Collaborative Study of Rapid Methods for Alkaline Phosphatase in Milk and Cream

Determination of alkaline phosphatase activity in milk and cream by the modified Scharer rapid test with use of photoelectric colorimeter for measuring absorbance was collaboratively studied. Milk samples (skim milk. milk and cream) with and without added raw milk were sent to 12 collaborators to be tested by (a) the modified Scharer rapid test using commercial standards and phenol standards for comparing colors, (b) the rapid colorimetric test and (c) the Rutgers method. The latter method was used for comparison only. In the modified Scharer rapid test, based on the category of standards, 73.3% of the samples using the commercial standards and 71.6% of the samples using phenol standards were correctly diagnosed. When the scoring was based on positive or negative, 98.4 and 92.6% of the samples were correctly diagnosed. Results with the phenol standards were significantly lower than those observed with the commercial standards. There were no false-positive results, as all incorrect readings were below limit of detection. Most of the errors occurred when the expected value was 1.0 μg phenol/ml. Results were 100% correct for the Rutgers method, but there are only two choices for this method, and they correspond to positive or negative. Compared to the theoretical values, data obtained by the colorimetric method ranged from 1.5 to 18.1% high, with a coefficient of variation of 4.4 to 13.4%. These variations were assumed satisfactory considering the levels at which phosphatase was tested.