Traceable measurement in clinical laboratories

National Institute of Metrology, Sos. Vitan-Barzesti No. 11, 75669 Bucharest, Romania 

Biological and Medical Research Department, MBC-03, P.O. Box 3354, Riyadh 11211, Saudi Arabia Fax +966-1-4427858, e-mail enein kfshrc.edu.sa 

Abstract Reliable, traceable and comparable measurements provide the rational basis for evaluation of the quality of a result and the starting point for recognized laboratory accreditation in any national area. Modern medical diagnostics and treatment involve rapidly rising numbers and types of clinical laboratory measurements, that are reliable. Therefore, the basic principles to be followed to assure the traceability of clinical measurements as required by the Romanian Laws of Metrology are reviewed. Main sources affecting the 

Clinical reference materials Clinical photometric system 

The increased attention paid to the concept of traceability and its implementation in the world of chemical measurements has been one of the major goals of metrological activity in recent years. In Romania traceability of chemical measurement has been closely connected with the accreditation of analytical chemistry laboratories. However, this concept has only recently been adopted in this country for clinical measurements. Within this framework an attempt is made to review what traceability means in terms of clinical measurements and what is now being done by the Romanian National Institute of Metrology (INM) to develop the principles of traceability in spectrophotometrical measurements performed in clinical chemistry laboratories. 

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Assessment of the present situation regarding quality of spectrophotometric measurements [5] suggested the need for basic principles to be followed to assure the required traceability. Lately, some efforts have been focused on elaborating traceability schemes for clinical chemistry measurements. The purpose of any traceability scheme is to provide comparability, compatibility and consistency between the huge numbers of chemical measurements needed everyday, universally in clinical laboratories. On the other hand, the quality accredita-... 

In cases where standardized methods are used in clinical laboratories to indicate bias, repeatability, or sensitivity, (for instance in enzymatic determinations) the measurement result is traceable to a reference method only if all the instruments involved in the method are appropriately calibrated against the proper physical standard. 

This paper has examined the role of calibration and evaluation of measurement uncertainty in clinical laboratories arising from the request for traceability assurance. To produce results which are accurate and reliable within the stated uncertainty, all uncertainties of the quality measurement process and the traceability chain should be demonstrated. Also, the quality of a spectrophotometric result depends critically on RMs and photometric systems whose traceability have been properly demonstrated. 

Though various methods for the determination of enzyme activity are used routinely in clinical laboratories, many of them are covered by patents, and the components of their test kits are not commercially available. In clinical chemistry, reference methods and certified enzyme reference materials (enzyme calibrators) for determination of activities of enzymes in human blood serum are recommended by international commissions (ISOTC/212, International Federation of Clinical Chemistry IFCC, Institute for Reference Materials and Measurements EU, etc.) at present for improved accuracy and establishment of traceability chains of the enzyme measurement system. By using these reference materials, the inaccuracy (imprecision between laboratories) has been minimized to within several per cent in several enzymes, such as lactate dehydrogenase (LDH) (EC 1.1.1.27), y-glutamyltrans-peptidase (y-GT) (EC 2.3.2.2), alanine aminotransferase (ALT) (EC 2.6.1.2), creatine kinase (CK) (EC 2.7.3.2), alkaline phosphatase (ALP) (EC 3.1.3.1), acid phosphatase (EC 3.1.3.2), and a-amylase (AMY)... 

Fig. 3 Traceability system for pH measurements in Germany. This traceability chain is basically similar to those for clinical chemistry and gas analysis, as DKD-accredited calibration laboratories act as multipliers in dissemination. It is a special feature of this structure that traceability of pH measurements does not extend to the SI but to a conventional reference frame recognized worldwide. Traceability to the SI is possible but would imply a considerable increase in uncertainty...

Traceability is not really a new fundamental concept in the field of laboratory medicine. Many years before the concept traceability had been mentioned in general chemical metrology, reference measurement procedures and reference materials had been established in clinical chemistry. Some basic experimental work for the development of reference measurement procedures and reference materials had already been undertaken in expert laboratories. 

To improve measurement capability of field laboratories, CENAM has also been offering a PT scheme, not only because there are few PT providers in Mexico, but also due to the need to promote traceable measurement by the use of reference value provided by CENAM. Following the successful implementation of a PT program for environmental measurement laboratory assessment made by authorities of three local governments [8], similar efforts have been made to promote among laboratories who could be considered in the future as reference laboratories in food, petrochemical, clinical [9] and industrial sectors. 

First, in most of the large-scale clinical and epidemiological studies that established the (1) relationships between lipids, lipoproteins, (2) risk for CHD, and (3) efficacy of cholesterol lowermg, the measurements were made in standardized laboratories in which the accuracy of the measurements was traceable to CDC reference methods. This included studies such as the National Diet Heart Study in the 1960s, the various LRC program studies (early 1970s to 1990), Specialized Centers of Research in Atherosclerosis (early 1970s to present), and several NHANES studies conducted between 1960 and 1994, ... 

The Modification of Diet in Renal Disease (MDRD) equation was developed from an extensive sample of patients with known CKD, all of whom had a measured GFR <90 ml/min/1.73 m [4]. This equation is now widely reported by clinical laboratories around the world whenever Scr is measured. Since the MDRD equation overestimates measured GFR in subjects with values >60 ml/min/1.73 m, values are only reported for GFR <60 ml/min/1.73 m [3]. Use of IDMS-traceable creatinine values in the IDMS-MDRD Study equation results in a more accurate eGFR. 

It is relevant to ask how often the routine measurement procedures currently used in laboratory medicine provide results that are traceable to high-level calibrators and reference measurement procedures (Lequin personal communication). It turns out that primary reference measurement procedures and primary calibrators are only available for about 30 types of quantity such as blood plasma concentration of bilirubins, cholesterols and sodium ion. International reference measurement procedures from the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) and corresponding certified reference material from BCR are available for the catalytic activity concentration of a few enzymes such as alkaline phosphatase and creatine kinase in plasma. For another 25 types of quantity, such... 

Fig. 1 Traceability system for the determination of the most important diagnostic markers in human body fluids in Germany. The clinical reference laboratories at the intermediate level providing calibration means to the routine medical laboratories are accredited as calibration laboratories in the framework of the German Calibration Service (DKD) and are firmly linked to the national metrology institute, PTB, by comparison measurements carried out on actual laboratory samples. Accreditation is in part required by the Federal Physicians Council (BAK) or is voluntary. The traceability system is still under development...

Method-dependent measurements can be grouped by sector. For example, in the clinical fields there are cases where some higher order reference materials are required for IVD methods, such as for determination of glucose in human serum. It is also required of reference laboratories in specific measurement methods. These issues are now under the responsibility of JCTLM (Joint Committee on the Traceability of Laboratory Medicine of CCQM). CENAM has developed a reference material for glucose and cholesterol determination in human serum, and certified by IDMS, which is under review by JCTLM for the use by reference laboratories in any country applying a reference method. 

This paper discusses a number of practical problems arising from the request for and use of clinical reference materials for the validation of the performance of photometric systems used in national clinical chemistry laboratories. It shows that uncertainties in the measurement step of photometric analysis have largely been ignored. Uncertainties associated with this step can and do contribute significantly to overall analytical uncertainty. Thus, for a knowledge of trueness and measurement uncertainty, an adequate certified reference materials system and an attempt at a traceability chain are of the utmost importance, since the quality of clinical chemistry results depends critically on the use of reliable reference materials and properly validated instruments. 

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