Commutable calibrator with value assigned by the IFCC reference procedure to harmonize serum lactate dehydrogenase activity results measured by 2 different methods

BACKGROUND: The availability of commutable calibrator materials may ease considerably the task of harmonizing assay results and ensuring their traceability to reference procedures. We sought to verify the commutability of potential calibrator materials and evaluate their effectiveness in harmonizing LDH results by 2 measurement methods. METHODS: We measured LDH in 109 serum samples and 31 materials, including frozen serum pools (with either normal or abnormal isoenzyme patterns), commercial stabilized materials, and the ERM-AD453/IFCC reference material. We assayed LDH activity with the IFCC reference procedure and with 2 commercial methods, 1 using the lactate-to-pyruvate (LP) reaction, and the other the pyruvate-to-lactate (PL) reaction. We selected a commutable material, with LDH value assigned by the reference procedure, as a calibrator for recalculating the results for patient sera by both LP and PL, thereby making them traceable to the IFCC reference procedure. RESULTS: Original values for patient sera (n = 109) by the 2 commercial methods showed a mean (SD) PL/LP ratio of 1.97 (0.03); this ratio changed to 1.06 (0.02) after recalculation of results. Linear regression of PL vs LP recalibrated values gave y = 1.108x - 9.7. At the clinically important concentration of 250 U/L (upper reference limit), the systematic difference between methods was 6.8%, which met our proposed quality specifications for inaccuracy and total error. CONCLUSIONS: By properly selecting the calibrator, the results of serum LDH measurement by 2 different methods may be harmonized and made traceable to the selected highest (reference) metrological level.