SCE Nordic alpha-amylase method selection and calibration study. A report by the Committee of Enzymes of the Scandinavian Society for Clinical Chemistry (SCE)

Evaluation of a direct alpha-amylase assay using 2-chloro-4-nitrophenyl-alpha-D-maltotrioside

We present the adaptation of an IFCC method for alpha-amylase using 2-chloro-4-nitro-phenyl-alpha-D-maltotrio-side as substrate (1) suited for routine work at 37 degrees C. In the assay, a constant proportion of substrate, i. e. 92%, is directly converted to 2-chloro-4-nitrophenol and maltotriose. The method is based on multi- and univariate optimization leading to following measurement conditions: substrate, 2.25 mmol/l; chloride, 310 mmol/l; calcium 5.0 mmol/l; 4-morpholinoethanesulphonic acid, 50 mmol/l; pH 6.28. The assay may be carried out manually or by mechanized procedures, with substrate or sample start, and it shows these analytical properties in measuring amylase activity of sera: no lag phase, detection limit 2.9 U/l, linear range < or = 820 U/l (for 300 s) or < or = 1450 U/l (for 120 s of measurement), and total manual imprecision 3.2% (CV) at 46 U/l. Bilirubin < or = 630 micromol/l, haemoglobin < or =6 g/l, triacylglycerols < or =30 mmol/l, heparin < or =100 kU/l, and glucose < or =120 mmol/l do not interfere. For adults, we established a preliminary 0.95-reference interval of 30-90 U/l not dependent on sex or age. A close association with the IFCC method demonstrates the reliable transfer of its measurement conditions to a robust routine method with minimal changes.

Interassay calibration as a major contribution to the comparability of results in clinical enzymology

OBJECTIVE

Factors contributing to the applicability of interassay calibration of methods measuring enzyme catalytic activities are described. Also discussed are the properties essential for such a material. Similarity of specificity for the methods to be calibrated as well as commutability between the material(s) intended to be used as calibrator are the main criteria to be satisfied.

RESULT

Several examples demonstrated that interassay calibration is feasible but a multi-enzyme calibrator with a wide commutability for the most popular methods remains to be developed. This is the project of the IFCC Working Group on Calibrators in Clinical Enzymology (WG-CCE). Several experimental data are also presented that indicate that the temperature at which the reaction is carried out is not a limiting factor in the implementation of interassay calibration in clinical enzymology.

Support of the diagnosis of pancreatitis by enzyme tests--old problems, new techniques

Computerized tomography and ultrasound are helpful in the diagnosis of acute pancreatitis. The procedures, however, are not always available and so laboratory tests continue to play an important role. Serum amylase measurement is the most widely used test, despite its lack of sensitivity (75-92%) and specificity (20-60%). A cut-off point of 3-6 times the upper reference limit increases the specificity greatly, but at the expense of sensitivity. A method using chloronitrophenyl-maltotrioside as substrate has recently been rejected by the IFCC. A method using ethylidene-protected 4-nitrophenyl-maltoheptaoside and a new alpha-glucosidase has emerged as the method of choice. Amylase isoenzyme determinations have higher specificity than total amylase measurements. Serum lipase methods are more sensitive and specific, but methodological problems persist. Cationic trypsin-1 measurements yield high sensitivity but low specificity. Elastase-1 is claimed to correlate best with the clinical symptoms. Reasons for the widely differing reports on sensitivity and specificity of tests for pancreatitis are discussed.

Calibration of the Ektachem Amylase method using human reference materials and the Phadebas Blue Starch method as an interim reference method

In a multilaboratory study a consensus was established to calibrate the Ektachem Amylase method to reproduce the results of the Phadebas Blue Starch method. The calibration graph has a slope = 3.39 and intercept = 25. For the Ektachem Amylase method the reaction-rate ratio between salivary and pancreatic amylase was calculated to be 0.89, relative to that of the Phadebas Blue Starch method. A calibration value for the Nordic Amylase Calibrator to be used on Ektachem analysers was determined to be close to 469 U l-1 for the current batch. However, since the difference from the stated value of 460 U l-1 is negligible, the authors recommend the use of the stated value for this and future batches of the Nordic Amylase Calibrator. An error of around 10% introduced by the presence of salivary amylase is comparable to the methods accepted by the Scandinavian Committee on Enzymes. Introduction of a consensus calibration reduced the interlaboratory variation by up to 40% at all levels.

