Accuracy of the H₂S test: a systematic review of the influence of bacterial density and sample volume

The presence/absence hydrogen sulphide test (P/A H₂S) is widely used as a low-cost alternative faecal indicator test in remote and resource-poor settings. The aim of the paper is to assess how bacterial density and sample volume affect its accuracy. Based on a systematic search, we identified studies that tested water samples (n = 2,034) using both the P/A H₂S test and recognised tests for thermotolerant coliforms (TTC) or Escherichia coli. We calculated P/A H₂S test specificity and sensitivity against a range of TTC and E. coli densities. For two studies, we compared this with sensitivity and specificity estimates for simulated 100 and 20 ml presence/absence tests. For most of the 19 included studies, as the threshold used to define contamination increased from 1 to 100 cfu/100 ml, P/A H₂S test sensitivity increased but specificity decreased. Similarly, the simulation indicated that increasing test volumes from 20 to 100 ml increased sensitivity but reduced specificity. There was potential for bias, for example from lack of blinding during test interpretation, in most of the studies reviewed. In assessing the P/A H₂S test as an alternative to standard methods, careful consideration of likely indicator bacteria levels and sample volume is required.