STUDY SIZE AND DOCUMENTATION TO DETECT INJECTION-RELATED HEPATITIS-C IN PRISON

We used existing data on hepatitis C prevalence, injection-related hep atitis C transmission and needle use in prisons and new data on infect iousness, to estimate the size of study required to detect injection-r elated hepatitis C in UK prisons. A pilot study of 500 prisoners follo wed for 10 weeks would have a 65% chance of detecting a hepatitis C se roconversion, conservatively assuming one injection per prisoner per w eek, and a 3% transmission rate per injection, but uncertainty might p ersist as to whether transmission had occurred during a short incarcer ation or before it. If the actual transmission rate was 10%, as recent ly documented, then such a study would have more adequate statistical power. A definitive study of 3000 prisoners for 10 weeks would expect to detect about six seroconversions, even with conservative estimates of injection frequency and transmission rate. Adequate design and powe r of these studies is important because of the complacency that could result from false-negative findings. We suggest six risk-factor themes that studies should document.