LDH Test: Lactate Dehydrogenase — Tissue Damage and Disease Activity Marker

LDH Test: Lactate Dehydrogenase — Tissue Damage and Disease Activity Marker — scientific infographic poster

Lactate dehydrogenase (LDH) is an enzyme found in nearly every cell of the human body, where it catalyzes the reversible conversion of pyruvate to lactate — the final step of anaerobic glycolysis. When cells are damaged or die, LDH leaks out of them and into the bloodstream, so a raised serum LDH is a sensitive but non-specific sign of tissue injury: it shows that cells somewhere are breaking down, not which ones or why. The test is used to follow hemolysis, to stage and follow lymphoma, germ-cell tumours and several other cancers, to classify pleural effusions, and as one of many severity markers in critical illness, including COVID-19. Its old role in diagnosing heart attacks has been taken over by cardiac troponin. Which LDH isoenzyme is raised, read together with companion tests, narrows down the source.

Table of Contents

  1. Overview — What LDH Measures
  2. LDH Isoenzymes
  3. Reference Ranges
  4. Causes of Elevated LDH
  5. LDH in Hemolysis and Megaloblastic Anaemia
  6. LDH in Cancer and Lymphoma
  7. LDH in Lung Disease: Pulmonary Embolism, Pleural Fluid and Pneumocystis
  8. LDH in Critical Illness and COVID-19
  9. Key Research Papers
  10. Connections
  11. Featured Videos

Overview — What LDH Measures

LDH (also written LD) is a four-subunit enzyme built from two kinds of subunit — M, encoded by the LDHA gene, and H, encoded by LDHB — which combine in different ratios to form five isoenzymes, LDH-1 through LDH-5. The enzyme interconverts pyruvate and lactate, reducing NAD+ to NADH or the reverse, a step that lets cells keep producing energy when oxygen is short, as in intense exercise or hypoxia.

When cell membranes are disrupted by injury, ischemia, inflammation or cell death, LDH passes into the extracellular fluid and the bloodstream. The total serum LDH test measures the sum of all five isoenzymes. Because LDH is present in red blood cells, white blood cells, heart, liver, kidney, skeletal muscle and lung, many kinds of significant cell injury raise the total. That breadth makes LDH useful for showing that something is wrong; isoenzyme testing or the clinical context is needed to show what. MedlinePlus notes that an LDH test alone cannot show what is damaging tissue or where the damage is, so it is usually done alongside other tests.

The main uses today are following hemolytic anaemia, staging and following lymphoma and germ-cell tumours, classifying pleural effusions as exudates or transudates, and, more loosely, as a severity marker in sepsis, organ failure and COVID-19.

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LDH Isoenzymes (LDH-1 through LDH-5)

The five isoenzymes differ in their mix of H (heart-type) and M (muscle-type) subunits, and their tissue distributions differ, which is what lets an isoenzyme test point toward the source of cell injury. The tissue sources below are those listed by MedlinePlus (U.S. National Library of Medicine):

The LDH-1/LDH-2 “flip” and myocardial infarction (historical): a pattern in which LDH-1 rises above LDH-2, reversing the usual order, was once used as a late laboratory sign of acute myocardial infarction. A 1992 clinical review described a raised CK-MB together with a “flipped” LDH isoenzyme pattern as reliable confirmation of a heart attack. That role has ended. The 2000 joint ESC/ACC redefinition of myocardial infarction built its biochemical criterion on troponin (or CK-MB), with no place for LDH, and the 2018 Fourth Universal Definition of Myocardial Infarction rests on cardiac troponin. Isoenzyme testing is now ordered mainly when the source of a raised total LDH is unclear. The cardiac troponin and CK-MB tests are covered on their own pages in the Lab Tests section.

Isoenzymes in red-cell disorders: red blood cells carry mainly LDH-1 and LDH-2; isoenzyme testing in a sickle cell cohort confirmed that these two predominated. A reversed pattern (LDH-1 above LDH-2) is also described in megaloblastic anaemia, where one study found it helped separate megaloblastic from hemolytic anaemia (see the next sections). By contrast, LDH-5 points toward liver or skeletal muscle.

