BT-PABA test with plasma PABA measurements: evaluation of sensitivity and specificity

Urinary recovery and plasma time curve of PABA were determined in 15 control subjects and 65 patients during a six hour period after ingestion of a meal containing 1 g BT-PABA (2.333 mmol PABA). In controls, the plasma PABA time curve registered a peak at two hours followed by a rapid decrease to reach its lowest value at six hours. In patients with chronic pancreatitis (n=32), the peak was lower and was followed by a slow decrease; furthermore, it was delayed in those with severe pancreatic insufficiency. The best discrimination between controls and patients with chronic pancreatitis was obtained by using the maximal value of plasma PABA (MPPABA) at two or three hours. In 56 subjects a hyperbolic relationship between MPPABA and duodenal lipase output stimulated by a meal was mathematically demonstrated and it was calculated that the lower limit of normal MPPABA (mean -2 SD) corresponded to a lipase output equal to 20.6% of the mean value of normal subjects (lower limit of normal lipase output = 40%). By comparison, steatorrhoea occurred when lipase output was less than 10%. Consequently, MPPABA was low not only in all patients with steatorrhoea but also in some who had pancreatic deficiency but normal daily faecal fat. With lipase output as the reference, 12 subjects who had normal MPPABA were proved to have falsely abnormal urinary results. Urinary PABA excretion after oral administration of 2.333 mmol free PABA was also determined in 27 subjects. The PABA excretion index (PEI) was calculated: PEI = PABA urinary excretion after BT-PABA/PABA urinary excretion after free PABA. Seven patients with normal MPPABA but low urinary excretion after BT-PABA had a normal PABA excretion index. It was also observed that, in five patients with intestinal disease, free PABA absorption was not impaired. In conclusion, BTPABA test with MPPABA determination used as an index of exocrine function is (1) less sensitive than Lundh's test, but more sensitive than steatorrhoea; (2) more specific than the test with urinary recovery determination alone, and as specific as PABA excretion index, which requires a double test.