Impact of Cost Sharing Increases on Continuity of Specialty Drug Use in Medicare Patients with Multiple Sclerosis or Rheumatoid Arthritis

Specialty tiers typically subject patients to high levels of cost sharing, but the impact on specialty drug treatment continuity is not well understood. Using a quasi-experimental study design that took advantage of the widespread use of specialty tiers and the variable cost sharing structure under Medicare Part D, we examined the impact of cost sharing increases on continuity of specialty drug use among a group of beneficiaries without low-income subsidies (non-LIS) as they transitioned from a 5% cost sharing pre-period to a 25% or more cost sharing post-period. We then compared this change with use among a contemporaneous control group of full LIS patients who faced minor cost sharing ($0-$5 copay) in both the pre- and post-periods. Data were drawn from 2007-2010 Chronic Conditions Data Warehouse (CCW) 5% Medicare files. The study samples consisted of patients with diagnoses for multiple sclerosis (ICD-9-CM code 340) or rheumatoid arthritis (ICD-9-CM code 714). To focus on patients eligible for treatment with Part D specialty drugs, we included only those patients who had used Part D-covered disease-modifying therapies (DMTs) for MS or biologics for RA during the pre-period. Our outcomes measures were a continuous gap of ≥30 days in availability of any disease-specific (i) Part D and (ii) Part D or Part B specialty drug.  We used 2-period, difference-in-difference logistic regression models with the generalized estimating equation method to estimate changes in the odds of having a continuous gap during the post-period vs. the pre-period, for the non-LIS group relative to the LIS control group, while adjusting for a series of beneficiary sociodemographic and clinical characteristics as well as Part D plan formulary characteristics (e.g., proportion of the covered Part D specialty drugs that were subject to prior authorization, step therapy, or quantity limits) and year indicators (to control for any underlying time trends). Models were run separately in MS and RA samples. We repeated all analyses in the subgroup of LIS and non-LIS patients aged <65 years (i.e., Medicare-eligible due to disability) and using within-person panel data fixed-effects logit models. In additional analyses, we included control variables for the number of days spent in the pre-period and in the post-period. Relative to the LIS control group, the non-LIS group had a greater increase in incidence of gaps and greater odds of having a gap in any Part D treatment from the pre- to the post-period (MS: 10%, OR=1.55 [95% CI, 1.16-2.08]; RA: 23%, OR=2.83 [95% CI, 2.22-3.61]. The increase in Part D treatment gaps was not offset by increased Part B use (typically available at lower out-of-pocket costs to most patients). Subgroup and sensitivity analyses showed consistent findings. In summary, increased cost sharing with transition into a period of specialty tier-level cost sharing was associated with interruptions in MS and RA specialty drug treatment. RA patients were more price sensitive than MS patients. No significant substitution effects were observed with Part B specialty drugs, raising concern about potential negative clinical consequences of lack of availability of any specialty treatment during treatment interruptions.