Collaborative Evaluation of the Plate Loop Technique for Determining Viable Bacterial Counts in Raw Milk

A collaborative evaluation of the plate loop count technique was undertaken involving 13 laboratories with 29 technologists and 27 plate loops, The log variance of the Plate Loop Count (PLC) was 0.007, compared with a log variance of 0.001 for the reference Standard Plate Count (SPC), both of which fell within the tolerance limit of 0.012 considered acceptable for reproducibility by Standard Methods. The overall log10 mean PLC (1.975) compared favourably with the log10 mean SPC (1.960). Despite this overall image of precision and accuracy of the PLC, comparative data between individual technicians and/or loops was significantly different. Factors which contributed to this inaccuracy and imprecision of the PLC included improper loop calibration and significant variation in technique not only between analysts but also by individual analysts between loops.