p-aminobenzoic acid; PAS, p-aminosalicylic acid; EPI, exocrine pancreatic insufficiency; and AUC, area under the curve. Presented in part at the annual meeting.
CLIN. CHEM. 41/4, 599-604(1995)
#{149} General
Clinical Chemistry
Simultaneous Assessments of Exocrine Pancreatic Function by Cholesteryl[14C]Octanoate Breath Test and Measurement of Plasma p-Arninobenzoic Acid Marco J. Bruno,”5 Frans J. Hoek,1’2 Barend Delzenne,2 Dirk J. van Leeuwen,3 Man F. Hofmann,4 and Guido N. J. Tytgat’ Two noninvasive tests for assessing pancreatic exocrine function, the cholesteryl-[14C)octanoate breath test and the HPLC N-benzoyl-tyrosyl-p-aminobenzoic acid/p-am inosalicylic acid (NBT-PABA/PAS) test, were simultaneously performed in nine patients with pancreatic exocrine insufficiency due to chronic pancreatitis and in nine healthy volunteers. ‘4C02 output in breath and plasma PABA concentration rose slowly in patients but increased rapidly in healthy subjects. The measurement time giving the best discrimination between both groups was 120 mmfor the cholesteryl-[14C]octanoatebreath test and 90 mm for the plasma PABA test. At these points, both single-sample tests had essentially identical diagnostic sensitivity. The diagnostic sensitivities of the two singlesample tests were equal to that of the cumulative 6-h urinary PABA recovery and the cumulative 6-h urinary PABA/PAS ratio. We conclude that, for both the cholesteryl-[14C]octanoate breath test and the plasma PABA test, a single test sample is sufficient for rapid detection of impaired exocrine pancreatic function. Indexing Terms:
panclatic
bromide/p-aminosahcykc
function/intermethod acid/,aoass/chmmatog,hy,
companson/ liquid
Many tests have been developed to assess exocrine pancreatic function (1). Tests in which pancreatic enzyme secretion is measured directly by duodenal sampling during stimulation by cholecystokinin or after a test meal are regarded as the gold standard. However, such procedures are expensive as well as time-consuming for medical personnel and are uncomfortable for the patient. Thus, continuing efforts have been made to develop nornnvasive methods for quantifying exocrine pancreatic function. One test that has been licensed in many countries is the p-aminobenzoic acid (PABA; bentiromide)6 test, which is based on hydrolysis of the peptide N-benzoyltyrosyl-p-aminobenzoic acid (NBT-PABA) by pancreDepartments
of’ Gastroenterology
and
Hepatology
and2 Clin-
ical Chemistry, Academic Medical Center, University of Amsterdam, Meibergdreef 9, 1105 AZ, Amsterdam, The Netherlands. 3Division of Gastroenterology and Hepatology, University of Alabama, 412 Lyons-Harrison Research Bldg., 701 S. 19th St., Birmingham, 4Division
AL 35294-00007.
University of California, San Diego, La Jolla, CA 92093-08 13. Corresponding author. Fax + 31-20-6917033. 6Nonstandard abbreviations: NBT-PABA, N-benzoyl-tyrosylp-aminobenzoic acid; PAS, p-aminosalicylic acid; EPI, exocrine pancreatic insufficiency; and AUC, area under the curve. Presented in part at the annual meeting of the American Gastroenterological Society, New Orleans, LA, 1994. Received October 28, 1994; accepted December 27, 1994. of Gastroenterology,
Claudio D. Schteingart,4
atic chymotrypsin
(2, 3). The esterolytic activity of is the rate-limiting step for the absorption of the PABA. PABA, often referred to as “free PABA,” is rapidly absorbed by the small intestinal mucosa, metabolized in the liver, and excreted in urine. Assessments of plasma PABA concentration or urinary PABA recovery after ingestion of NBT-PABA provide an indirect assessment of chymotrypsin activity. Nonetheless, a false-positive test result may occur despite normal hydrolysis of the peptide by chymotrypsin. For example, PABA may be poorly absorbed because of intrinsic intestinal disease, its hepatic metabolism may be impaired because of parenchymal cell disease, or its urinary excretion may be deficient because of renal chymotrypsin
impairment
or incomplete collection of urine. Diabetic
gastroparesis with delayed gastric emptying may also give false-positive test results. To correct for these defects in the absorption of PABA or for its altered postabsorptive metabolism, free PABA can be readministered separately, so that the ratio of PABA recovery after NBT-PABA administration to that after free PABA administration, referred to as the PABA excretion index, can be determined (4). Alternatively, [‘4CIPABA, as a marker for uptake of free PABA, can be administered simultaneously with NBTPABA (5, 6), which provides values for the PABA excretion index after only a single test procedure. In another
modification
of the procedure,
p-aminosalicylic
acid (PAS),
an analog of free PABA with a similar chemical structure and identical pharmacokinetic properties, is administered with the bentiromide (7). The combined administration of PAS with NBT-PABA allows calculation of the PABA/PAS index, an index similar to the PABA excretion index, after a single test procedure and no use of radioactive label. In yet another modification, PABA recovery is measured in serum or plasma (8-15). Plasma or serum measurements may offer a shorter test procedure than urinary measurements by obviating the need for an accurate collection of urine. A relatively new test for noninvasive assessment of exocrine pancreatic function is the cholesteryl[‘4Cloctanoate breath test (16-18), which is based on the intraluminal hydrolysis of cholesteryl-[14C]octanoate by cholesterol esterase (EC 3.1.1.13). [14C]Octanoic acid is rapidly (and passively) absorbed by the intestinal mucosa, transported to the liver via the portal vein, and rapidly oxidized to 14C02. Because the lipolytic activity of cholesterol esterase is the rate-limiting step for the absorption of [14C]octanoate, the rate at which appears in breath is proportional to the rate of CLINICAL CHEMISTRY. Vol. 41, No. 4,
1995
599
Hence, measurement of 14C02 output in breath after ingestion of cholesteryl-[’4C]octanoate provides a noninvasive way to assess the rate of hydrolysis of the ester substrate and thereby assess exocrine hydrolysis.
pancreatic
function.
To date, a direct comparison between the cholesterylbreath test and the NBT-PABA test has not been performed in humans. We report here the results of a study comparing the efficacy of these two tests in patients with exocrine pancreatic insufficiency (EPI) caused by chronic pancreatitis. We performed the same tests in healthy volunteers to define the rate of hydrolysis of the two substrates when pancreatic function is unimpaired. [14C]octanoate
Materials and Methods Subjects The group of patients (one woman and eight men; ages 37 to 76 years) had well-documented EPI caused by chronic pancreatitis and attended our outpatient clinic regularly. The diagnosis of chronic pancreatitis was based on clinical presentation and characteristic abnormalities detected during endoscopic retrograde cholangiopancreatography and abdominal ultrasound. The etiologies of chronic pancreatitis were alcohol abuse (5), trauma (1), and idiopathic (3). All patients had a fecal fat excretion (without enzyme supplementation) of at least 15 g/day and abnormal recovery of PABA in urine (
