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Choosing an Index Date for Untreated Patients in External Comparator Studies.
Luis Antunes1, Gerd Rippin2, Eleanor Ralphs3
1IQVIA, Porto, Portugal.
Drug Safety
|September 29, 2025
Summary
Selecting an index date for external comparator studies is crucial. Emulated index dates (IDE) for untreated patients offer an unbiased approach, outperforming other methods in simulation studies for external comparator analysis.
Area of Science:
- Biostatistics
- Observational Studies
- Comparative Effectiveness Research
Background:
- Defining a baseline (index date) is challenging in external comparator (EC) studies, particularly when comparing treated patients to untreated ones.
- Existing methods for index date selection in observational studies are not specifically tailored for the unique demands of EC studies.
Purpose of the Study:
- To evaluate different methods for assigning index dates in external comparator studies.
- To assess the bias and performance characteristics of various index date assignment and analytical approaches.
Main Methods:
- A simulation study compared two main approaches: index date emulation (IDE) and early index date (EID).
- IDE involved assigning observed treatment start times from treated cohorts to untreated cohorts.
- EID involved starting follow-up at diagnosis and using time-dependent Cox models, including modified versions.
Main Results:
- The index date emulation (IDE) and modified early index date (mEID) approaches demonstrated unbiased performance.
- The modified index date emulation (mIDE) and early index date (EID) approaches exhibited significant bias.
Conclusions:
- The proposed EC analysis approach using emulated index dates for untreated patients is a valid and potentially advantageous concept.
- This method offers a robust solution for index date selection in external comparator studies.
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