Methods compared for determining total amylase activity and amylase isoenzymes in serum

We evaluated two kinetic methods for determining total amylase activity and isoenzyme composition in serum. Stability studies of reagents for measuring total activity indicate that reagents containing 4-nitrophenyl-alpha-glucosides or enzyme-linked reagents can be stored only for seven days at 4 degrees C. Methods based on 4-nitrophenyl-alpha-glucoside substrates cannot be used if the reagent absorbance at 405 nm exceeds 2. However, in the alpha-amylase EPS method (Boehringer Mannheim) an ethylidene-protected 4-nitrophenyl-alpha-D-maltoheptaoside substrate is stable for up to 28 days after reconstitution. Further studies indicated that the Amylase-DS (Beckman) and the alpha-Amylase EPS standard curves are linear to at least six times the upper limit of the reference interval. Within-batch imprecision (CV less than 1.1%) and between-batch imprecision (CV less than 3.3%) for these two methods are comparable with those for other kinetic methods, and there is excellent correlation (r2 = 0.983) between the two methods. The reference interval, determined by use of samples from 90 healthy blood donors, is 31 to 141 U/L for the amylase-DS method, 22 to 92 U/L for the alpha-Amylase EPS method. We also used these two methods to measure amylase isoenzymes after inhibiting the salivary isoenzyme with either a lectin or a monoclonal antibody. We found the monoclonal antibody method more specific than the lectin inhibition method for determining the isoenzymes.