Validity and comparison of two measures of days supply in Medicaid claims data

In claims-based pharmacoepidemiologic and health services research studies of chronic medications, days supply can be determined using either an observed measure or a measure calculated from quantity dispensed and dosing directions. The two measures should yield the same duration, but if not, the preferred method is unknown.

We aimed to determine whether the observed or calculated measure of days supply more closely approximated the actual refill interval.

Using California Medicaid data from 1998 to 2001, we identified all prescriptions followed by > or = 1 refill for nevirapine and abacavir tablets. For each record, we calculated the difference between the days supply and the refill interval for each method. In a subgroup analysis, we compared the measures only in records where the methods were discordant.

Of 29 646 records, in 27 230 (92.5%) the measures were identical. The difference between the days supply and the refill interval was identical for each measure with a median difference of 4 days (range 0-1142 days), p = 0.5 for the difference between them. In the discordant subset, the median difference for the observed measure was 10 days (range 0-1053 days) and for the calculated measure it was 12 days (range 0-1023 days), p = 0.003 for the difference between them.

In California Medicaid, observed and calculated measures of days supply generally provided identical data which closely approximated the refill interval. In the few discordant records, the observed measure was slightly more accurate. These findings suggest that both variables provide valid data for temporal relations between anti-retroviral prescriptions and events.