Patterns of lactate dehydrogenase isoenzymes 1 and 2 in serum of patients performing an exercise test

Values for total lactate dehydrogenase (LD, EC 1.1.1.27) activity and LD isoenzymes were determined in serum from 56 patients and 40 healthy subjects before and 24, 48, and 72 h after they performed an exercise test. The mean (for all four times) total LD activity concentration and proportion of LD-2 were within the normal range for all 96 subjects. Mean LD-1 values for serum, although within the normal range in all subjects, were significantly higher in patients with positive exercise test results than in subjects with negative results: 75 (SD 12) U/L in 35 patients with ST depression >2 mm; 63 (SD 14) U/L in 16 patients with ST depression of 1-2 mm; 43 (SD 11) U/L in subjects with negative test results, by 48 h after the test. The LD 1:2 ratio was also markedly higher in the group of patients with positive test exercise results, especially in those with ST depression >2 mm (1.02, SD 0.06), compared with those subjects with negative results (0.60, SD 0.04). A similar trend was also found 24 and 72 h after the exercise test. We conclude that exercise-myocardial ischemia may lead to an increased LD 1:2 ratio in serum, and demonstrate a correlation between the degree of ischemia and the LD 1:2 ratio. Determination of the LD 1:2 ratio, even in the presence of normal total LD activity, may assist in the clinical evaluation of patients performing an exercise test.