A diagram of the five LDH isoenzymes as rows of four subunit blocks, heart-type H and muscle-type M: LDH-1 is four H blocks and is found mainly in heart muscle and red blood cells, LDH-2 three H and one M in white blood cells, LDH-3 two and two in the lung, LDH-4 one H and three M in the kidney, placenta and pancreas, and LDH-5 four M blocks in the liver and skeletal muscle. A right-hand column explains the flip, LDH-1 rising above LDH-2: in healthy serum LDH-2 is higher; red-cell breakdown releases mainly LDH-1 and LDH-2; one study used a flipped pattern to help separate megaloblastic from hemolytic anaemia; and the flip as a late sign of heart attack was replaced by cardiac troponin in the 2000 and 2018 definitions of myocardial infarction. FIVE ISOENZYMES, FIVE TISSUE PATTERNS each LDH molecule has four subunits · H = heart type (LDHB gene) · M = muscle type (LDHA gene) H subunit M subunit ISOENZYME SUBUNITS MAIN TISSUE SOURCE LDH-1 heart muscle, red blood cells LDH-2 white blood cells LDH-3 lung LDH-4 kidney, placenta, pancreas LDH-5 liver, skeletal muscle tissue sources as listed by MedlinePlus; the total LDH test adds all five together, so a raised total says cells are breaking down, not which ones THE “FLIP”: LDH-1 ABOVE LDH-2 healthy serum LDH-2 is normally higher than LDH-1 red cells break down they release mainly LDH-1 and LDH-2 megaloblastic anaemia one study used a flipped pattern to help tell it apart from hemolytic anaemia heart attack — a retired use the flip was once a late sign of infarction; the 2000 and 2018 definitions of myocardial infarction rest on cardiac troponin instead subunit blocks show composition only, not amounts; isoenzyme testing is ordered when the source of a raised total is unclear

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Reference Ranges

LDH — Adults, MedlinePlus example range (IU/L)

LOW < 125
NORMAL 125 — 220
HIGH > 220

The MedlinePlus Medical Encyclopedia gives a normal range of 125 to 220 international units per liter (IU/L) as an example. Reference ranges vary between laboratories and assay methods, so a result is read against the range printed on the reporting laboratory's own report.

What MedlinePlus and the studies cited on this page report about results:

Pregnancy: HELLP syndrome (Hemolysis, Elevated Liver enzymes, Low Platelets) is a pregnancy complication in which a raised LDH is one of the laboratory findings.

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Causes of Elevated LDH

Because LDH is present in nearly every tissue, the list of conditions that raise it is long. Interpretation rests on grouping a raised LDH with companion findings that narrow the source.

Hemolytic conditions:

Red blood cells carry mainly LDH-1 and LDH-2, so these isoenzymes predominate when red cells break down.

Megaloblastic anaemia: vitamin B12 or folate deficiency causes ineffective red-cell production in the bone marrow, and the destruction of precursor cells there raises LDH, often strikingly (see the Hemolysis and Megaloblastic Anaemia section).

Cancer and hematologic malignancy:

Myocardial injury (historical role):

Liver disease:

Pulmonary conditions:

Skeletal muscle injury:

Renal conditions:

Critical illness and systemic conditions:

Falsely high LDH from a hemolyzed sample: red cells that break open during blood drawing, transport or processing release their own LDH into the tube, raising the measured value without any disease. In a 2006 laboratory study that lysed blood cells in samples from 12 people on purpose, the false rise in LDH grew roughly in step with the amount of lysate, and clinically meaningful changes in LDH appeared even in samples whose hemolysis was mild or almost invisible to the eye (free hemoglobin below 0.6 g/L). The authors found the response too variable to correct mathematically; for in-vitro hemolysis they suggested measuring free hemoglobin, alerting the clinician and collecting a new sample. A laboratory comment such as “specimen hemolyzed” flags this. MedlinePlus likewise notes that a high LDH can result from red cells breaking open when the sample was collected and tested.

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LDH in Hemolysis and Megaloblastic Anaemia

Detecting and following hemolysis is one of the main uses of LDH. When red blood cells break down in the body, they release LDH, mainly LDH-1 and LDH-2. A raised LDH, a raised indirect (unconjugated) bilirubin and a low haptoglobin together form the classic laboratory picture of hemolysis.

Each part of this picture reflects a different consequence of red-cell breakdown:

The peripheral blood smear adds morphological evidence: schistocytes (fragmented red cells) in microangiopathic hemolytic anaemia (TTP, HUS, HELLP), spherocytes in autoimmune hemolytic anaemia, and sickled cells in sickle cell disease.

Findings by condition:

Megaloblastic anaemia: in vitamin B12 or folate deficiency, DNA synthesis in developing blood cells is impaired. A 2026 review describes how the resulting ineffective erythropoiesis destroys red-cell precursors inside the bone marrow, raising LDH and indirect bilirubin and lowering haptoglobin, so the picture can mimic a hemolytic anaemia, while the reticulocyte count stays inappropriately low for the degree of anaemia. In a 2000 study of 75 patients with macrocytic anaemia classified by bone marrow examination, a total LDH above 3,000 IU/L was diagnostic of megaloblastic anaemia in that series; for totals between 451 and 3,000 IU/L, a reversed isoenzyme pattern (LDH-1 above LDH-2) helped confirm megaloblastic anaemia and separate it from hemolytic anaemia.

