Random mandatory drugs testing of prisoners: a biassed means of gathering information

Our objective was to develop and test a methodology for inferring the percentage of prisoners currently using opiates from the percentage of prisoners testing positive for opiates in random mandatory drugs testing (rMDT).

The study used results from Willing Anonymous Salivary HIV (WASH) studies (1994-6) in six adult Scottish prisons, and surveys (1994-5 and 1997) in 14 prisons in England and Wales. For Scottish prisons, the percentage of prisoners currently using opiates was determined by assuming, with varying empirical support, that: current users of opiates in prison were 1.5 times as many as current inside-injectors; and current inside-injectors were 0.75 times as many as ever injectors in prison. We also assumed that current inside-users' frequency of use of opiates (by any route) was equal to the frequency of inside-injecting by current inside-injectors in Aberdeen and Lowmoss Prisons in 1996, namely six times in 4 weeks. We assumed that some scheduling of heroin-use prior to weekends takes place, so that only 50% of current inside-users of opiates would test positive for opiates in rMDT: these assumptions allow us to arrive at WASH-based expectations for the total percentage of prisoners testing positive for opiates in rMDT. For England and Wales, a multiplier of 118/68 was applied which was derived from prisoners' interviews, to convert the results from ever inside-injectors, as determined by WASH studies, to the percentage of current inside users of opiates. We made the same assumptions on frequency of inside-use of opiates as in dealing with the Scottish results.

We expected 202.7 opiate positive results in April to September 1997 in rMDTs at six adult prisons in Scotland, 226 were observed. We expected 227.0 at a set of 13 adult prisons and one other in England and Wales; 211 were observed.

Further testing of the methodology for prisons in England and Wales will be possible when 1997 WASH data are released. So far, the methodology has performed well. From it, we infer that 24% of inmates at the six adult prisons in Scotland were current inside-users of opiates, compared to 11% at the 14 adult prisons where survey data were available in England and Wales. The corresponding April to September 1997 percentage of opiate positives in rMDT were: 13% (results from the six Scottish prisons) and 5.4% (results from 14 prisons in England and Wales), a two-fold under-estimate of % current users of opiates in prison (24% and 11%). Planning of drug rehabilitation places for prisoners should thus be based on twice the percentage of prisoners testing opiate positive in rMDT. This correction factor of two should be kept under review.