title: Weight-for-age z-score as a proxy marker for diarrhoea in epidemiological studies.
abstract: 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.
  
  
  CONCLUSIONS
  Repeated WAZ measures appear to be a suitable proxy marker for diarrhoea in children, but have disadvantages in terms of specificity and study power.
authors: Department of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, Keppel Street, WC1E 7HT London, UK. Wolf-Peter.Schmidt@lshtm.ac.uk
authors: Wolf-Peter
authors: WP
authors: Schmidt
authors: Sophie
authors: S
authors: Boisson
authors: Bernd
authors: B
authors: Genser
authors: Mauricio L
authors: ML
authors: Barreto
authors: Kathy
authors: K
authors: Baisley
authors: Suzanne
authors: S
authors: Filteau
authors: Sandy
authors: S
authors: Cairncross
mesh_terms: D001827:Body Height; D001835:Body Weight; D001938:Brazil; D002675:Child, Preschool; D003967:Diarrhea / Q000175:diagnosis* / Q000188:drug therapy / Q000453:epidemiology / Q000209:etiology; D019587:Dietary Supplements; D016021:Epidemiologic Studies; D005869:Ghana; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D009752:Nutritional Status; D015995:Prevalence; D016032:Randomized Controlled Trials as Topic; D012044:Regression Analysis; D014801:Vitamin A / Q000627:therapeutic use*; D014802:Vitamin A Deficiency
chemical_list: D014801:Vitamin A