In order to determine the prevalence of latent infection due to Mycobacterium 
tuberculosis in drug users and to provide centres for drug users with a 
practical tool for tuberculosis screening, 237 drug users were subjected to the 
Monotest and, for reference purposes, to the Mantoux test. The overall 
prevalence of subjects with a tuberculin skin reaction size > or = 5 mm in the 
Mantoux test was 25.7%; utilizing a cut-off of > or = 10 mm, the prevalence was 
11.4%. Irrespective of cut-off, the Monotest showed a sensitivity of > 90% and a 
specificity of > 80%. At a prevalence of 25.7%, and with cut-offs of > or = 5 or 
> or = 10 mm, the positive predictive value was 83% or 62.2%, respectively. 
Irrespective of cut-off, the negative predictive value was > 97%. In conclusion, 
the Monotest proved satisfactory as a tool for epidemiological screening in a 
population with a high prevalence for latent tuberculosis, namely drug users.