Unit Converter
Lactate Dehydrogenase (LDH)
(Widely Distributed Intracellular Enzyme - Marker of Tissue Injury, Cell Turnover & Hemolysis)
Synonyms
- Lactate dehydrogenase
- LDH
- Lactic dehydrogenase
- LD
- Total LDH activity
Units of Measurement
LDH is reported as enzyme activity, not concentration.
- nkat/L
- µkat/L
- nmol/(s·L)
- µmol/(s·L)
- U/L
- IU/L
- µmol/(min·L)
- µmol/(h·L)
- µmol/(h·mL)
(All units express the same enzymatic activity.)
Unit Conversions
Core Relationships
1 U/L=1 IU/L=1 μmol/(min\cdotpL)1\ \text{U/L} = 1\ \text{IU/L} = 1\ \mu\text{mol/(min·L)}1 U/L=1 IU/L=1 μmol/(min\cdotpL) 1 U/L=16.67 nkat/L1\ \text{U/L} = 16.67\ \text{nkat/L}1 U/L=16.67 nkat/L 1 μkat/L=60 U/L1\ \mu\text{kat/L} = 60\ \text{U/L}1 μkat/L=60 U/L 1 nkat/L=0.06 U/L1\ \text{nkat/L} = 0.06\ \text{U/L}1 nkat/L=0.06 U/L 1 μmol/(s\cdotpL)=60 U/L1\ \mu\text{mol/(s·L)} = 60\ \text{U/L}1 μmol/(s\cdotpL)=60 U/L 1 μmol/(h\cdotpL)=0.0167 U/L1\ \mu\text{mol/(h·L)} = 0.0167\ \text{U/L}1 μmol/(h\cdotpL)=0.0167 U/L 1 μmol/(h\cdotpmL)=16.7 U/L1\ \mu\text{mol/(h·mL)} = 16.7\ \text{U/L}1 μmol/(h\cdotpmL)=16.7 U/L
Description
Lactate Dehydrogenase (LDH) is a cytoplasmic oxidoreductase enzyme present in almost all human tissues.
It catalyzes the reversible conversion of lactate to pyruvate, playing a key role in anaerobic glycolysis.
Because LDH is released into the bloodstream whenever cells are damaged or destroyed, it serves as a non-specific but sensitive marker of tissue injury, hemolysis, and high cell turnover.
LDH Isoenzymes
LDH exists as five isoenzymes, formed by H (heart) and M (muscle) subunits:
| Isoenzyme | Predominant Tissue |
| LDH-1 | Heart, RBCs |
| LDH-2 | Reticuloendothelial system |
| LDH-3 | Lung |
| LDH-4 | Kidney, pancreas |
| LDH-5 | Liver, skeletal muscle |
Routine practice now measures total LDH, not isoenzymes.
Physiological Role
- Maintains NAD⁺/NADH balance
- Enables energy production during hypoxia
- Supports anaerobic metabolism
Clinical Significance
INCREASED LDH
Major Causes
1. Hemolysis
- Hemolytic anemia
- Transfusion reactions
- Sample hemolysis (most common false cause)
2. Tissue Injury / Necrosis
- Myocardial injury (historical marker)
- Hepatitis
- Liver congestion
- Muscle trauma
- Burns
3. Malignancy
- Lymphoma
- Leukemia
- Germ cell tumors
- Solid tumors (tumor burden & prognosis)
4. Hypoxia & Ischemia
- Shock
- Severe anemia
- Sepsis
5. Lung Disease
- Pneumonia
- Pulmonary embolism
6. Renal Disease
- Acute kidney injury
- Renal infarction
DECREASED LDH
- Rare
- Seen in congenital LDH deficiency
- Usually clinically insignificant
Reference Intervals
(Method-dependent; IFCC, 37 °C)
Serum LDH
- Adults: 120 – 250 U/L
- Children: Higher than adults
- Newborns: Physiologically elevated
Always interpret using laboratory-specific reference ranges.
All units represent catalytic enzyme activity.
Diagnostic Uses
1. Hemolysis Work-up
High LDH + low haptoglobin + indirect hyperbilirubinemia.
2. Oncology
- Disease severity
- Tumor burden
- Prognostic scoring (e.g., lymphoma)
3. Liver Disease
Hepatocellular injury marker (non-specific).
4. Muscle Injury
Rhabdomyolysis, trauma.
5. Pleural / Ascitic Fluid Analysis
Used in Light’s criteria to differentiate:
- Exudate vs Transudate
6. Critical Illness
Severity marker in sepsis and ARDS.
Analytical Notes
- Hemolysis falsely elevates LDH - most critical pre-analytical issue.
- Serum preferred; plasma acceptable depending on method.
- Prompt processing recommended.
- Standardized IFCC assay performed at 37 °C.
Clinical Pearls
- LDH is highly sensitive but non-specific.
- Isolated LDH elevation → think hemolysis or malignancy.
- In anemia, high LDH strongly suggests hemolysis.
- LDH is essential for Light’s criteria interpretation.
- LDH isoenzymes are largely obsolete in routine practice.
Interesting Fact
Before troponins, an LDH-1 > LDH-2 “flip” was used to diagnose myocardial infarction.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Enzymes
- IFCC Recommendations on Enzyme Measurement
- Mayo Clinic Laboratories - LDH
- ARUP Consult - Enzyme Interpretation
- MedlinePlus - LDH Test