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LDH in Cancer and Lymphoma

In oncology LDH is used in two ways: as a factor in established staging and prognostic systems, and as a value followed over the course of treatment.

Aggressive non-Hodgkin lymphoma (International Prognostic Index): the 1993 International Prognostic Index was built from 2,031 patients treated with doxorubicin-containing chemotherapy. Its five factors are age, tumour stage, serum LDH, performance status and number of extranodal sites. The four risk groups had predicted five-year survival of 73%, 51%, 43% and 26%; an age-adjusted index for patients 60 or younger, based on stage, LDH and performance status, gave 83%, 69%, 46% and 32%. A 2014 enhanced index for diffuse large B-cell lymphoma in the rituximab era (NCCN-IPI, 1,650 patients) uses LDH normalized to the upper limit of normal and separated low- and high-risk groups more widely than the IPI (five-year overall survival 96% vs 33%, against 90% vs 54% for the IPI).

Germ cell tumours: the 1997 International Germ Cell Consensus Classification (data on 5,202 patients with non-seminomatous and 660 with seminomatous germ cell tumours) combines the degree of elevation of the three serum markers — alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG) and LDH — with the primary site (mediastinal) and the presence of non-pulmonary visceral metastases in non-seminoma. It defined good, intermediate and poor prognosis groups with 5-year survival of 91%, 79% and 48% across all germ cell tumours (the poor group contained only non-seminomas). A 2021 update in 9,728 men with metastatic non-seminoma treated between 1990 and 2013 found 5-year overall survival of 96%, 89% and 67% in the three groups, and identified an LDH above 2.5 times the upper limit of normal, older age and lung metastases as additional adverse factors.

Multiple myeloma: the 2015 Revised International Staging System (R-ISS) adds chromosomal abnormalities and LDH to the original ISS. Stage I requires a normal LDH, and a high LDH is one of the features that places ISS stage III patients in R-ISS stage III.

Metastatic solid tumours:

Tumour lysis syndrome (TLS): when treatment kills a large mass of fast-growing cancer cells, their contents flood the blood. An international expert panel defines TLS by its metabolic abnormalities — high uric acid, high phosphate, high potassium and low calcium — which can lead to kidney failure, arrhythmias and seizures. The 2008 evidence-based guidelines describe prevention with hydration and rasburicase in high-risk patients and hydration plus allopurinol or rasburicase at intermediate risk.

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LDH in Lung Disease: Pulmonary Embolism, Pleural Fluid and Pneumocystis

Pulmonary embolism: LDH can be raised in pulmonary embolism, but it is not part of current diagnosis or risk assessment. The 2019 ESC guideline on acute pulmonary embolism builds its early risk classification from hemodynamic status, the Pulmonary Embolism Severity Index (PESI or sPESI), right-ventricular dysfunction on imaging and cardiac biomarkers (troponin, with natriuretic peptides as additional markers); LDH is not among them. BNP/NT-proBNP, troponin and D-dimer have their own pages in the Lab Tests section.

Pleural effusion (Light's criteria): LDH measured in pleural fluid, together with serum LDH and protein, is used to classify effusions as transudates or exudates. Under Light's criteria (1972), an effusion is classed as an exudate when any one of the following is present:

A 2014 systematic review of 48 studies found that an exudate was unlikely when all of Light's criteria were absent (likelihood ratio 0.04), and that a pleural fluid LDH above 200 U/L (likelihood ratio 18), a pleural fluid cholesterol above 55 mg/dL, or a pleural-to-serum cholesterol ratio above 0.3 were among the most accurate single indicators of an exudate. Exudates arise from causes such as pneumonia, cancer, rheumatoid disease and empyema; transudates from fluid shifts in conditions such as heart failure, cirrhosis and nephrotic syndrome.

Pneumocystis jirovecii pneumonia (PCP): in a 1988 study of 84 HIV-infected patients with lung problems, mean serum LDH was 361 IU in the 54 with PCP versus 224 IU in those with other lung diseases. Individual values overlapped, but levels above 450 IU consistently predicted PCP in that population, and only 7% of PCP patients had a normal LDH. Mean LDH was higher in patients who died (447 IU) than in survivors (340 IU), though the overlap was too great to predict death in an individual. During treatment, 75% of survivors had gradually falling LDH while 75% of non-survivors had rising values. Diagnosis rests on finding the organism, typically in bronchoalveolar lavage fluid.

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LDH in Critical Illness and COVID-19

In intensive care, a raised LDH is common and usually has several sources at once — liver ischemia from low blood pressure, hemolysis, muscle breakdown, lung injury and widespread inflammation. A trend over repeated measurements carries more information than a single value.

