Weight-for-age z-score as a proxy marker for diarrhoea in epidemiological studies

BACKGROUND: The validity of unblinded randomised trials testing interventions against diarrhoea is severely compromised by the potential for bias. Objective proxy markers for diarrhoea not relying on self-report are needed to assess the effect of interventions that cannot be blinded. Short-term changes in weight-for-age z-score (WAZ) may (due to catch-up growth) not be a clinically important marker for nutritional status. However, even a transient decrease in WAZ could indicate recent diarrhoea, and be interpreted as the effect of an intervention. METHODS: Using data from two large vitamin A trials from Ghana and Brazil, the immediate effect of the cumulative diarrhoea occurrence over 14 and 28 day time windows on WAZ was explored. RESULTS: A very strong linear association was found between the number of days with diarrhoea over the last 14–28 days and WAZ. In both trials, differences in diarrhoea between the trial arms were associated with corresponding differences in WAZ. CONCLUSION: Repeated WAZ measures appear to be a suitable proxy marker for diarrhoea in children, but have disadvantages in terms of specificity and study power.