Bilardi et al. BMC Public Health 2010, 10:70 http://www.biomedcentral.com/1471-2458/10/70 audit and an education session prior to the commence- ment of the trial. The self-completed pre-trial question- naire collected information about GPs characteristics, knowledge, attitudes and practices regarding chlamydia testing and was conducted both pre and post trial by all participating GPs. A clinical audit was undertaken at each practice to collect details about issues likely to have an impact on chlamydia testing in that clinic. Audit data were used to develop an ideal individualized chlamydia testing pathway for each clinic, which incor- porated current best practice for testing in the primary care setting of annual chlamydia testing for sexually active women aged 16 to 24 years [23]. GPs were advised to collect specimens for testing by first pass urine, self-collected vaginal swab or endocervical swab. Participating GPs were eligible to enroll in related chla- mydia education activities accredited under the RACGP Quality Assurance and Continuing Professional Devel- opment Program (QA&CPD) [24]. Following the audit, an education session was held at each practice to further inform GPs about chlamydia test- ing, management of test results and methods of introdu- cing the subject of testing to patients. Practices were provided with waiting room chlamydia posters, pamphlets and chlamydia screening flow charts. A DVD recording of the education session was available for doctors unable to attend. At the request of GPs, tear off pads with brief information sheets for patients specifically about the rea- sons for testing and the simplicity of testing and treatment were produced and distributed to the practices. Mid trial, GPs in the intervention group received a let- ter to remind them of the incentive offered for chlamy- dia testing. They were not provided with any information about the number of tests performed to date nor the amount of money they had accrued through testing. Payment was made to GPs at the end of the trial period. All practices received an honorarium amount of $AUD1000 in recognition of GPs time spent out of usual roles in participating in the trial. Outcome measures and data collection The primary study outcome was the difference in the proportion of women tested for chlamydia in the control and intervention groups before and during the interven- tion. Consultation and chlamydia testing data were col- lected for two time periods: 12 months prior to and 6 months of the intervention. Data were not collected on the number, characteristics or identities of women who were offered or declined testing during the trial. With the permission of all participating GPs, pathol- ogy laboratories were provided with each GPs unique provider number, specific to their place of practice. The total number of women aged 16 to 24 years tested for chlamydia at least once in each study time period was Page 3 of 9 ascertained for each GP as was the total number of women who had at least one positive chlamydia test result. Consultation data were collected from each prac- tice’s computerised patient record system on the num- ber of individual women aged 16 to 24 years each GP consulted in the two time periods. Sample size At trial commencement, approximately 6% of Australian women aged 16 to 24 years were being tested each year for chlamydia by GPs [25]. We hypothesized that the rate of individual women tested for chlamydia would rise to 30% in the intervention group (an increase of 24%) and 15% in the control group (change of 9%). Assuming a conservative intracluster correlation of 0.05 for a cluster size of 100 (which assumes a design effect of 6), a total of 12 practices (six in each group) would be required to give 80% power to detect the hypothe- sized difference with a type 1 error of 5%. Statistical analysis In both groups, the proportion of 16 to 24 year old women tested at least once for chlamydia was estimated using the observed proportion calculated separately for the twelve months prior and six months of the trial. The observed proportion numerator was the number of women tested at least once for chlamydia and the denominator, the individual number of women con- sulted during the time period. The chlamydia positivity was also calculated and defined as the proportion of patients tested with at least one positive test. Confidence intervals for all proportions (using the binomial distribu- tion) and a p-value for the comparison of estimated pro- portions between study groups (using the chi-squared test) separately for the 12 month and 6 month periods were calculated, adjusted for clustering by GP. A mixed effects logistic regression model, with two level hierarchy (patient and individual GP) was fitted to the data including results from both the intervention and control group for both time periods. The model included fixed regression effects for GP age, GP gender, whether the GP was aware of the RACGP chlamydia testing guidelines (RACGP ‘Red Book’), whether the GP had postgraduate qualifications and for an interaction between the intervention group and the time period. Statistical analysis was performed using Stata statistical software 10.0 [26]. Questionnaire data were analyzed using Statistical Package for the Social Sciences (SPSS) version 17.0 [27]. Ethical approval Ethical approval for this study was granted by the human research ethics committee of The University of Melbourne (HREC No: 050747).