External quality assessment in the measurement of haemoglobin by blood gas analysers in Belgium.

OBJECTIVE: The Belgian national External Quality Assessment Scheme (EQAS) for haematology organized a survey to assess the reliability of haemoglobin (Hb) measurements with the blood gas analysers (BGAs) currently available in Belgian hospitals. MATERIAL AND METHODS: All hospital laboratories received two specimens of fresh EDTA anticoagulated whole blood and were asked to determine the Hb concentration using both the conventional haematology analyser (HA) and all BGAs in the hospital. Ninety-seven hospital laboratories participated in the study and a total of 166 results were reported. The BGAs used (grouped according to technology) were Rapidlab 845, 855, 865 (Bayer 1, n = 41), Rapidlab 1245, 1265, Rapidpoint 405 (Bayer 2, n = 19), GEM Premier 3000 (Instrumentation Laboratory, IL, n = 13), ABL 500 and 600 series (Radiometer 1, n = 13), ABL 700 and 800 series (Radiometer 2, n = 35), Omni C, S5 (Roche 1, n = 7), Omni 3, 6, 9, S2, S4, S6 (Roche 2, n = 21). RESULTS: For the BGAs from Bayer, Radiometer and Roche, interlaboratory variation ranged from 0.6 % to 4.1 %, indicating good precision and close agreement between centres. A significant negative bias observed on the GEM Premier 3000 using the EDTA anticoagulated blood samples did not appear to be present in fresh heparinized whole blood samples. There was no significant difference in imprecision and bias between Hb measurements on BGA situated in and outside the central laboratory.

Positive interference of merthiolate in the TDx digoxin assay in control samples used in the Belgian External Quality Assessment (EQA) Scheme.

We investigated the possible origin of the spuriously high results observed with the Abbott TDx Immunoassay in the 1991 Belgian external quality assessment scheme for digoxin. The present work ascribes this discrepancy to a matrix effect induced by the addition of merthiolate as preservative to the control samples. It consequently stresses the importance of avoiding the use of this compound for preparing such samples.

Analytical evaluation of twenty assays for determination of thyrotropin (TSH).

This paper summarises the results of the evaluation of several assays of thyrotropin in serum. The authors assessed 8 radioimmunoassays (IRMA techniques) and 10 enzyme immunoassays, including ELISA, luminescent immunometry and fluoroimmunometry, also covering a broad spectrum of actual assays. An unique protocol based on identical human samples for all assays was used in order to compare precision, linearity, detection limit, matrix effects and correlation. The authors found a wide spread in precision results between the evaluated assays; as distinct from enzyme immunoassays, discrepant analytical results of the radioimmunoassays are mainly due to differences in standardization procedures. All assays were linear in a smaller or larger range. Sensitivity was generally in agreement with the manufacturer's specification. One assay was not acceptable for clinical use because the practical detection limit only reached 1.5 mU/l. Three other assays (from the same origin) demonstrated particular problems with the stability of the chromogenic reagent. The study of matrix effects was performed using different artificial matrices and the results varied from one assay to another; in the test conditions, some assays were not sensible to matrix manipulations, others were extremely influenced by ion strength or protein concentration. The correlation study confirmed the results of the precision study: assays with a good intraassay precision had also the best correlations. In a general appreciation of the evaluated methods the authors notice that most of the evaluated "classical" enzyme immunoassays operate around the sensitivity limits of spectrophotometry, explaining also the inferior results. Some radioimmunoassays and the enhanced luminescent technique gave the best results in this evaluation study.

Simultaneous determination of p-aminobenzoic acid, acetyl-p-aminobenzoic acid and p-aminohippuric acid in serum and urine by capillary gas chromatography with use of a nitrogen-phosphorus detector.

In various studies during recent years, the use of p-aminobenzoic acid has been described in screening tests for exocrine pancreatic function. A synthetic three-unit compound N-benzoyl-L-tyrosyl-p-aminobenzoic acid has been administered orally and hydrolysed in the small intestine in the presence of chymotrypsin to N-benzoyl-L-tyrosine and p-aminobenzoic acid. This study describes a convenient procedure in which, after a selective extraction and derivatization with diazomethane, capillary gas chromatography is used combined with nitrogen-sensitive detection. With the proposed procedure, p-aminobenzoic acid and its major metabolites, acetyl-p-aminobenzoic acid and p-aminohippuric acid, can be monitored in serum and in urine samples.