To the Editor: The trial of buprenorphine-assisted heroin detoxification in primary care and a specialist clinic by Gibson et al1 was intended to compare the effectiveness and cost-effectiveness of buprenorphine-assisted withdrawal in a specialist clinic with treatment by general practitioners. However, of the average $191 for primary care staff costs, $69 was incurred at the clinic. As at least a third of interactions between patients and staff actually took place in the clinic, the primary care arm of the trial was really a combination of specialist clinic and primary care.

Another design problem was the study’s lack of statistical power. A study would need 550 participants to have an 80% chance of identifying (at the 0.05 level of statistical significance) a difference of 50% in self-reported abstinence during the 8-day detoxification (ie, improving the percentage reporting abstinence from 22% to 33%). The trial by Gibson and colleagues had only 115 participants.

As expected, the trial produced statistically non-significant results. Yet, the authors highlight the finding that 23% of primary care patients reported being abstinent during the 8-day detoxification, compared with 22% of the clinic patients, (95% CI risk difference, –14.1% to 16.5%; P = 0.9 [χ2]). Moreover, the clinic group performed better on an objective and more reliable measure of abstinence: 20% of clinic patients versus 14% of primary care patients gave morphine-free 8th day urine specimens, (95% CI risk difference, –7.7% to 19.8%; P = 0.4 [χ2]).

As the confidence intervals for these risk differences include zero, the confidence interval for any estimate of incremental cost-effectiveness includes infinity. It is quite misleading for Gibson and colleagues to claim that “it costs $20 to achieve a 1% improvement in outcome in primary care”, as this ignores both the conflict and the variability in their clinical outcomes.1 Moreover, the statement ignores the variability in the estimated costs of treatment (eg, mean cost per clinic patient, $332; SD, $70).

Surprisingly, Gibson and colleagues did not collect any information on continuing abstinence at the 13-week follow-up. Rather, they collected information on patients’ current treatment. While patients in whom detoxification therapy fails should be offered other treatment, post-withdrawal engagement in maintenance treatment is not a meaningful measure of the effectiveness of detoxification. If anything, it is a measure of failure.

The trial needed sufficient statistical power to identify clinically meaningful differences in abstinence at the end of the 8-day detoxification and at 13 weeks. Staff working in the specialist clinic should not have been extensively involved in the delivery of primary care. Gibson and colleagues should have summarised their findings using appropriate estimates of clinical effect and cost-effectiveness with 95% confidence intervals.2

In reply: The primary focus of our study1 was retention in treatment, and not differences in abstinence. Caplehorn has previously argued compellingly that an orientation to abstinence can have an adverse impact on treatment outcomes in opioid dependence.2 We were using buprenorphine to redefine detoxification, not as a treatment producing lasting abstinence but as a way of promoting engagement in ongoing treatment. The power of our study was calculated on the basis of the proportion of subjects entering post-detoxification treatment, not on their self-reported abstinence levels.

During the detoxification stage in the primary care setting, we used a shared-care dosing arrangement. This was primarily because of the need to give an initial research assessment to all participants before they were randomly allocated to treatment arms — something that would only occur in the context of a research study, and noted in the discussion. Further details of the health economic analysis are soon to be published.3

Ours was a study of the setting for buprenorphine treatment. Its critical finding was that patients were equally as likely to be engaged in maintenance treatment with practitioners in primary care as in specialist clinics.