Biological variance of total lactate dehydrogenase and its isoenzymes in human serum

We measured the variance components of total lactate dehydrogenase (LD; EC 1.1.1.27) activity and isoenzymes in sera of 24 healthy laboratory staff, ages 23-50 years, over a 12-month period. We used the first six weeks' results to establish baseline values for each analyte in each individual. These values were "normally" distributed for total LD and isoenzymes 1 through 4, but values for isoenzyme-5 were skewed slightly to the left. The overall means and variances determined for the remaining 10 months were not significantly (p greater than 0.05) different from the corresponding baseline values. Average variances (SD2) for the longer-term (10-month) study were: intra-individual, 732, 1.92, 2.25, 1.09, 1.11, and 2.25, and inter-individual, 1988, 5.03, 1.76, 1.57, 1.21, and 2.01 for total LD (U/L) and LD-1 through LD-5 (% of total), respectively. Average long-term analytical variability was less than 35% of the total variance for the five isoenzymes and 6% for total LD, and was characteristic of the individual. There were no significant differences in mean activities with respect to age or sex. All six analytes exhibited slight seasonal variations.