In vitro and in vivo analysis of the PABA test compared with the Lundh test--influence of intraluminal pH.

Lundh test and PABA test results were compared in 50 patients. In the resulting correlation curve three areas of interest were differentiated: (1) an area with mean tryptic activity, found in the Lundh test, below 4 U/ml, where an abnormal PABA test result was also seen; (2) an intermediate area from 4 to 9 U/ml, where PABA test results may be found in discordance with the Lundh test; (3) the normal level, where PABA excretion was shown to be independent of intestinal chymotrypsin activity. In experiments in vitro on the kinetics of the hydrolysis of BTPABA by chymotrypsin the profound influence of the pH on Km and Vmax was shown. This influence of the pH explains why in the intermediate area of pancreatic dysfunction normal PABA test results were found in a number of cases. A higher mean pH level of the Lundh test aspirates was found for patients with a normal PABA test result than for patients with an abnormal PABA test.