David Burnett

Sodium Measurements in the Presence of Paraproteins by Four Direct ISE Methods and Flame Photometry Compared

Sodium was determined by flame photometry and by direct potentiometry using four different systems (Ektachem DT60, Corning 614, Orion 1020 and Nova-1) in serum samples from patients with normal proteins and paraproteins. Flame photometric and direct potentiometric differences (ISE-flame), showed the expected relationship with total protein whether or not the sample contained paraprotein, an exception being the Ektachem DT60 on normal protein. With the exception of the Ektachem DT60 with samples with normal protein levels, correlation between different ISE-flame estimates was good. Differences between direct potentiometric measurements are probably more attributable to efforts made by manufacturers to relate them to flame photometric measurements in ‘normal samples’, than to the volume occupied by increased protein content. The apparent lack of influence of protein on ISE-flame for the Ektachem DT60 was shown to relate to the original external reference electrode salt bridge used. A second study showed that changing this fluid to an equi-transferrant high salt content caesium chloride based reference fluid partially unmasked the protein effect.

Collaborative trial of an enzyme-based assay for the determination of paracetamol in plasma

From the Division of Clinical Chemistry, MRC Clinical Research Centre, Harrow, Middx HAl 3UJ t Department ofClinical Biochemistry, Addenbrookes Hospital, Cambridge CB22QR; ••Poisons Unit, New Cross Hospital, London SE14 5ER; ttDepartment ofClinical Biochemistry, Queen Elizabeth II Hospital, Welwyn Garden City AL7 4HQ; •••Department ofClinical Biochemistry, St Albans City Hospital, St Albans, Herts AL3 5PN;ttt Microbial Technology Laboratory, PHLS Centre for Applied Microbiology and Research, Porton Down, Salisbury, Wilts SP40JG

Quality control in the simultaneous assay of anticonvulsants using an automated gas chromatographic system with a nitrogen detector

An automated gas-liquid chromatographic technique for the routine determination of phenobarbitone, primidone and phenytoin using a nitrogen-detector is described. The high selectivity of the detector for nitrogen-containing compounds allows a direct extraction procedure and the use of a phenytoin analogue as an internal standard obviates the need for accurate aliquot measurement after extraction. The anticonvulsants are chromatographed as methyl derivatives produced by on-column methylation with trimethylphenyl ammonium hydroxide following automatic liquid injection. Quality control data from an internal quality control scheme is presented and the performance of the laboratory in an inter-laboratory control scheme is reported and discussed.

ISO standards for pathology – a step too far?

The International Standard, ISO 15189 which specifies requirements for quality and competence in medical laboratories was written by medical laboratory professionals. Since its first publication in 2003, it has raised awareness of the importance of medical laboratories with governments throughout the world and provided a tool with which laboratories may develop their quality management systems and assess their own competence. This paper examines the use of the Standard in the context of self-assessment, accreditation and regulation of medical laboratories.

Interference by Caffeine in Polarization Fluoroimmunoassays for Theophylline

We read with interest the short report by Wicks et al. I on interference by caffeine in polarization fluoroimmunoassays for theophylline in external quality assurance samples. We have also looked at the question of caffeine interference using reagents from three manufacturers: Abbott Laboratories Ltd (Maidenhead, UK), Sigma Diagnostics (Sigma Chemical Co. Ltd, Poole, UK) and International Bioclinical (lBC, obtained from Biostat Ltd, Stockport, UK). Fourteen samples distributed by UKNEQAS for respiratory drugs during 1992 were used in the study. Theophylline was measured on the Abbott TDx analyser using Abbott FPIA reagents, Sigma reagents and IBC reagents. The bias in theophylline measurements (difference between measured theophylline and the spiked theophylline

Method comparisons, influence of the number, distribution and range of samples on performance claims.

Because of the difficulties associated with obtaining patient samples and the labile nature of some analytes, manufacturers will always require the assistance of clinical chemistry laboratories in the establishment of performance claims, but our experience suggests that this work should not be undertaken lightly by laboratories and that manufacturers would be advised to assess the resources of any chosen site carefully before proceeding.

An evaluation of the Kodak glucose/ BUN analyser including experience with proposed testing protocols.

concept in clinical chemistry, namely the use of dry chemistry films to be used initially for colorimetric analysis [2,3]. The glucose and urea methods were evaluated in this study following the guidelines by the National Committee for Clinical Laboratory Standards (NCCLS) [1] which are recommendations for goals to be sought by manufacturers. This evaluation protocol is designed to provide a multipurpose evaluation framework for a wide range of methods and instruments and is in three sections. PSEP-2 (proposed standard for establishing performance claims) describes the four-week baseline period which is used to establish confidence limits for the controls used throughout the study. PSEP-3 details the precision study and PSEP-4 describes the comparison of methods experiment. The Kodak Ektachem GLU/BUN analyser is a microprocessor controlled discrete analyser which operates in single or dual test mode. The instrument used in the present study was an engineering model used in the USA and Europe and was designed to evaluate the concept of quantitative chemical analysis using multilayered reagents. It was used according to the operators manual. The instrument was modified to give urea values, as opposed to blood urea nitrogen values, and was calibrated in mmol/L.

Evaluation of the Kodak Ektachem DT60 analyser for sodium, potassium, glucose and urea.

The Kodak Ektachem DT60/E analyser is a desk-top analyser which uses thin-film ’dry’ chemistry technology to perform colorimetric and potentiometric analyses. The analyser is simple to operate and the slides are claimed to be stablefor up to oneyear if stored at 4 C. Calibration is said to be required only once every three months. Results presented in this paper substantiate these claims and show that the Ektachem DT60/E produces results which are comparable in terms ofbias andprecision with currently used methods for sodium, potassium, urea and glucose. The instrument might have an application within the laboratory for emergency work as well as for out-of-laboratory testing.