Just how compliant must women on bisphosphonates be?

They need to take their medication at least about 75% of the time if they want to make a significant impact on fracture rates, according to the results of a retrospective cohort study.

Ethel S. Siris, MD, and colleagues identified over 35,000 women 45 years and older taking either alendronate or risedronate. They estimated patients' adherence by calculating their medication possession ratio (MPR), which measured the number of times they had their prescription refilled over time. For instance, a patient given an initial 3-month prescription who then refilled 2 more 3-month prescriptions over 1 year had in her possession 3/4 of the medication she needed. That translated as a 75% MPR. The researchers found that among patients with an MPR of 0% to 50%, the probability of a fracture remained constant at about 11%. When MPR exceeds 50%, the probability of fracture decreases as adherence increases.

Using an MPR of 80% or higher to indicate adherence, the authors calculated that fewer than half (43%) of the women in the study were adherent with therapy. Those who were compliant enjoyed a 21% overall reduction in fractures, a 37% reduction in vertebral and hip fractures, a 20% reduction in nonvertebral fractures, and a 9% reduction in wrist fractures.

The authors of an accompanying editorial emphasize that perhaps the most important point of the study is that although compliance rates with bisphosphonate therapy are low, marginal increases in adherence rates can substantially impact fracture risk and associated morbidity, mortality, and cost.

Siris ES, Harris ST, Rosen CJ, et al. Adherence to bisphosphonate therapy and fracture rates in osteoporotic women: relationship to vertebral and nonvertebral fractures from 2 US claims databases. Mayo Clin Proc. 2006;81:1013-1022.

Badamgarav E, Fitzpatrick LA. A new look at osteoporosis outcomes: the influence of treatment, compliance, persistence, and adherence. Mayo Clin Proc. 2006;81:1009-1012.