Table 3.
Sensitivity, specificity, positive predictive value and negative predictive value of H2S test against Escherichia coli by membrane filtration, for different Escherichia coli risk categories in stored household water samples in rural Bangladesh (N = 525)
| Duration of H2S test incubation (hour) | Escherichia coli level (CFU/100 mL) by membrane filtration | N | Number of H2S positive samples | Number of H2S negative samples | Sensitivity % (95% exact CI) | Specificity % (95% exact CI) | PPV % (95% exact CI) | NPV % (95% exact CI) |
|---|---|---|---|---|---|---|---|---|
| 24 hours | No risk (< 1) | 55 | 8 | 47 | – | 85 (73, 94) | – | – |
| Low risk (1–10) | 144 | 37 | 107 | 26 (19, 34) | 82 (68, 92) | 31 (23, 38) | ||
| Moderate risk (11–100) | 208 | 94 | 114 | 45 (39, 52) | 92 (85, 97) | 29 (22, 37) | ||
| High risk or above (> 100) | 107 | 81 | 26 | 76 (67, 84) | 91 (83, 96) | 64 (52, 75) | ||
| All positive (≥ 1)* | 470 | 221 | 249 | 47 (42, 52) | 97 (93, 98) | 16 (12, 21) | ||
| 48 hours | No risk (< 1) | 55 | 28 | 27 | – | 49 (35, 63) | – | – |
| Low risk (1–10) | 144 | 104 | 40 | 72 (65,80) | 79 (71, 85) | 40 (28, 53) | ||
| Moderate risk (11–100) | 208 | 175 | 33 | 84 (79, 89) | 86 (81, 91) | 45 (32, 58) | ||
| High risk or above (> 100) | 107 | 99 | 8 | 93 (87, 98) | 78 (70, 84) | 77 (60, 90) | ||
| All positive (≥ 1)* | 470 | 388 | 82 | 83 (79, 86) | 93 (90, 95) | 25 (17, 34) |
CFU = colony forming units; CI = confidence interval; H2S = hydrogen sulfide; NPV = negative predictive value; PPV = positive predictive value.
*
The sum of the sample numbers in the low, moderate and high risk categories is smaller than the total number of positive samples because of 11 confluent (positive but not countable) samples.