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Persistent User Bias in Case-Crossover Studies in Pharmacoepidemiology.

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    Area of Science:

    • Pharmacoepidemiology
    • Biostatistics
    • Observational Study Design

    Background:

    • Case-crossover studies are susceptible to upward bias in odds ratios due to chronic medication exposure.
    • Persistent medication use can contaminate case-crossover analyses, particularly when associations are null.

    Purpose of the Study:

    • To assess the upward bias in odds ratios from case-crossover designs when studying chronic medication exposure.
    • To evaluate the effectiveness of the case-time-control design in remedying this bias.

    Main Methods:

    • Danish registry data (1995-2012) were used for case-crossover and case-time-control analyses.
    • Three medications (statins, insulin, thyroxine) and three outcomes (retinal detachment, wrist fracture, ischemic stroke) were examined.
    • Controls were matched on age, sex, and index date; 12-month medication exposure was ascertained.

    Main Results:

    • Case-crossover analyses showed elevated odds ratios for all medications and retinal detachment.
    • Case-time-control analyses yielded near-null estimates for retinal detachment, suggesting bias correction.
    • Residual bias was observed for wrist fracture and ischemic stroke, with case-time-control odds ratios remaining above unity.

    Conclusions:

    • Case-crossover studies of chronic medication use may exhibit moderate upward bias due to persistent users.
    • The case-time-control design partially mitigates this bias, but does not eliminate it entirely for all outcomes.
    • Optimal strategies for addressing bias in medication exposure studies require further investigation.