The comparison between the two methods was made using criteria set by international standard ISO 17994 (2004). The combined original E. coli count results from all the laboratories showed that the relative recovery of the Colilert-18 method after 18 hours of incubation was 6.96% and 8.58% lower than that of the MMPN method after 48 hours and 72 hours of incubation, respectively (Table 4). The confirmation changed the relative difference between the methods only slightly, with the average confirmed recovery of the Colilert-18 method being 8.81% lower than the MMPN method recovery. After the correction of false negatives, the relative difference between the Colilert-18 and MMPN methods was reversed; the corrected recovery of the Colilert-18 method was 8.30% higher than the recovery of the MMPN method (Table 4).
A quantitative comparison between the relative differences of the E. coli results obtained using the Colilert-18 and MMPN-48 and -72 methods per laboratory according to ISO 17994 (2004). Combined results of six laboratories
| Material . | N . | M . | SD . | SE . | U . | LO . | HI . | Evaluation* . |
|---|---|---|---|---|---|---|---|---|
| Original 18-48 | 263 | −6.96 | 67.4 | 4.16 | 8.32 | −15.28 | +1.36 | Methods equivalent |
| Original 18-72 | 263 | −8.58 | 67.2 | 4.14 | 8.29 | −16.87 | −0.30 | Methods different |
| Confirmed 18-72 | 263 | −8.81 | 67.8 | 4.18 | 8.36 | −17.17 | −0.45 | Methods different |
| Corrected 18-72 | 263 | +8.30 | 71.2 | 4.39 | 8.78 | −0.48 | +17.08 | Methods equivalent |
| Material . | N . | M . | SD . | SE . | U . | LO . | HI . | Evaluation* . |
|---|---|---|---|---|---|---|---|---|
| Original 18-48 | 263 | −6.96 | 67.4 | 4.16 | 8.32 | −15.28 | +1.36 | Methods equivalent |
| Original 18-72 | 263 | −8.58 | 67.2 | 4.14 | 8.29 | −16.87 | −0.30 | Methods different |
| Confirmed 18-72 | 263 | −8.81 | 67.8 | 4.18 | 8.36 | −17.17 | −0.45 | Methods different |
| Corrected 18-72 | 263 | +8.30 | 71.2 | 4.39 | 8.78 | −0.48 | +17.08 | Methods equivalent |
N, number of samples; M, mean relative difference value (unweighted); SD, standard deviation; SE, standard error of the mean value; U, expanded uncertainty of measurement; LO, lower confidence limit, approximately 95%; HI, higher confidence limit, approximately 95%.
*When evaluating the equivalence of the methods in this comparison, a maximum deviation of D = 20% was used. In a two-sided evaluation, the methods were considered equivalent if neither of the methods gave significantly higher or lower results than the other one.