HBD has been used as a substitute for LDH-1 (heart) isoenzyme measurement. Actually, HBD is total LDH that is forced to act on a a-ketobutyric acid substrate instead of pyruvic or lactic acid. Under these conditions, LDH-1 and LDH-2 show relatively greater activity than LDH-5, so that HBD therefore indirectly measures LDH-1 (heart) activity. However, if the LDH-5 (liver) value is elevated sufficiently, it will also produce measurable HBD effect. Therefore, HBD is not as specific as electrophoresis or heat fractionation in separating heart from liver isoenzymes. Nevertheless, since HBD assay is easier to perform (and therefore cheaper) than LDH isoenzyme assay, some follow the practice of using a more specific isoenzyme method if in doubt about LDH heart versus liver contribution. Once there is proof that the heart fraction is elevated, they follow subsequent activity levels with HBD.
Causes of lactic dehydrogenase fraction 1 elevation
Acute MI
Cardiac muscle hypoxia without definite acute MI
Blood specimen hemolysis
Hemolytic anemia
Megaloblastic anemia
Renal cortex infarct
Germ cell tumors (some patients)
LDH isomorphic isoenzyme pattern
Multiorgan hypoxia
Neoplasia
Other conditions (less common; see Fig. 21-1, F)