In this study we found that offering GPs a $AUD5 testing payment did not increase testing for chlamydia. This was the first RCT to test incentive payments for chlamydia testing. Systematic reviews of RCT's assessing the effectiveness of financial incentives to improve health care quality are limited, and often show mixed results [17, 18, 28]. Observational studies have found large incentive payments of £10-25 (approximately $AUD20-50) increased chlamydia testing significantly . Our data suggest that a small incentive will not substantially improve chlamydia testing rates.
Strengths and Limitations
The major strength of this study was that it was the first RCT to our knowledge to examine the effect of providing GPs with an incentive payment to increase chlamydia testing in general practice. Previous Australian studies aimed at increasing testing in general practice have examined the effectiveness of a variety of multi-faceted and singular method interventions, including the use of computer alerts , concurrent testing with pap smears  and online sexual health assessment tool referral . Retention rates in the trial were very high, with a 96% response rate for testing data and questionnaire data. The study provided a good representative sample of GPs across regional/rural and metropolitan Victoria.
There were a number of limitations to our study, most notably, that the observed change in testing was considerably lower than hypothesized. An imbalanced distribution of variables was also evident at trial commencement, with GPs in the control group more likely to be younger and have a postgraduate qualification. GPs in the intervention group were also more than twice as likely to be testing for chlamydia at trial commencement. Although our multivariate analysis did adjust for these baseline differences, the lower than hypothesized effect size and the baseline imbalance in key confounders, did reduce our statistical power. The comparison of different pre-trial (12 months) and trial (6 months) time periods may not have allowed for seasonal changes in patient load and possible changes in testing frequence. In interpreting these results, it is important to note that one GP in the intervention group was testing very high numbers of young women before the trial commenced, accounting for nearly half the total number of chlamydia tests (44%) in the pre-trial period, but less than a quarter (23%) in the trial period. Despite thorough investigation into the decrease in this GP's testing figures during the trial period, no practicable explanation could be found. If this GP is removed from the multivariate analysis, a greater increase in testing in the intervention group compared to the control group is evident suggesting a positive impact of the incentive payment (from OR = 0.9; 95% CI: 0.5, 1.6 to OR = 1.22; 95% CI: 0.8, 2.0).
A further limitation of our study is that we did not provide GPs in the intervention group with ongoing feedback on their testing performance, nor were incentive payments made until the completion of the trial. It is likely that GPs forgot they were in the trial and consequently that they would receive payment for each eligible woman tested. The additional use of testing feedback on the number of tests performed during the trial period may have been useful in prompting behaviour change in GPs .
It is likely that testing increased in both groups due to GPs raised awareness from the educational components of the study [30–32] - both the intervention and the control group received the same educational package prior to commencement in the trial. Educational interventions as part of a multi-faceted intervention , and which combine strategies such as outreach visits and printed material have been shown to be effective in changing physician behaviour  without financial incentive. The significant decrease in positive chlamydia diagnosis in the intervention group is likely to be as a result of GPs testing higher numbers of low risk or asymptomatic women.
Lastly, female GPs and part time GPs were over-represented in the sample compared with the Australian GP population and therefore it is possible that the results of our study may not be generalizable to the wider population.
While it appears that a small financial incentive did not motivate behaviour change in GPs, large financial incentives have shown to be effective in other settings. In the UK, as part of the 1999/2000 pilot chlamydia testing program undertaken in two healthcare authorities, financial incentives of up to £25 pounds (approximately $AUD50) were offered to practices for the opportunistic chlamydia testing of young women aged 16 to 24 years. General practices comprised a high proportion of participating health sites (72%), from which an effective screening rate (ESR) of 46% in the target female population was achieved in the Portsmouth authority, where testing was fully implemented from the beginning of the program . However, since the introduction of the National Chlamydia Testing Program (NCSP) in 2003, when financial incentives were discontinued, the ESR dropped significantly in general practice to around 10% . Santer et al , in a similar study examining opportunistic chlamydia testing in primary care among women, reported an ESR of 30% in women attending for a cervical smear and 23% for contraception, however conceded it was unlikely higher rates could be achieved among teenage women (23%) without the use of a financial incentive scheme and an education campaign to raise public and professional awareness . In recent Australian studies, in which GPs have been asked about how best to facilitate increased testing in general practice, the need for financial incentives has been stressed [13, 15].
The size of the financial incentive offered in this study may have been insufficient to motivate GPs to offer increased testing. Systematic reviews have shown small financial incentives have mixed results in improving the quality of physician care [17, 18, 28]. Town et al , in a systematic review of the randomized trials examining the effect of financial incentives on provider preventative care (n = 8), deduced that small rewards did not appear to induce change in doctors preventative care practices. A higher incentive payment or different incentive scheme i.e. target screening rate bonuses or practice level incentives, may have proved more effective however, a cost-effectiveness analysis would need to be undertaken to compare alternative strategies to increase testing . Payments in the UK pilot chlamydia testing program were considerably higher than those offered here and were offered to the practice, not the GP, however they were not sustained as part of the NCSP. In stating this, there has been evidence to suggest that small incentive payments to GPs, as part of a multi-faceted package including regular feedback, have increased childhood immunization rates in Australia .
The lack of reminders may have also have had a negative impact on the uptake of testing by GPs. Reminders are generally accepted as an effective way to promote behaviour change [37, 38] and reviews have shown that computer based alerts can be effective in increasing GPs preventative care practices [39–41]. In a recent Australian RCT, in which intervention group GPs received a computer generated alert to remind them to test young women for chlamydia, a 27% greater increase in chlamydia testing was evident in the intervention group . The authors concluded that while the computer alert increased chlamydia testing, the reminder alone would not be sufficient to increase testing to the levels required; however, it would be useful as part of a multi-faceted intervention .