title: Potential sources of bias in the use of Escherichia coli to measure waterborne diarrhoea risk in low-income settings.
abstract: Escherichia coli is the standard water quality indicator for diarrhoea risk. Yet, the association between E. coli and diarrhoea is inconsistent across studies without a systematic assessment of methodological differences behind this variation. Most studies measure water quality cross-sectionally with diarrhoea, risking exposure misclassification and reverse causation. Studies use different recall windows for self-reported diarrhoea; longer periods increase potential outcome misclassification through misrecall. Control of confounding is inconsistent across studies. Additionally, diarrhoea measured in unblinded intervention trials can present courtesy bias. We utilised measurements from a randomised trial of water interventions in Bangladesh to assess how these factors affect the E. coli-diarrhoea association.
  
  
  
  We compared cross-sectional versus prospective measurements of water quality and diarrhoea, 2-versus 7-day symptom recall periods, estimates with and without controlling for confounding and using measurements from control versus intervention arms of the trial.
  
  
  
  In the control arm, 2-day diarrhoea prevalence, measured prospectively 1 month after water quality, significantly increased with log10E. coli (PR = 1.50, 1.02-2.20). This association weakened when we used 7-day recall (PR = 1.18, 0.88-1.57), cross-sectional measurements of E. coli and diarrhoea (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.
  
  
  
  By systematically addressing potential sources of bias, our analysis demonstrates a clear relationship between E. coli in drinking water and diarrhoea, suggesting that the continued use of E. coli as an indicator of waterborne diarrhoea risk is justified.
authors: Division of Epidemiology, School of Public Health, University of California, Berkeley, CA, USA.
authors: Ayse
authors: A
authors: Ercumen
authors: Division of Epidemiology, School of Public Health, University of California, Berkeley, CA, USA.
authors: Benjamin F
authors: BF
authors: Arnold
authors: Infectious Disease Division, International Centre for Diarrhoeal Disease Research, Dhaka, Bangladesh.|Rollins School of Public Health, Emory University, Atlanta, GA, USA.
authors: Abu Mohd
authors: AM
authors: Naser
authors: Infectious Disease Division, International Centre for Diarrhoeal Disease Research, Dhaka, Bangladesh.
authors: Leanne
authors: L
authors: Unicomb
authors: Division of Epidemiology, School of Public Health, University of California, Berkeley, CA, USA.
authors: John M
authors: JM
authors: Colford
authors: Infectious Disease Division, International Centre for Diarrhoeal Disease Research, Dhaka, Bangladesh.|School of Medicine, Stanford University, Stanford, CA, USA.|Centers for Disease Control and Prevention, Atlanta, GA, USA.
authors: Stephen P
authors: SP
authors: Luby
mesh_terms: D001459:Bangladesh / Q000453:epidemiology; D003430:Cross-Sectional Studies; D003906:Developing Countries; D003967:Diarrhea / Q000453:epidemiology*; D060766:Drinking Water / Q000382:microbiology*; D004926:Escherichia coli / Q000302:isolation & purification*; D006801:Humans; D011446:Prospective Studies; D062486:Public Health Surveillance / Q000379:methods; D015203:Reproducibility of Results; D013997:Time Factors; D000069578:Waterborne Diseases / Q000453:epidemiology*
chemical_list: D060766:Drinking Water
keywords: 
  E. coli
  ; 
  E. coli
  ; Bangladesh; diarrhoea; enfermedad transmitida por el agua; maladie d'origine hydrique; medida de calidad del agua; mesure de la qualité de l'eau; riesgo de diarrea; risque de diarrhée; water quality; waterborne disease