Estimating the number of opiate users in Amsterdam by capture–recapture: The importance of case definition

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