COVID-19: a 2020 pooled analysis of 9 studies (1,532 patients) found that an elevated LDH at the earliest point in hospital was associated with about a 6-fold increase in the odds of severe disease and about a 16-fold increase in the odds of death; the authors called for larger studies. A later meta-analysis of 21 studies (10,399 patients) found an elevated LDH in 44% of patients and linked it with a composite poor outcome (odds ratio 5.33) and with death (odds ratio 4.22); as a predictor, elevated LDH had a sensitivity of 0.74 and specificity of 0.69. Proposed sources of LDH in severe COVID-19 include direct lung injury and wider inflammatory damage to other organs.

Sepsis: LDH is not part of the 2016 Sepsis-3 definitions. Septic shock is identified by the need for vasopressors to keep mean blood pressure at 65 mm Hg or higher together with a serum lactate above 2 mmol/L (18 mg/dL) after fluid resuscitation; that group had a hospital mortality of 42.3% in the data the task force examined. Serum lactate and LDH are different tests: lactate measures the product of anaerobic metabolism, LDH the enzyme that interconverts lactate and pyruvate.

Ischemic hepatitis (“shock liver”): sudden loss of blood flow to the liver — from cardiac arrest, cardiogenic shock or severe low blood pressure — raises LDH steeply along with the aminotransferases. In the 1994 study described in the Causes section, the mean ALT/LDH ratio was 0.87 in ischemic hepatitis against 4.65 in acute viral hepatitis, so a low ALT/LDH ratio points toward ischemia or acetaminophen injury rather than viral hepatitis.

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Key Research Papers

The following are peer-reviewed publications, guidelines and reference chapters behind the statements on this page.

  1. Wroblewski F, LaDue JS. Lactic dehydrogenase activity in blood. Proc Soc Exp Biol Med. 1955;90(1):210–213. — PubMed PMID: 13273400
  2. Farhana A, Lappin SL. Biochemistry, Lactate Dehydrogenase. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; updated 2023 May 1. NCBI Bookshelf NBK557536 — PubMed PMID: 32491468
  3. The International Non-Hodgkin's Lymphoma Prognostic Factors Project. A predictive model for aggressive non-Hodgkin's lymphoma. N Engl J Med. 1993;329(14):987–994. — PubMed PMID: 8141877
  4. International Germ Cell Cancer Collaborative Group. International germ cell consensus classification: a prognostic factor-based staging system for metastatic germ cell cancers. J Clin Oncol. 1997;15(2):594–603. — PubMed PMID: 9053482
  5. Lippi G, Salvagno GL, Montagnana M, Brocco G, Guidi GC. Influence of hemolysis on routine clinical chemistry testing. Clin Chem Lab Med. 2006;44(3):311–316. — PubMed PMID: 16519604
  6. Henry BM, Aggarwal G, Wong J, Benoit S, Vikse J, Plebani M, Lippi G. Lactate dehydrogenase levels predict coronavirus disease 2019 (COVID-19) severity and mortality: A pooled analysis. Am J Emerg Med. 2020;38(9):1722–1726. — PubMed PMID: 32738466
  7. Thygesen K, Alpert JS, Jaffe AS, Chaitman BR, Bax JJ, Morrow DA, White HD; Executive Group on behalf of the Joint ESC/ACC/AHA/WHF Task Force for the Universal Definition of Myocardial Infarction. Fourth Universal Definition of Myocardial Infarction (2018). J Am Coll Cardiol. 2018;72(18):2231–2264. — PubMed PMID: 30153967
  8. Brodsky RA. Paroxysmal nocturnal hemoglobinuria. Blood. 2014;124(18):2804–2811. — PubMed PMID: 25237200
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  10. Cassidy WM, Reynolds TB. Serum lactic dehydrogenase in the differential diagnosis of acute hepatocellular injury. J Clin Gastroenterol. 1994;19(2):118–121. — PubMed PMID: 7963356
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  17. Hillmen P, Young NS, Schubert J, et al. The complement inhibitor eculizumab in paroxysmal nocturnal hemoglobinuria. N Engl J Med. 2006;355(12):1233–1243. — PubMed PMID: 16990386
  18. Parakh N, Dewan P. Revisiting the hematological manifestations of vitamin B12 deficiency. Front Nutr. 2026;13:1898533. — PubMed PMID: 42657046
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  20. Ruzich RS. Cardiac enzymes. How to use serial determinations to confirm acute myocardial infarction. Postgrad Med. 1992;92(7):85–89, 92. — PubMed PMID: 1437917
  21. Levinson SS, Hobbs GA. Usefulness of various lactate dehydrogenase isoenzyme 1 profiles after myocardial infarction. Ann Clin Lab Sci. 1994;24(4):364–370. — PubMed PMID: 7944273
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