Usefulness of reference materials in calibration of enzyme activities

alpha-Amylase, alkaline phosphatase and gamma-glutamyltransferase were studied in a multicentre evaluation. Analyses were performed on different patient samples. Each enzyme was assayed in two different laboratories at both 30 and 37 degrees C, with widely used reagent kits and with the IFCC reference method (if in existence). Results differed considerably according to the measurement procedure. Data also showed that it was not possible to employ a constant conversion factor for one enzyme and different techniques between 30 and 37 degrees C. Calibration with three reference materials extensively improved the intermethod consistency for most of the tested measurement procedures. It was possible to transfer accuracy from the method used for the certification of the reference material to routine procedures, by using the reference material as calibrator. Temperature did not seem to be a crucial variable for the implement of the enzyme calibrator approach.

Effects of oleic acid and endogenous bile on duodenal secretion of somatostatin in man

We studied the effects of intraduodenal oleic acid on the release of somatostatin to plasma and the correlation between endogenous bile output and plasma somatostatin. In five normal persons infusion of 0, 5, 10, 20, and 40 mM oleic acid dose-dependently increased the levels of somatostatin during as well as after gallbladder emptying. The difference between somatostatin concentration during and after gallbladder emptying was not significant. The amylase secretion also was significantly correlated to the dose of fat, whereas the output of bile salts was the same for all fat doses used. Our observations indicate that intraduodenal oleic acid--and not bile salts--releases somatostatin from the gut.

Advances in the enzyme diagnosis of pancreatic diseases

This paper reviews recent developments of analytical methods for the determination of alpha-amylase, of its isoenzymes, and of lipase. The evaluation of severity and etiology of acute pancreatitis by enzyme assays, e.g., pancreatic elastase 1, phospholipase A2, and routine enzymes are discussed. The limited significance of enzyme determinations as compared to imaging and endoscopic procedures for the diagnosis of chronic pancreatitis is demonstrated. Indirect "tubeless" tests for the evaluation of pancreatic exocrine insufficiency with respect to the secretion of chymotrypsin (chymotrypsin in stool and NBT-PABA test) and cholesterol esterase (pancreolauryl test) are reviewed. Finally, the superiority of morphologic investigations over biochemical tests for the timely detection of pancreatic carcinoma is shown.

Comparison of two methods for the determination of the isoenzymes of alpha-amylase in serum and urine

In this paper we describe our results of the determination of the isoenzymes of alpha-amylase in serum and urine with two monoclonal antibodies, using 4,6-ethylidene-protected 4-nitrophenylmaltoheptaoside as substrate. For comparison we have used the wheat-germ inhibitor method with blue starch as substrate. The differences observed between each method were small. The technique using monoclonal antibodies is easy and reliable and can, therefore, replace the wheat-germ inhibitor method.

Effects of oleic acid and oleyl alcohol on cholecystokinin and secretin in plasma and pancreatobiliary secretion

To evaluate the importance of the terminal carboxy group of the oleic acid molecule in the release of secretin and cholecystokinin (CCK) to plasma, six normal persons were stimulated twice with duodenal perfusates containing either 20 mM oleic acid or 20 mM oleyl alcohol. Oleic acid increased the pancreatic secretion of bicarbonate and amylase and the concentrations of secretin and CCK in plasma and provoked gallbladder emptying. Oleyl alcohol only increased the amylase output slightly, but significantly. It is concluded that the carboxy group of the fat molecule has an important role in triggering the release of secretin and CCK to plasma.

Fat and pancreatic secretion

In 10 healthy volunteers we investigated the effects of intraduodenal oleic acid at various concentrations (0-40 mM) and at various degrees of emulsification on pancreaticobiliary secretion and the release of secretin and cholecystokinin (CCK) into plasma. We found that the release of the two hormones was directly related to the dose of fat and to the degree of emulsification. The threshold of CCK release (and amylase output) was low in comparison with the threshold for secretin release (and bicarbonate or volume output). When the degree of emulsification of the fat was increased, no simple relation was observed between hormone levels and pancreatic exocrine secretion. The output of bile salts was identical at various fat concentrations. We conclude that both secretin and CCK are dose-dependently released by emulsions of oleic acid in normal man and that the thresholds for release are probably different.

