Recall bias in a prospective cohort study of acute time-varying exposures:Example from the herpetic eye disease study

Recall bias is possible in a prospective cohort study when exposure status is transient and must be periodically recalled, and ascertainment occurs af ter symptom onset. We know of no published demonstration of such bias at pl ay in a prospective cohort study. In a substudy of a randomized clinical tr ial, 308 participants were prospectively followed to investigate potential acute triggers of ocular herpes simplex virus (HSV) recurrences. Participan ts reported on the presence of systemic infection or high psychological str ess (exposures) on a home log that was completed weekly for up to 15 months and mailed to the study's coordinating centers. By protocol, exposure repo rting was to occur on the last day of the week (Sunday) so that a prospecti ve 1-week lag period between exposure and outcome in the following week cou ld be assessed. The study outcome was development of a recurrence of ocular HSV disease documented by clinical examination. Using 35 weekly reports of exposure properly completed before the week of an outcome, there was no ev idence of higher risk of HSV recurrence associated with systemic infection (rate ratio = 0.62, 95% confidence interval [CI]: 0.19-2.02) or high psycho logical stress rate (ratio = 0.0, 95% CI: 0.0-undefined). In contrast, when the analysis was based on 26 weekly reports of exposure improperly complet ed on or after the date of outcome, the risk of recurrence associated with systemic infection was estimated to be 4-fold (rate ratio = 4.07, 95% CI: 1 .84-8.98), and there was a suggestion of a 2-fold risk associated with high psychological stress (rate ratio = 2.02, 95% CI: 0.69-5.91). Without real- time monitoring of exposure reporting, preservation of, the temporal exposu re-disease relationship-an implicit assumption of the prospective cohort st udy design-may be particularly tenuous when transient exposures are investi gated longitudinally. (C) 2001 Elsevier Science Inc. All rights reserved.