Lactate dehydrogenase (LDH or LD) is a key enzyme in most cells, catalysing the reversible conversion of pyruvate to L-lactate. Its contemporaneous main clinical uses are limited primarily to the investigation of haemolysis, serous collections and as a tumour marker.
Physiology
L-lactate dehydrogenase is an enzyme of the oxidoreductase class, found in the cytosol of almost every single cell in the human body. The enzyme is a tetramer of four subunits, with two main subunits H and M, therefore forming five possible isozymes, LD1 to LD5:
- LD1 (H4): cardiac, renal, red blood cells
- LD2 (H3M1): white cells, lymph nodes, spleen, pulmonary
- LD3 (H2M2): white cells, lymph nodes, spleen, pulmonary
- LD4 (H1M3): white cells, lymph nodes, spleen, pulmonary
- LD5 (M4): hepatic, skeletal muscle cells
LDH has an important role in glycolysis, mediating the metabolism of pyruvate to L-lactate. This reaction is reversible.
Clinical use
Historically lactate dehydrogenase was an important blood test for the delayed diagnosis of myocardial infarction, however troponin has rendered this usage obsolete. It has also been shown that its specificity for cardiac ischaemia was poor 1. It was also previously advocated as a marker for liver function, but markers such as the transaminases are superior in this regard. Its use as a marker for inflammation of the muscle is also discontinued in view of the widespread availability of assays for creatine kinase (CK) which has better specificity than LDH.
Its main clinical uses currently are:
- marker for haemolysis
- tumour marker
-
germ cell tumour of the testes
- mainly LD1 isozyme: diagnostic, prognostic and assessing response to therapy
-
metastatic melanoma: prognostic marker
- elevated LDH in metastatic disease upstages the tumour to M1c
-
germ cell tumour of the testes
-
pleural fluid evaluation
- Light criteria in distinguishing between exudative and transudative effusions
- may also be used to evaluate other serous fluid collections e.g. pericardial, ascites
NB: lactate dehydrogenase is elevated in many other pathologies (i.e. it has very poor specificity) hence its lack of clinical utility in contemporary clinical practice