A COMPARATIVE STUDY OF STAINS PROPOSED FOR THE DIRECT MICROSCOPIC EXAMINATION OF MILK

Five stains which have been proposed as substitutes for stains now specified in Standard Methods were compared by means of a collaborative study involving nine different laboratories. Stain I was the present Standard Methods alcohol-containing methylene blue stain; Stain II was a potassium - dichromate - sulfuric acid polychrome methylene blue stain; Stain III was an acid- and water-free methylene blue stain; Stain IV was a modification of Stain III containing hydrogen peroxide; Stain V was a methylene blue stain, the use of which was accompanied by a modification in fixation of the milk film; and Stain VI was a modified two color stain, details of which were not supplied. Examination of weighted averages of all samples from all laboratories showed that Stain III yielded the highest average count. Unfavorable comments pertaining to all stains except Stain III were received. Great variation in the results between laboratories was evident. One stain might show superior results in one laboratory and give inferior results in another laboratory. In general, Stain III showed the least variation in results among the collaborating laboratories, although it obtained the highest score in only one laboratory. The study pointed up the need for uniformity in laboratory procedure among different laboratories, particularly as regards the direct microscopic counting procedure.