Combined analysis of retrospective and prospective occurrences in cohort studies: HIV-1 serostatus and incident pneumonia.

Background: The authors show how information collected on retrospective occurrence times may be combined with prospective occurrence times in the analysis of recurrent events from cohort studies.

Methods: We demonstrate how the observed data can be expanded from one to two records per participant and account for the within-individual dependence when estimating variances. We illustrate our methods using data from the Women's Interagency HIV Study, which recorded 384 retrospective and 352 prospective occurrences of pneumonia in 9478 retrospective and 7857 prospective person-years among 2610 adult women.

Results: The hazard of non-Pneumocystis carinii pneumonia among the 2056 HIV-1 infected women was 2.24 times (95% confidence limits: 1.74, 2.89) that of the 554 uninfected women, independent of age. This hazard ratio was homogeneous across retrospective and prospective occurrences (P for interaction = 0.96) and combining occurrence types increased the precision by reducing the standard error by about a fourth.

Conclusions: As expected, HIV-1 infection increases the hazard of pneumonia, with more precise inference obtained by combining information available on bidirectional occurrences. The proposed method for the analysis of bidirectional occurrence times will improve precision when the estimated associations are homogeneous across occurrence types, or may provide added insight into either the data collection or disease process when the estimated associations are heterogeneous.