BACKGROUND
In the United Kingdom (UK), the National Treatment Agency for Substance Misuse (NTA) considers retention to be the best available measure of drug treatment effectiveness. Accordingly, the NTA has set local treatment systems the annual target of retaining 75% of clients for 12 weeks or more, yet little assessment of this target or factors that improve retention has occurred. This study aims to quantify the proportion of people retained in treatment for 12 weeks in the North West of England and to identify factors associated with premature drop out.

METHODS
The North West National Drug Treatment Monitoring System (NDTMS) was used to identify treatment durations for everyone beginning a treatment episode between 1st April 2005 and 31st March 2006 (N = 16626). Odds ratios, chi-square and logistic regression analyses compared clients retained for 12 weeks to clients whose discharge record showed they had prematurely dropped out before 12 weeks. Individuals with other outcomes were excluded from analyses.

RESULTS
75% of clients (N = 12230) were retained for 12 weeks and 10% (N = 1649) dropped out prematurely. Multivariate analysis showed drop out was more likely among Asian drug users (adjusted odds ratio 1.52, 95% CI 1.12 to 2.08) than their white equivalents. Drop out was more likely among residents of Cumbria and Lancashire (adjusted odds ratio 1.80, 95% CI 1.51 to 2.15) and Greater Manchester (adjusted odds ratio 2.00, 95% CI 1.74 to 2.29) than Cheshire and Merseyside and less likely among alcohol users (adjusted odds ratio 0.73, 95% CI 0.59 to 0.91). A significant interaction between age and deprivation was observed. For those aged 18 to 24 years and 25 to 34 years, drop out was significantly more likely among those living in affluent areas. For those in the older age groups the converse effect was observed.

CONCLUSION
In combination, the drug treatment systems of the North West achieved the Government's retention target in 2005/06. A number of factors associated with drop out were identified; these should be considered in strategies that aim to improve retention. Drop out and retention are measures that capture the joint effect of many factors. Further work is required to evaluate the effect of deprivation.