset of injecting drug use. We explored variation in results for selected probable scenarios of parameter variation for disease progression, as well as the stability of the results based on fewer years of cross-sectional samples.

RESULTS

The method yielded incidence estimates of problem drug use, over time. When applied to people in Oslo who inject drugs, we found a significant reduction of incidence of 63% from 1985 to 2008. This downward trend was also present when the estimates were based on fewer surveys (five) and in the results of sensitivity analysis for likely scenarios of disease progression.

CONCLUSION

This new method, which incorporates temporarily inactive problem drug users, may become a useful tool for estimating the incidence of problem drug use over time. The method may be less data intensive than other methods based on first entry to treatment and may be generalized to other groups of substance users. Further studies on drug use progression would improve the validity of the results.

A comprehensive model of treatment participation in chronic disease allowed prediction of opioid substitution treatment participation in Zurich, 1992-2012

OBJECTIVES

Chronic diseases are often associated with cycling in and out of treatment. We used data of a large opioid substitution treatment case register to (1) identify associated factors and (2) integrate retention and readmission into a model of overall participation over subsequent treatment episodes of various groups.

STUDY DESIGN AND SETTING

Data of all 9,407 patients undergoing 26,545 methadone or buprenorphine substitution treatment episodes between 1992 and 2012 in the canton of Zurich, Switzerland, were analyzed. We used extended survival analysis to estimate the duration of, and time between, treatment episodes, with the number of episodes, gender, nationality, administration route, age at onset of first regular heroin use, and provider type as independent variables. A similar analysis was applied to estimate overall participation (the probability of being in treatment at a given day after first entry independent of current number of treatment episode) and to test for group differences.

RESULTS

The time between treatment episodes shortened with the increasing number of episodes. Retention slightly increased after the first episode and then shortened for later treatment episodes. Effect sizes were generally rather weak (odds ratio ≤ 1.47). Effects were usually equal for all episodes, and if changing, weakened for later episodes.

CONCLUSION

The complex process of leaving and entering treatment as well as the daily probability of being in treatment independent of treatment episode can be predicted by comprehensible statistical models applied to patient-period data sets.

A multi-state model to estimate incidence of heroin use

Background

Existing incidence estimates of heroin use are usually based on one information source. This study aims to incorporate more sources to estimate heroin use incidence trends in Spain between 1971 and 2005.

Methods

A multi-state model was constructed, whereby the initial state “heroin consumer” is followed by transition to either “admitted to first treatment” or to “left heroin use” (i.e. permanent cessation or death). Heroin use incidence and probabilities of entering first treatment ever were estimated following a back-calculation approach.

Results

The highest heroin use incidence rates in Spain, around 1.5 per 1,000 inhabitants aged 10–44, occurred between 1985 and 1990; subdividing by route of administration reveals higher incidences of injection between 1980 and 1985 (a mean of 0.62 per 1.000) and a peak for non-injectors in 1990 (0.867 per 1,000).

Conclusions

A simple conceptual model for heroin users’ trajectories related to treatment admission, provided a broader view of the historical trend of heroin use incidence in Spain.

The effect of hepatitis C treatment and human immunodeficiency virus (HIV) co-infection on the disease burden of hepatitis C among injecting drug users in Amsterdam

AIMS

The hepatitis C virus (HCV) disease burden among injecting drug users (IDUs) is determined by HCV incidence, the long latency period of HCV, competing mortality causes, presence of co-infection and HCV treatment uptake. We examined the effect of these factors and estimated the HCV disease burden in Amsterdam.

DESIGN

A Markov model was developed, incorporating HCV and human immunodeficiency virus (HIV), and parameterized with data from the Amsterdam Cohort Studies, surveillance studies and literature.

SETTING

IDU population of Amsterdam.

MEASUREMENTS

HCV infection simulated from its acute phase to HCV-related liver disease (i.e. decompensated cirrhosis and hepatocellular carcinoma).

FINDINGS

The HCV prevalence among IDUs in Amsterdam increased to approximately 80% in the 1980s. From 2011 to 2025, the HCV-related disease prevalence will accordingly rise by 36%, from 57 cases (95% range 33-94) to 78 (95% range 43-138), respectively. In total, 945 (95% range 617-1309) individuals will develop HCV-related liver disease. This burden would have been 33% higher in the absence of HIV, resulting in 1219 cases (95% range 796-1663). In Amsterdam, 25% of HIV-negative IDUs receive successful HCV treatment, reducing the cumulative disease burden by 14% to 810 (95% range 520-1120). Further reduction of 36% can be achieved by improving treatment, resulting in 603 cases (95% range 384-851).

CONCLUSIONS

The hepatitis C virus burden among injecting drug users in Amsterdam has been reduced by a high competing mortality rate, particularly caused by HIV infection, and to a smaller extent by hepatitis C virus treatment. Improved hepatitis C virus treatment is expected to contribute to reduce the future hepatitis C virus disease burden.

Estimating incidence trends in regular heroin use in 26 regions of Switzerland using methadone treatment data

Background

Regional incidence trends in regular heroin use are important for assessing the effectiveness of drug policies and for forecasting potential future epidemics.

Methods

To estimate incidence trends we applied both the more traditional Reporting Delay Adjustment (RDA) method as well as the new and less data demanding General Inclusion Function (GIF) method. The latter describes the probability of an individual being in substitution treatment depending on time since the onset of heroin use. Data on year of birth, age at first regular heroin use and date of admission to and cessation of substitution treatment was available from 1997 to 2006 for 11 of the 26 regions (cantons) of Switzerland. For the remaining cantons, we used the number of patients in 5-year age group categories published in annual statistics between 1999 and 2006.

Results

Application of the RDA and GIF methods on data from the whole of Switzerland produced equivalent incidence trends. The GIF method revealed similar incidence trends in all of the Swiss cantons. Imputing a constant age of onset of 21 years resulted in almost equal trends to those obtained when real age of onset was used. The cantonal incidence estimates revealed that in the mid 80s there were high incidence rates in various regions distributed throughout all of the linguistic areas in Switzerland. During the following years these regional differences disappeared and the incidence of regular heroin use stabilized at a low level throughout the country.

Conclusion

It has been demonstrated that even with incomplete data the GIF method allows to calculate accurate regional incidence trends.

A note on heroin epidemics

We show that the steady states of the White and Comiskey [E. White, C. Comiskey, Heroin epidemics, treatment and ODE modelling, Math. Biosci. 208 (2007) 312-324.] model of heroin epidemics are stable.