Estimating true burden of disease detected by screening tests of varying validity.

ABSTRACT

Timely and accurate information on disease load is essential for planning health programs. Unfortunately, complexity, cost and need of skilled personnel limit the use of screening tools of high validity in developing countries. The disease load estimated with tools of low validity differs considerably from true disease load, particularly for diseases of extreme levels of prevalence/incidence. A tool of 70% sensitivity and specificity may yield a prevalence/incidence rate of 34% (CI 32.23-35.67%) for a disease whose true rate is only 10.0% (CI 8.94-11.06%). We proposed a procedure to derive the true estimate in such cases, based on the concepts of sensitivity and specificity of a diagnostic/screening test. It is applied on two sets of real data--one pertaining to incidence rate of low birth weight (LBW) and the other to prevalence rate of obesity--where multiple screening tests of varying validity were used to estimate the magnitude. Different screening tests yielded widely varying incidence/prevalence rates of LBW/obesity. The prevalence/incidence rates derived by using the proposed estimation procedure are similar and close to the true estimate obtained by screening tests considered as gold standard. Further, sample size determined on the basis of the results of a tool of low validity may be either larger or smaller than the required sample size. Estimation of true disease load enables determination of correct sample size, thus improving the precision of the estimate and, in some instances, reducing the cost of investigation.