Collaborative trial of the direct epifluorescent filter technique (DEFT), a rapid method for counting bacteria in milk.

The direct epifluorescent filter technique (DEFT) is a new rapid method which uses membrane filtration and epifluorescent microscopy for counting in milk. A collaborative trial of the DEFT was conducted between six laboratories. Each laboratory obtained a highly significant relationship between the DEFT count and plate count with a correlation coefficient generally greater than 0.9 but there were significant differences between these relationships. The repeatability of the DEFT, although ca 1.5 times worse than that of the plate count, was of a level acceptable in practice. Reproducibility of the DEFT was ca 3 times that of the plate count. This poor reproducibility was probably mainly due to counting errors. Possible reasons for this and ways of reducing counting errors are discussed.