Patient compliance with drug therapy for diabetic nephropathy

Dyfrig Hughes and Braden Manns suggest that there are 3 important assumptions regarding compliance that require further clarification in our decision and cost-utility analysis.1

First, we assumed that noncompliers lose renal function at the same rate as patients in the placebo arm of a diabetic nephropathy trial.2 We selected 80% adherence as the threshold required for antihypertensive drug effect on the basis of studies3^,4^,5^,6 we referenced in our article.1 However, some degree of renoprotection may still occur at adherence levels below 80%, as the renoprotective effects of the drug therapy may be independent of the blood pressure effects in this particular disease. Therefore, we do concur that a sensitivity analysis could have been carried out.

Second, Hughes and Manns question whether cost really is the primary barrier for drug adherence for 34% of patients. This assumption is based on a Canadian study that indicated that 34% of the compliance failure was due to cost, representing 17% of patients.7 We indicated in our article that this was a conservative estimate, as price elasticity has been demonstrated to be as high as 64% in a large randomized controlled study and a very large population study.8^,9 We would contend that the figure we used describing the relationship between drug cost and adherence is conservative. Hughes and Manns also indicate that the relationship was less clear in view of a study by Caro and colleagues that looked at patients in Saskatchewan between 1989 and 1994.10 They may not be aware that in Saskatchewan during that time period there was a fairly comprehensive pharmacare program, which might explain variations between expensive and inexpensive antihypertensive agents.11 However, we agree that factors other than drug costs must not be ignored when evaluating the implications of noncompliance.

Finally, we feel that our assumption concerning the proportion of patients already being covered through provincial or private insurance is valid. We concur with Hughes and Manns that the effect on adherence of providing medications free at the point of delivery should be more thoroughly assessed. We also hope that if such studies are undertaken and do show significant improvements in adherence, there would be consideration to developing a national pharmacare program whereby cost-effective medications such as ACE inhibitors for diabetic nephropathy would be provided free to all Canadians.

Competing interests: See original article.1