Ercumen A, Arnold BF, Naser AM, Unicomb L, Colford JM Jr. Potential sources of bias in the use of Escherichia coli to measure waterborne diarrhea risk in low-income settings. Trop Med Int Health. 2016 Oct 31. doi: 10.1111/tmi.12803
OBJECTIVES: Escherichia coli is the standard water quality indicator for diarrhea risk. Yet the association between E. coli and diarrhea is inconsistent across studies without a systematic assessment of methodological differences behind this variation. Most studies measure water quality cross-sectionally with diarrhea, risking exposure misclassification and reverse causation. Studies use different recall windows for self-reported diarrhea; longer periods increase potential outcome misclassification through misrecall. Control of confounding is inconsistent across studies. Additionally, diarrhea measured in unblinded intervention trials can present courtesy bias. We utilized measurements from a randomized trial of water interventions in Bangladesh to assess how these factors affect the E. coli-diarrhea association.
METHODS: We compared cross-sectional versus prospective measurements of water quality and diarrhea, two- versus seven-day symptom recall periods, estimates with and without controlling for confounding and using measurements from control versus intervention arms of the trial.
RESULTS: In the control arm, two-day diarrhea prevalence, measured prospectively one month after water quality, significantly increased with log10 E. coli (PR=1.50, 1.02-2.20). This association weakened when we used seven-day recall (PR=1.18, 0.88-1.57), cross-sectional measurements of E. coli and diarrhea (PR=1.11, 0.79-1.56) or did not control for confounding (PR=1.20, 0.88-1.62). Including data from intervention arms led to less interpretable associations, potentially due to courtesy bias, effect modification and/or reverse causation.
CONCLUSIONS: By systematically addressing potential sources of bias, our analysis demonstrates a clear relationship between E. coli in drinking water and diarrhea, suggesting that the continued use of E. coli as an indicator of waterborne diarrhea risk is justified.
