Differences in molar absorptivity of 4-NP with the reaction solution and apparatus affect ALP measurement

We examined the differences in molar absorptivity of 4-NP obtained using different kits for ALP measurement and different instruments. The apparent molar absorptivity of 4-NP in the same reaction solution determined by six different instruments was 15.98, 16.72, 16.06, 17.00, 16.27, 17.62 and that using four different reaction solution kits for ALP with the same instrument was 16.90, 17.38, 17.72, 16.11. We measured ALP in three serum samples with six instruments using the same kit and in twelve serum samples with the same instrument using four kits. ALP activities measured using the same molar absorptivity value differed with the instrument(p < 0.01). However, those measured using the apparent molar absorptivity value for each instrument revealed no significant differences(p > 0.05). In conclusion, we suggest that standard material should be contained in each kit for enzyme measurement and the apparent epsilon for each kit and instrument should be obtained to minimize the systematic error caused by using the same epsilon in different laboratories.