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  • Author: Anders Kallner x
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Abstract

The performance of all measurement procedures used in routine clinical laboratories shall be verified; a minimum is to verify the precision and trueness of the results. This is well established and adequate recommendations and procedures are available. Conveying this information in a form that is adequate and understandable for the practical end-user in the health care sector is still a much debated issue. By tradition, since several decades, the “total error” (TE) is presented, a quantity that is the linear sum of an imprecision and bias. Since any combination of the two can yield the same TE it may not be very helpful in finding and correcting a root-cause for an unacceptable value. Also, an acceptable TE may hide an unacceptable level of its constituents. An alternative is the measurement uncertainty (MU), which is recommended by accreditation and standardizing bodies The MU separates the imprecision and bias and expresses an interval around a best estimate within which the true value is expected with a certain probability. We describe the reporting the best estimate of a measurement result and describe how the uncertainty of the result, can be calculated, using simple custom-made software.

Abstract

Revised recommendations for diagnosis of diabetes introduce the intermediary risk group of impaired fasting glucose (IFG), defined as individuals with a fasting blood-glucose concentration between 5.6 and 6.0 mmol/l. We apply the concept of uncertainty to identifiable steps of sampling and measuring blood-glucose. Since many instruments in primary health care measure plasma-glucose and report results as blood-glucose and vice versa, factors affecting the transformation are also considered. The study identifies the measurement procedure as the major source of uncertainty, closely followed by preanalytical sources. The estimated uncertainties indicate that the presently available procedures do not allow identification of IFG by a single investigation. The approach to establish an uncertainty budget can be used to evaluate the clinical usefulness of measurements.

Abstract

Reference intervals in clinical chemistry are commonly based on results of measurements in reference populations or are taken from the literature. A reference population should represent a defined group of individuals and be as similar as possible to the patients under investigation. Frequently, reference populations have been recruited from institutionalised healthy young people who do not necessarily fulfill these criteria. In the present study we describe the temporal changes in 37 commonly measured quantities in men and women from childhood to late in life. The samples were collected in the primary health care and sorted according to an assumed decision by the physician. The emerging group of individuals forms a reference population that was regarded as “non-diseased” and the results of measurements in this population are reference values. A remaining group of “non-healthy” were likewise identified for comparison. The central 95 percentile was wider than those usually assigned to the quantities whereas the medians almost coincided. In the “non-healthy” group the medians were shifted in a direction that would be expected from pathophysiology aspects.

Abstract

Background: Fibrinogen and C-reactive protein (CRP) concentrations are predictors of outcome in the atherosclerotic patient. It is important in risk stratification that these quantities are measured reproducibly in routine and research.

Method: In the present study, we compare measurements of fibrinogen and high-sensitivity CRP in EDTA and citrate plasma samples (n=150) using nephelometric immunoassays. Fibrinogen was also measured in citrate plasma using a clotting method.

Results: In approximately one-third of the samples, the fibrinogen concentration measured by immunoassay was higher in citrate plasma than in EDTA plasma, in spite of the dilution by citrate. The immunoassay results of fibrinogen concentration measurements in EDTA and citrate plasma differed significantly and also differed from those of functionally measured fibrinogen concentrations. A difference was found between the concentration of CRP in EDTA plasma and citrated plasma which also did not correspond to the dilution.

Conclusions: Reproducibility of results is essential in risk stratification by fibrinogen or high-sensitivity CRP concentrations and small differences close to the decision limits may have a decisive impact. Immunological measurements are liable to confounding effects that may be difficult to foresee, qualitatively and quantitatively. Great care should be observed when measuring the concentration of calcium containing analytes in anticoagulated samples. Fibrinogen concentrations should preferably be measured functionally in citrate plasma.

Clin Chem Lab Med 2008;46:1175–9.

Zusammenfassung

Einleitung: Die Leistungsfähigkeit von Messverfahren in medizinischen Laboratorien wird im Allgemeinen überwacht durch das regelmäßige Messen von einem oder mehreren Qualitätskontrollmaterialien, sogenannten internen Qualitätskontrollen. Die Bewertung des Messergebnisses der Qualitätskontrolle führt zur Annahme oder zur Sperrung von Patientenmessergebnissen und erfolgt gemäß anerkannten Regeln und gesetzlicher Vorgaben.

Methoden: Varianzen von Qualitätskontrollmessungen sowie die Zielwerte für die Westgardauswertung wurden während einer ersten Phase (Vorperiode) ermittelt. In der darauf folgenden zweiten Phase (Messphase) wurden die Qualitätskontrollergebnisse gemäß der deutschen Rili-BÄK und gemäß den Westgard 1:2s und 1:3s Regeln für vier beispielhaft ausgewählte Messverfahren verglichen, die folgende Charakteristika aufwiesen: Geringe Unpräzision und geringe Unrichtigkeit (Glukose im Plasma), große Unpräzision und geringe Unrichtigkeit (Phenobarbital im Serum), geringe Unpräzision und große Unrichtigkeit (Natrium im Urin) sowie große Unpräzision und große Unrichtigkeit (Magnesium im Plasma).

Ergebnisse: Das Ergebnis der Bewertung gemäß der Westgard-Regeln (1:2s und 1:3s) ist stark abhängig von der Vorperiode, wohingegen die Bewertung durch die Rili-BAEÄK auf den einheitlich vorgegebenen Grenzen und den bindenden Zielwerten beruht. Die Anzahl der Grenzverletzungen, die von der Rili-BÄK bzw. den Westgard 1:2s und 1:3s Regeln erkannt wurden, unterscheiden sich zum Teil deutlich. Die Rili-BÄK Grenzen für die Plasma-Glukose erscheinen großzügig vor dem Hintergrund der Leistungsfähigkeit des verwendeten Messverfahrens.

Schlussfolgerung: Durch die definierten Zielwerte und Bewertungsgrenzen hat die Rili-BÄK das Potential, die Harmonisierung von Patientenergebnissen klinisch-chemischer Untersuchungen, die in unterschiedlichen Laboratorien erstellt werden, zu verbessern. Laboratorien, die gemäß der Westgard-Regeln eine Bewertung der internen Qualitätskontrolle durchführen und Zielwerte und Bewertungsgrenzen demzufolge intern und unabhängig von anderen Laboratorien ermitteln, müssen sich für die Harmonisierung von Patientenergebnissen auf die externe Qualitätskontrolle verlassen.

Abstract

Fibrinogen concentration is routinely measured by several methods and the results may influence diagnostic and treatment strategies. It is therefore necessary that results are compatible and transferable between laboratories. In the present study, it is shown that commonly used immuno-nephelometric methods, a commercial variant of the Clauss clotting rate method, and the classical syneresis method, do not differ significantly using patient material, in the interval 2–12 g/l. A research ELISA method that measures intact fibrinogen is not linearly correlated to the syneresis method. The commutability of available calibrators and reference materials (including the WHO 2nd IS) was only 50–80% except for one of the calibrators for which the virtual concentration coincided with that obtained by the