Pooled Prevalence Calculator – Demonstration analyses

Fixed pool size and perfect tests

Two methods are provided for fixed pool sizes and perfect tests, one producing asymptotic confidence intervals (Method 1) and the other exact confidence intervals (Method 2). These methods should only be used if you can be confident that the sensitivity and specificity of the test are both close to 100%. If the true prevalence is likely to be close to zero, Method 2 (with exact confidence limits) should be used in preference to Method 1, because Method 1 could produce a negative lower confidence limit.

For this analysis, it was assumed that samples from 210 individual fruit bats were aggregated into 42 pools of 5 samples each, that 22 pools produced a positive test result and that the test sensitivity and specificity were both 100%. Input values and results for this analysis are summarised in the table below.

 Method 1 2 Input values: Number of pools tested 42 42 Number of pools positive 22 22 Pool size 5 5 Lower CL 0.025 0.025 Upper CL 0.975 0.975 Results: Est. Prevalence 0.1379 0.1379 2.5 percentile 0.0866 0.0832 97.5 percentile 0.2038 0.1926 Std. Error 0.0279 0.0279