A number of different outcome measures have been proposed for use in prospective studies of morbidity associated with childhood diarrhea. These include the number of episodes experienced by each child over a defined period (a measure of incidence) and the number of days of diarrhea divided by the total number of days of observation for each child (a measure denoted "longitudinal prevalence"). The authors examined data from Ghana to determine which of these measures is more strongly associated with weight gain over a 4-month period and subsequent mortality. Both diarrhea incidence and longitudinal prevalence were associated with weight gain in children aged 6-23 months, but a statistically stronger association was observed with longitudinal prevalence (likelihood ratio statistic 28.95 on 1 degree of freedom against 19.70 for incidence). Neither measure was associated with weight gain in younger or older children. Longitudinal prevalence, but not incidence, was strongly associated with subsequent mortality (p = 0.002 for longitudinal prevalence; p = 0.557 for incidence). Although many epidemiologic studies of diarrhea focus on incidence, these data suggest that longitudinal prevalence is more strongly predictive of long-term health outcome. The authors conclude that longitudinal prevalence merits greater attention as a measure of outcome in diarrhea studies.
PIP: During June 1990 to June 1991, weekly home visits and periodic clinical examinations were conducted among 1872 children 0-5 years old living in northern Ghana. The data obtained from the home visits and the clinical examinations were analyzed to determine whether diarrhea incidence (number of episodes experienced by each child over a defined period) or longitudinal prevalence (number of days of diarrhea divided by the total number of days of observation for each child) had the strongest association between weight gain over a 4-month period and subsequent mortality. Diarrhea burden, be it diarrhea incidence or prevalence, was strongly associated with weight gain in children 6-23 months old only. Yet, longitudinal prevalence of diarrhea had more explanatory power with regards to weight gain than did incidence (likelihood ratio statistic: 28.95 on 1 degree of freedom vs. 19.70). When the researchers adjusted for longitudinal prevalence, the strength of the association between the number of incident episodes and weight gain declined. On the other hand, when they adjusted for incidence, the strength of the association between longitudinal prevalence and weight gain remained very strong. Longitudinal prevalence was also a strong predictor of subsequent mortality (5% absolute increase in longitudinal prevalence associated with a 17% relative increase in the risk of mortality; p = 0.002 vs. p = 0.557 for incidence). These findings suggest that longitudinal prevalence of diarrhea is a stronger predictor of long-term health outcome than incidence. Longitudinal prevalence may be best suited for studies of interventions expected to improve host response to diarrhea (e.g., vitamin A supplementation or treatment of acute diarrhea with zinc) or those seeking to quantify the burden on children of morbidity from diarrhea. In conclusion, longitudinal prevalence should receive greater attention as a measure of outcome in studies of childhood diarrhea.