Low Dual-Eligibility Reporting in Puerto Rico in the MBSF Information

The monthly buy-in indicators in the Medicare Master Summary Beneficiary File (MBSF) flag concurrent Medicare and Medicaid enrollment, commonly called “dual eligibility”. It came to my attention that the MBSF indicates an unbelievably low dual eligibility rate for Puerto Rico. For example, in June 2022, 98.53% of Puerto Ricans enrolled in Medicare are categorized as “Medicare only”. This is much lower than the overall US rate of 81.08%.

Table 1. MBSF Dual Status* for Puerto Rico Medicare Enrollees, June 2022

*Code values for DUAL_STUS_CD_06 are aggregated into the above categories following the specification in the MBSF Codebook variable description comment.

I found similar distributions to Table 1 for years 2015-2021 and across months.

Because Puerto Rico participates in Medicaid, I decided to see how many Puerto Ricans have dual status according to the Medicaid and CHIP T-MSIS Analytic File (TAF) Demographic and Eligibility file (DE). The TAF DE contains monthly DUAL_ELGBL_CD variables that indicate dual status with a value of “08 - Eligible is entitled to Medicare-Other Dual Eligibles (Non QMB, SLMB, QDWI or QI) including prescription drugs”. In the 2022 TAF DE I found 354,254 residents of Puerto Rico are considered dually eligible in June. 97.69% of these TAF DE records link successfully to the MBSF via BENE_ID; however, 97.11% of them have an MBSF dual status indicating Medicare only.

Table 2. MBSF Dual Status* for Puerto Rico TAF DE Dual Eligibles Linked by BENE_ID, June 2022

*Code values for MBSF DUAL_STUS_CD_06 were aggregated into “Full Medicaid” and “Partial Medicaid” following the specification in the MBSF Codebook variable description comment.

This data quality issue was addressed in 2025 with the release of Version 2 of the MBSF for data years starting in 2023. The comment for DUAL_STUS_CD_01-12 in the Medicare Beneficiary Summary File (MBSF) Codebook was embellished to include notes on the topic such as, “Prior to MBSF ABCD V2, the MBSF data did not include complete information from territories such as Puerto Rico and the Virgin Islan Islands, because these territories do not submit dual eligibility data to CMS through the MMA [Medicare Modernization Act] files.” To improve the data for the territories, Version 2 adds a value to DUAL_STUS_CD_01-12 of “10 - Territory buy-ins (Subsidized months for beneficiaries in Puerto Rico, Virgin Islands, and other territories). Reference Comment below “. The information in the extended comment shows that the Puerto Rico Medicaid program works differently than in the 50 states (see the Codebook for the complete comment).

Table 3 shows that with the addition of code value 10 in 2023, the V2 MBSF reports 45.89% of Puerto Ricans enrolled in Medicare were also enrolled in Medicaid (i.e., dually eligible) via territory buy-in.

Table 3. Version 2 MBSF Dual Status* for Puerto Rico, June 2023

*Code values for MBSF DUAL_STUS_CD_06 were aggregated into “Full Medicaid” and “Partial Medicaid” following the specification in the MBSF Codebook variable description comment.

Summary

Dual-eligibility reporting in Puerto Rico and other U.S. Territories is an excellent example of the impact of upstream data decisions. Territories were missing eligibility information as they were not required to submit dual eligibility information to CMS through the MMA. For studies interested in these populations, different methods are required for identification prior to 2023 and starting in 2023. Prior to 2023, researchers should consider combining both MBSF: Base and TAF DE for more complete eligibility information. Beginning in 2023, the Version 2 MBSF adds a new value (10) to the DUAL_STUS_CD_01-12 that captures dual eligibility for U.S. territories. The Codebook contains helpful commentary on the topic under the variable description.