This study analyzed the relationship between adherence to RAAS-Is and statins and spending on traditional Medicare services for disabled and aged Medicare beneficiaries with diabetes between 1997 and 2005. The study is unique in several important respects. First, unlike medication adherence studies that rely solely on prescription claims, the MCBS permitted us to draw an exceptionally rich picture of the personal characteristics, health service utilization and spending, and diabetes knowledge and self-management practices of a nationally representative sample of community-dwelling diabetics treated in fee-for-service settings. Second, the MCBS samples can be tracked longitudinally to capture medication adherence patterns and potential outcomes over durations of up to 3 years. Typical medication adherence studies track utilization patterns over a single year. A third strength of the study is the inclusion of observations for persons who died and were otherwise lost to follow-up. This means that the study findings can be generalized beyond the survivor cohorts typical of traditional adherence studies. A fourth strength of the study design is the multiple controls for potential confounding due to indication bias and healthy adherer bias.
We found that median MPR adherence rates were 0.88 for RAAS-Is and 0.77 for statins. An MPR of 0.80 is often cited as reasonably good adherence behavior for chronic medications (Vink et al. 2009
). By that standard slightly more than half of RAAS-I users were adherent with therapy over 3 years, whereas slightly fewer than half of statin users were adherent. Nonadherence can take the form either of intermittent use with gaps in therapy or discontinuance of the drug. Lack of dispensing dates in the MCBS dataset precluded analysis of therapy gaps, but we were able to determine discontinuance rates from 1 year to the next. For individuals surviving all 3 years, 15 percent of both the RAAS-I and statin user groups discontinued therapy by the start of the second year, and an additional 15 percent discontinued therapy by the third year (results not shown). This would suggest that therapy gaps and drug discontinuance contributed roughly equal shares of observed nonadherence for the two groups. Gaps in therapy are usually ascribed to lapses in patient behavior. Discontinuance could be due either to patient behavior or physician response to treatment failure or adverse drug reactions.
Whatever the cause of nonadherence, we found that beneficiaries with better MPRs had lower spending on traditional Medicare Part A and Part B services. For statin users, a 10 percentage point increase in MPR was associated with U.S.$832 lower Medicare expenditures in the multivariate model (p
<.01). A 10 percentage point increase in MPR for RAAS-Is was associated with U.S.$285 lower Medicare costs (p
<.05). To put these estimates in context, during our study timeframe a 30-pill statin fill averaged U.S.$80.95 and a 30-pill RAAS-I fill averaged U.S.$30.68 in 2006 dollars.7
For noncensored survivors in our sample, a 10 percentage point change in MPR would be equivalent to 3.6 months of use, which would cost U.S.$291.44 and U.S.$110.45 for statin and RAAS-I users, respectively. In other words, at the margin, utilization of both drugs was associated with net savings to the Medicare program. Results from our sensitivity tests suggest that the savings potential from increased adherence is concentrated among relatively poor adherers, but additional analysis using larger samples is necessary to confirm that finding.
As noted in the introduction, other studies have reported cost savings from improved adherence with these drugs. Although differences in study design and sample populations make direct comparisons difficult, it is possible to examine variation in overall effect sizes across studies. For example, Sokol et al. (2005)
computed the relationship between annual adherence and medical costs for nonelderly patients with diabetes, hypertension, hypercholesterolemia, and heart failure. In models controlling for comorbidity but no additional drug use or patient behavior variables, medical costs among the poorest adherers (MPR=0.1–0.19) were reported to be two to three times higher than for the highest adherers (MPR=0.80–1.00) for all cohorts except heart failure. It is difficult to imagine that drug adherence was the only or even the primary cause of cost differences of this magnitude. Even studies with longer timeframes report implausibly high expenditure cost offsets from drug adherence. A study of aged diabetics enrolled in a Medicare HMO for up to 5 years reported that a 10 percent increase in MPR for antidiabetic medications was associated with lower total medical cost of between 8.6 percent and 28.9 percent (Balkrishnan, Rajagopalan, and Camacho 2003
). By contrast, our estimated adherence impacts are modest. We estimate that a 10 percent increase in MPR would reduce Medicare outlays by approximately 0.7 percent for RAAS-I users and by about 2.1 percent for statin users over 3 years. Short of replicating our research design on these other data sources, it is impossible to determine the true source of such discrepant findings. However, we believe that our sample frame and expansive confounder domains address limitations in other work in this area.
That said, our results should be interpreted in the light of several important limitations. The lack of service dates and days supply in the MCBS prescription drug event files means that our MPR measures are less accurate than measures based on actual claims data. We believe that the standardized 30-pill measure used to compute MPR is accurate for statins and most RAAS-I prescriptions. However, some patients receive prescriptions for ACE-inhibitors that require two or even three pills per day. For this reason, our estimated MPR for RAAS-Is may be slightly overestimated, and to this extent, our estimates of RAAS-I adherence effects may be understated. Second, because our samples are relatively small, the estimated standard errors are large compared with those using large claims datasets. Small sample sizes limited our ability to conduct detailed analyses of Medicare savings associated with different levels of MPR. Third, while we were able to track adherence in medication use for 3 years, diabetes is a chronic and progressive condition, and studies with longer timeframes are needed to corroborate our findings. Fourth, because the MCBS does not gather data on prescribing, we were unable to identify beneficiaries who were prescribed RAAS-Is or statins and failed to fill them (nonfulfillment). Fifth, the lack of Medicare claims data for Medicare Advantage enrollees means that our results cannot be generalized to this segment of the Medicare population.
Finally, our estimates of potential Medicare savings do not include any costs associated with improving adherence other than the added cost of the drugs themselves. It goes without saying that additional savings will be difficult to obtain unless cost-effective methods are available to increase drug adherence among Medicare beneficiaries with diabetes. Value-based insurance designs (VBID) with lowered copayments—or even free drugs—have been suggested as one approach to increasing medication utilization rates among diabetics (Rosen et al. 2005
; Nicholson 2006
; Fitch, Iwasaki, and Pyenson 2008
; Spaulding et al. 2009
;). At this point, the evidence base on VBID effectiveness is meager and such programs would be difficult to implement under current Medicare payment rules because drug copay policies are made by private Part D prescription drug plans. Moreover, nonfinancial barriers to adherence may be even more difficult to surmount (Gellad, Grenard, and McGlynn 2009
). One potential way to spur better adherence is through focused diabetes self-management courses. Our analysis showed that beneficiaries who attended these courses had lower Medicare spending. Although these results were not statistically significant, the fact that only a third of the respondents had taken such a class provides ample room for improvement.