Informative Presence Bias in Comorbidity Data of Medicare Advantage-Enrolled Beneficiaries

BackgroundAnalyses of Medicare administrative claims data are faced with methodological challenges, including accounting for the potential effect of insurance status on documented comorbidities. We present an example of how failing to account for informative presence bias related to beneficiary enrollment status in such analyses may lead to flawed results. MethodsIn this retrospective observational study of Medicare beneficiaries undergoing isolated coronary artery bypass grafting (CABG) from 1999-2019, we compare the distribution of documented comorbidities between beneficiaries with Medicare Advantage (MA) and Traditional Medicare (TM) plans. Long-term survival was then compared in both unweighted and overlap weighted analyses with and without the inclusion of documented comorbidities. ResultsAmong 3,015,066 Medicare beneficiaries undergoing CABG from 1999-2019, 2,345,476 underwent isolated CABG and had suitable data for analysis. The annual proportion of MA-enrolled beneficiaries undergoing CABG remained stable from 1999-2007 (1.1-4.5%) and then progressively increased annually, reaching 38.2% in 2019. The incidences of documented comorbidities were substantially lower among MA-enrolled versus TM-enrolled beneficiaries. Among MA-enrolled and TM-enrolled beneficiaries, respectively, the unweighted median survival difference was only 8 [-12,28] days (10.02 [9.96,10.07] vs 10.00 [9.98,10.01] years); the weighted (adjusted for demographics and procedural characteristics, but not beneficiary comorbidities) median survival difference was also minimal at -2 [-28,24] days (10.00 [9.95,10.06] vs 10.01 [9.98,10.04] years). However, the weighted (with adjustments including beneficiary comorbidities) median survival difference demonstrated a substantial survival disadvantage for MA-enrolled beneficiaries compared to their TM-enrolled counterparts: -604 [-626,-575] days (9.78 [9.73,9.83] vs 11.44 [11.41,11.47] years), respectively. Conclusions and RelevanceComorbidities in MA-enrolled beneficiaries may be severely under-reported in Medicare data. Studies failing to account for this are susceptible to informative presence bias with a significant treatment effect. In the absence of policy changes, increasing enrollment in MA plans will continue to decrease the population of Medicare beneficiaries with suitable data for study in comparative analyses.