Effect of using information from only one system for dually eligible health care users.

OBJECTIVE: We sought to determine whether all diagnoses and total illness burden of patients who use both the VA and Medicare health care systems can be obtained from examination of data from only one of these systems.
METHODS: Cohorts included all age-eligible Medicare users who also used the VA health care system in fiscal years 2000-2002 but were not enrolled in a Medicare HMO. Relative risk scores (RRS; a measure of illness burden developed by DxCG, Inc., Boston, MA) were calculated using VA, Medicare, and all diagnoses from both VA and Medicare data sources. The relationship between RRS and reliance on Medicare versus the VA system also was explored. We explored whether differences in VA and Medicare RRS were caused by veterans who mainly used pharmacy services or by an underweighting in the RRS calculation of mental health diagnoses. Finally, we explored the relationship between inpatient utilization and RRS in each system.
RESULTS: On average for a given patient who used both VA and Medicare services, more diagnoses were recorded in Medicare ( approximately 13-15) than in the VA system ( approximately 8) for dual users. On average only 2 diagnoses were common to both the VA and Medicare. Medicare data alone accounted for approximately 80% of individuals' total illness burden, and VA data alone lead to RRSs that capture one-third of the total illness burden. The ratio of RRS when calculated using Medicare and VA separately was approximately 2.4. RRS was only weakly to moderately correlated with inpatient utilization in each system.
CONCLUSION: Using data from just Medicare or VA data sources when conducting research on dually eligible veterans may seriously underestimate total illness burden of the population and also may lead to an underidentification of individuals in a particular disease class.