IFCC Method for Lactate Dehydrogenase Measurement in Heparin Plasma Is Unreliable

Various societies for clinical chemistry have proposed recommendations for the measurement of the activity of lactate dehydrogenase (LDH; l-lactate:NAD+ oxidoreductase; EC 1.1.1.27) (1)(2)(3)(4)(5). Depending on the pH of the buffer, the activity of LDH can be measured by the increase as well as the decrease in NADH. The optimum pH for the pyruvate-to-lactate conversion is 7.4–7.8. The German (2) and French (5) Societies for Clinical Chemistry recommend this reaction. The IFCC (4) recommends the lactate-to-pyruvate conversion, for which the optimum pH is 8.8–9.8. In our laboratory, we have measured LDH according to the recommendations of the French society (SFBC) for years. This measurement was routinely performed with a Modular analyzer (Roche GmbH). When this SFBC method for LDH measurement needed to be replaced because our supplier stopped producing the necessary reagents, we decided to introduce the method based on the recommendations of the IFCC. Because we did not want to change our reference values (<450 U/L), we introduced a conversion factor to make the results obtained with the IFCC method directly comparable to the results obtained with the SFBC method. When we compared both methods using patient samples for which LDH was requested, we found numerous outliers, which forced us to do some additional investigations. To determine which method was responsible for the outliers, we began measuring duplicates for LDH with both methods. For these measurements we used lithium-heparin-plasma samples collected in plastic tubes with plasma separator (product no. 367994; Becton Dickinson). All measurements took place in the primary tubes, i.e., LDH measurements were performed directly in the lithium-heparin plasma after …