An interlaboratory study of measurement of aspartate aminotransferase activity with use of purified enzyme materials.

The Center for Disease Control (CDC), the New York State Department of Health (NYSDH), the College of American Pathologists, and 23 manufacturers of diagnostic products participated in an interlaboratory study of aspartate aminotransferase (EC 2.6.1.1) methodologies. Six different lyophilized materials were prepared and characterized and then distributed to 293 laboratories for aspartate aminotransferase measurements. The specimens included one human serum; four catalytic concentrations of the cytoplasmic isoenzyme, two purified from human erythrocytes, and two from porcine heart; and one matrix bovine serum albumin (30 g/liter) blank. The purified isoenzymes were prepared in the matrix. We present data on Michaelis parameters (Km and Vmax), Arrhenius plots, activation with pyridoxal 5-phosphate, vial-to-vial variability, and stability on reconstitution. The 281 responses showed that most of the laboratories used NADH-detection methods (91.1%), monitored at 340 nm (79.4%), and reported results in U/liter (89.4%). The percentage of laboratories reporting use of reaction temperatures of 30 and 37 degrees C was evenly divided, i.e., 42.7 and 42%, respectively. Analytical values reported by participating laboratories were categorized by reporting temperature, instrument, and method. Results were most consistent for a selected group of laboratories that supplemented optimized reaction solutions with pyridoxal 5-phosphate.