Escherichia coli a Reliable Indicator of Waterborne Illness Potential

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.

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