The results of the MTF technique are expressed in
terms of the most probable number (MPN) of microorganisms
present. This number is a statistical estimate
of the mean number of coliforms in the sample.
As a consequence, this technique offers a semi-quantitative
enumeration of coliforms. Nevertheless, the
precision of the estimation is low and depends on the
number of tubes used for the analysis: for example, if
only 1 ml is examined in a sample containing 1
coliform/ml, about 37% of 1-ml tubes may be
expected to yield negative results because of the
random distribution of the bacteria in the sample.
But, if five tubes, each with 1 ml sample, are used,
a negative result may be expected less than 1% of the
time (APHA et al., 1998).