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

Authors

  • Robert Gross MD, MSCE,

    Corresponding author
    1. Center for Clinical Epidemiology and Biostatistics, University of Pennsylvania School of Medicine, USA
    2. Division of Infectious Diseases, Department of Medicine, University of Pennsylvania School of Medicine, USA
    3. Center for Education and Research on Therapeutics, University of Pennsylvania School of Medicine, USA
    • Center for Clinical Epidemiology and Biostatistics, Center for Education and Research in Therapeutics, University of Pennsylvania School of Medicine, 804 Blockley Hall, 423 Guardian Drive, Philadelphia, PA 19104-6021.
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  • Warren B. Bilker PhD,

    1. Division of Infectious Diseases, Department of Medicine, University of Pennsylvania School of Medicine, USA
    2. Center for Education and Research on Therapeutics, University of Pennsylvania School of Medicine, USA
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  • Brian L. Strom MD, MPH,

    1. Division of Infectious Diseases, Department of Medicine, University of Pennsylvania School of Medicine, USA
    2. Center for Education and Research on Therapeutics, University of Pennsylvania School of Medicine, USA
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  • Sean Hennessy PharmD, PhD

    1. Division of Infectious Diseases, Department of Medicine, University of Pennsylvania School of Medicine, USA
    2. Center for Education and Research on Therapeutics, University of Pennsylvania School of Medicine, USA
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  • No conflict of interest was declared.

Abstract

Background

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.

Objectives

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

Methods

Using California Medicaid data from 1998to 2001, we identified all prescriptions followed by ≥ 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.

Results

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.

Conclusions

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. Copyright © 2008 John Wiley & Sons, Ltd.

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