Two-phase surveys often are used to estimate prevalence, in particular when the disease is rare or the case ascertainment procedure difficult and/or costly. However, few authors of such surveys take into account the sensitivity error associated with the use of a screening procedure in the first phase and its imprecision in correcting the prevalence estimate and confidence interval.
Two examples of two-phase surveys of rheumatic diseases (hip and knee osteoarthritis, rheumatoid arthritis and spondyloarthropathies) are used to present methodological approaches to obtain corrected prevalence estimates. Two methods for assessing the accuracy of the screening procedure are described--two-phase pilot and case-control designs--that are best suited for frequent and rare diseases, respectively, and naive and corrected estimates of prevalence compared.
When the sensitivity error is not taken into account, prevalence is underestimated, as is, especially, the width of its confidence interval. In our examples, the corrected confidence interval width increased up to 50% as compared with naïve one.
The screening procedure accuracy should be thoroughly assessed in two-phase prevalence surveys and prevalence estimates and their confidence intervals corrected accordingly.