Is it necessary to do both the presumptive and confirmation tests when testing for coliforms?
Are the presumptive and confirmation tests both necessary
Microbiological tests are often conducted in two stages. The first stage, called the presumptive test, uses media that allows stressed or injured organisms to grow. The media will indicate by color change, turbidity, or gas formation if the target organism is present. However, a non-target organism can sometimes interfere and cause a false positive by also changing the color of the media or forming turbidity or gas. To be sure that a positive result from the presumptive test is indeed a target organism (such as a coliform bacteria), confirm the result using confirmation media. The media used for confirmation is very restrictive to non-target organisms and contains nutrients only for the target organisms. If the target organisms are stressed or injured, they may not grow in the confirmation media and will lead to false negatives. That is why the presumptive media is always used first. Some media, such as the m-ColiBlue24® broth for total coliforms and E. coli, and broth containing MUG do not require confirmation because the non-target organisms do not interfere.