Quantitative and categorical results from water and hand-rinse sampling
| Colilert result | Petrifilm result | Concentration of E. coli per 100 mL | Category |
| Absent | 0 | <1 CFU or MPN/100 mL | Not detected |
| Present | 0 | 1–99 MPN/100 mL | Low |
| Present | 1–9 CFU/1 mL | 100–999 CFU/100 mL | Medium |
| Present | 10-49 CFU/1 mL | 1000–4999 CFU/100 mL | High |
| Present | ≥50 CFU/1 mL | ≥5000 CFU/100 m | Very high |
We use a combination of 1 mL Petrifilm quantitative tests and 100 mL Colilert presence/absence tests to estimate colony forming units or most probable number. This approach was chosen to optimise the information obtained while staying feasible and within fieldworker capabilities. Combining the 2 tests provides a categorical measure of household contamination. Note that we assume that any sample positive for Petrifilm will also be positive for Colilert, as the sensitivity is much higher for Colilert (Colilert has a lower detection limit of 1 MPN/100 mL, whereas Petrifilms have a lower detection limit of 100 CFU/100 mL). If we detect growth on Petrifilm in the absence of a positive result on the Colilert, we will rely on the Petrifilm result.
CFU, colony forming unit; MPN, most probable number.