What is the Difference Between Reporting Microbiology Testing per CFU or MPN?
CFU vs. MPN
The difference between reporting as CFU or MPN per unit of measure is due to their respective methods. CFUs or "colony forming units" are determined by directly counting visible colonies of bacterial growth on the plate or film. MPN, or Most Probable Number, on the other hand measures growth statistically. In this method, sample is added to a series of test tubes with special media at different concentrations. The MPN value is then measured based on the reaction levels in the tubes and the value is then calculated with a 95% confidence interval.
MPN Performance and Sensitivity
MPN methods generally have a slightly lower limit of quantification than do CFU methods. If increased sensitivity is desired, then an MPN method can be considered.
Special Considerations in Method Selection
Occasionally there is a regulatory requirement for an MPN method, such as drinking water portability standards for coliform count. Alternatively, if a sample has known antimicrobial activity then CFU methods may be a better choice due to greater dilution during sample preparation. There also may be cost differences between the two methods.
Peteris Spels, Business Unit Manager for Eurofins Microbiology Laboratories, Inc. of Providence area, Rhode Island, explains the key details of these methods in the following video:
For further questions on this topic:
