There are numerous ways to measure microbial contamination of water, and USP recommends specific ways for pharmaceutical waters.
Traditionally, plate count methods are widely used for releasing water; however, they are prone to error and can take 5-7 days to obtain results. Plate counting also requires the formation of a colony of bacteria on a plate, and hence, the measurement unit is called the 'colony forming unit' or CFU, which estimates the total number of bacteria present in the sample plate.
Another way of measuring microbial contamination in water is through laser-induced fluorescence with a microbial detection analyzer. These analyzers use laser-induced fluorescent technology to measure the total number of cells that are present in the sample volume of water. Systems that use this technology allow at-line, real-time monitoring of microbial contamination in pharmaceutical waters by identifying cells present in the sample water immediately and are reported as auto-fluorescent units (AFUs).