Reliability of IFCC Method for Lactate Dehydrogenase Measurement in Lithium-Heparin Plasma Samples

In a recent Technical Brief on the IFCC-recommended method for lactate dehydrogenase (LD) measurement, Bakker et al. (1) questioned the reliability of this method for heparin samples. According to their findings, the IFCC method produced an excessive frequency of significantly discordant duplicate measurements (17.8%; 27 of 152 samples) compared with the formerly applied French Society (SFBC)-recommended method (1.4%; 2 of 140 samples). Neither increased centrifugation time and speed or the use of plasma separators to reduce the plasma contamination with blood cells, nor LD measurements without concurrent measurements of other analytes to eliminate possible interferences lowered the reported high rate of duplicate errors. The problem was also not attributable to the clinical chemistry analyzer because measurements on both Hitachi 911 and 717 analyzers (Roche Diagnostics GmbH) showed results similar to those obtained with the previously used Modular analyzer (Roche Diagnostics GmbH). Only when serum samples or secondary tubes for centrifuged plasma samples were used was there a significantly lower frequency of duplicate errors: 2.6% (3 of 114) for serum and 1.1% (1 of 94) for secondary plasma tubes. The authors pointed out that the differences in buffer composition of the IFCC (pH 9.40 ± 0.05) (2) and SFBC (pH 7.4–7.8) methods influenced the integrity of platelets and erythrocytes. In this case, the presence of cells might play a role in the high frequency of duplicate errors. We have been measuring LD activity on three Roche Hitachi 917 analyzers according to the recommendations of the IFCC (2) in our laboratory since February 2003. The within-run imprecisions (CV) for a heparin-plasma sample (n = 20) were 0.8% (197.4 U/L) and 0.7% (321.3 U/L), and the between-run CV were 1.4% for the Roche Precipath U (178.3 U/L; n = 28) and 1.1% for the Roche Precinorm U calibrators (241.0 U/L; n = 28). …