No, Lactate and Lactate Dehydrogenase Are Distinct Molecules
Lactate and lactate dehydrogenase (LDH) are two completely different biochemical entities: lactate is a metabolic end product, while LDH is the enzyme that catalyzes its formation.
Biochemical Distinction
- Lactate is a small organic molecule (the ionized form of lactic acid) that serves as the end product of anaerobic glycolysis when oxygen supply is inadequate 1, 2
- Lactate dehydrogenase (LDH) is an oxidoreductase enzyme composed of protein subunits that catalyzes the reversible conversion between pyruvate and lactate, with concomitant interconversion of NADH and NAD+ 1, 3
- LDH exists as five different isoenzyme forms (composed of M and H subunits) with varying tissue distributions and biochemical properties, including different substrate affinities and electrophoretic mobility 1, 3
Clinical Measurement Differences
- Serum lactate is measured as a metabolite concentration (typically in mmol/L or U/L) and reflects tissue oxygenation status, metabolic stress, or shock states 2, 4
- Serum LDH is measured as enzymatic activity (in IU/L or U/L) and serves as a marker of tissue damage, cell death, or high tumor burden since it leaks from damaged cells into circulation 5, 1, 2
Distinct Clinical Applications
Lactate is used for:
- Assessing tissue perfusion and shock severity 2, 4
- Predicting mortality in sepsis and critical illness 2
- Monitoring treatment response in inflammatory diseases 2
LDH is used for:
- Distinguishing exudative from transudative pleural effusions (Light's criteria require pleural fluid LDH >2/3 upper limit of normal serum LDH or pleural fluid/serum LDH ratio >0.6) 6, 5
- Prognostic stratification in testicular cancer (part of IGCCCG risk classification) 7
- Prognostic assessment in melanoma stage IV disease 7
- Evaluating malignant pleural effusions (LDH >1500 IU/L associated with worse prognosis in LENT score) 7
- Detecting tissue damage across multiple organ systems 1
Common Clinical Pitfall
Do not confuse elevated serum lactate with elevated serum LDH—they indicate different pathophysiological processes and require distinct clinical interpretations. Elevated lactate suggests inadequate tissue oxygenation or metabolic dysfunction, while elevated LDH indicates cellular damage, hemolysis, or high tumor burden 1, 2.