Effect of trypsin on the hormonal regulation of the fat-stimulated human exocrine pancreas

To study the effects of trypsin on the pancreaticobiliary secretion and the release of secretin and cholecystokinin (CCK) to plasma, seven normal subjects were stimulated twice with duodenal perfusates containing 20 mM oleic acid (pH 6.0) with and without 1 g of bovine trypsin added per liter. In addition, six patients with advanced pancreatic insufficiency who received only the oleic acid were compared with eight normal subjects. The concentrations of secretin and CCK in plasma and the pancreatic enzyme and volume secretions were unaffected by the addition of trypsin, but the initial bile acid output and the bicarbonate secretion in the period after gallbladder emptying were reduced during perfusion with trypsin. The severely reduced enzyme secretion in chronic pancreatitis did not influence the basal or oleic acid-stimulated concentrations of the hormones in plasma. The study does not support the hypothesis of a trypsin-mediated negative feedback control of human pancreatic enzyme secretion. Furthermore, the reduced duodenal output of bicarbonate found in response to trypsin is not explained by changes in the release of secretin or CCK.

Plasma secretin, plasma cholecystokinin, pancreaticobiliary secretion, and fat absorption: effect of duodenal osmolality and polysorbate 80

In 20 normal persons we investigated the effects of duodenal osmolality on the release of secretin and cholecystokinin (CCK), pancreaticobiliary secretion, and fat absorption after intestinal infusion of emulsified oleic acid (pH 6.0). The release of CCK was found to be unaffected by the changes in osmolality, whereas the plasma levels of secretin were affected in parallel with volume and bicarbonate secretion. An inverse relation was found between fatty acid absorption and release of secretin and bicarbonate secretion but not between fatty acid absorption and release of CCK. It is suggested that the secretin and CCK cells respond differently to emulsified oleic acid.

The significance of plasma CCK and secretin in the oleate-stimulated pancreatico-biliary secretion in man

In eight normal persons the plasma levels of secretin and cholecystokinin (CCK) measured after intraduodenal oleate were reproduced by intravenous (i.v.) infusion of synthetic secretin (3.4 pmol.kg-1.h-1) and CCK-8 (17.5 pmol.kg-1.h-1), either alone or in combination. Using an indicator dilution technique, the combined infusions of the two hormones were found to account for all the pancreatico-biliary secretion of amylase, bicarbonate, bile salts and volume elicited in response to intraduodenal oleate, pH 6. Furthermore, the bicarbonate secretion and the flow rate elicited by secretin were augmented by CCK, whereas no augmentation was found with regard to the CCK-stimulated enzyme and bile salts output.

SCE Nordic alpha-amylase study. II: Assessment of proposed calibration procedure. A report by the Scandinavian Committee on Enzymes (SCE)

Eighty-seven Nordic Hospital laboratories participated in a joint SCE-NORDKEM follow-up study of the long-term stability of the previously established calibration factors for a number of alpha-amylase routine methods based on six different substrates. Human control materials with 90% pancreatic, 90% salivary, and pure pancreatic alpha-amylases were measured by the participants. The data were plotted before and after calibration of each method using a human pancreatic calibrator with an assigned catalytic concentration of 390 U/l (Phadebas blue starch method, 37 degrees C). As in the previous study, carried out 9 months earlier, the pre-calibration values varied over a six-fold range. The post-calibration values of all methods except those based on a tetraose substrate showed an acceptable inter-laboratory comparability. As a temporary measure, SCE recommends that the Nordic laboratories calibrate the accepted routine methods by their individual calibration factor. Detailed suggestions for calibration procedures and a discussion of the principles of transferability will shortly be published by the SCE in this journal.

External quality assessment of serum enzyme activity assays and the effect of calibration on interlaboratory concordance

The design and results of a UK external quality assessment scheme for six enzymes are described, from 21 surveys over a period of 7 years. Improvements in interlaboratory agreement and the adoption of reliable methods are documented. The potential of enzyme calibration materials in further improving numerical concordance between laboratories using different assay conditions, including temperature, is demonstrated and discussed.