The dipslides estimate counts in CFU per mL. Drinking water is usually measured on a per 100 mL basis. The lowest dipslide number of 1000 CFU per mL which is equivalent to 100,000 CFU per 100 mL. This is well above the limit for swimming (200 CFU per 100 mL) and drinking water (< 1 CFU per 100 mL). Drinking water should be tested using EPA-accepted test methods such as pour plate and MPN.
If a dipslide is used as a substitute for a pour plate method, it is important to take the surface area of the dipslide into account. The surface area of the dipslide is 0.1 mL. A hard count of 1 colony is 1 per 0.1 mL or 10 CFU per mL. This is 1000 CFU per 100 mL – still 5x above the swimming limit and well above the drinking water limit. Note: The presence of a colony on a dipslide after incubation would be a FAIL for drinking water. The absence of a colony should NOT be considered a PASS. Due to the limited sample volume (or contact area) it is quite possible that water above the drinking water limit might not grow a colony on the dipslide.
The dipslides should be used to estimate gross contamination.