Prepared by: Dr. Omar Adwan – Medical Laboratory Technologist, MedLab Academy
Last Updated: September 12, 2026
Category: Clinical Chemistry
A lactate dehydrogenase (LDH) blood test measures the activity of an enzyme that is present in many tissues throughout the body. LDH may increase when cells are damaged and release their intracellular contents into the circulation.
Because LDH is widely distributed in red blood cells, liver, skeletal muscle, kidneys, lungs and other tissues, a high LDH result is nonspecific. It can indicate that tissue injury or cell turnover is occurring, but it cannot independently identify the exact organ, disease or cause.
One of the most important laboratory limitations is specimen hemolysis. Red blood cells contain substantial amounts of LDH, so in-vitro hemolysis during blood collection, transport or processing can produce a falsely elevated result.
Quick Laboratory Summary
- LDH stands for lactate dehydrogenase.
- LDH is found in many tissues and is therefore a nonspecific marker.
- High LDH may reflect tissue injury, hemolysis, liver disease, muscle injury, some infections, ischemia or other disorders.
- LDH may also have prognostic or monitoring roles in selected malignancies.
- A high LDH level alone does not diagnose cancer.
- Hemolysis of the blood specimen may falsely increase LDH.
- Always review the hemolysis index or specimen integrity before interpreting an unexpectedly high LDH.
- There is no single universal normal LDH range.
- An example adult range published by MedlinePlus is approximately 125–220 IU/L.
- The reference interval printed by the performing laboratory should take priority.
1. What Is Lactate Dehydrogenase (LDH)?
Lactate dehydrogenase is an intracellular enzyme involved in cellular energy metabolism. It catalyzes the reversible conversion between lactate and pyruvate.
LDH is present in many tissues rather than being restricted to a single organ. Important sources include:
- Red blood cells
- Liver
- Skeletal muscle
- Kidneys
- Lungs
- Heart
- Brain
- White blood cells
- Other tissues
When cells are injured, damaged or destroyed, LDH may be released into the bloodstream. This makes serum LDH useful as a general biochemical marker of tissue damage.
However, this widespread tissue distribution is also its major limitation:
LDH can tell you that tissue damage may be occurring, but total LDH usually cannot tell you exactly where that damage is occurring.
2. What Is an LDH Blood Test?
An LDH blood test measures total lactate dehydrogenase activity in a blood specimen. LDH can also be measured in selected body fluids, including pleural, peritoneal and cerebrospinal fluid in appropriate clinical situations.
The routine serum or plasma LDH result represents the combined activity of LDH released from multiple tissues.
Other names include:
- LDH
- LD
- Lactate dehydrogenase
- Lactic acid dehydrogenase
3. Why Is an LDH Test Ordered?
LDH may be ordered when a clinician is investigating possible tissue injury or wants additional information about a known disease.
Examples include evaluation or monitoring of:
- Hemolysis and hemolytic anemia
- Liver disease
- Muscle injury
- Selected blood disorders
- Selected cancers
- Severe infection or systemic illness
- Tissue ischemia or necrosis
- Selected pulmonary disorders
- Selected kidney disorders
LDH should generally be interpreted together with more organ-specific tests rather than used as a stand-alone diagnostic marker.
4. LDH Blood Test Normal Range
There is no universal LDH normal range. Reference intervals depend on factors including:
- Analytical method
- Analyzer and reagent system
- Measurement temperature
- Laboratory validation
- Population characteristics
- Age in some settings
A commonly cited MedlinePlus example is approximately:
| Test | Example Reference Interval | Important Note |
|---|---|---|
| Total LDH | Approximately 125–220 IU/L | Use the reference interval reported by the laboratory that performed the test. |
5. What Does a High LDH Level Mean?
A high LDH indicates increased enzyme activity in the blood, usually because LDH has been released from cells.
The challenge is determining why.
An elevated LDH can occur with:
- Red blood cell destruction
- Liver-cell injury
- Muscle injury
- Tissue ischemia
- Severe systemic illness
- Selected infections
- Some malignancies
- Other conditions associated with cell injury or increased cell turnover
An abnormal LDH result therefore acts more like a signal than a diagnosis.
6. LDH 300, 500 or 1000 — What Do These Levels Mean?
Searches such as LDH 300, LDH 500, LDH 1000 or LDH 2000 are common, but these numbers do not have one universal interpretation.
The better approach is to compare the result with the upper limit of normal (ULN) for the performing laboratory.
For educational purposes only, if a laboratory had a ULN of 220 U/L:
| LDH Result | Approximate Relationship to ULN | General Interpretation |
|---|---|---|
| 220 U/L | 1× ULN | At the example upper reference limit. |
| 300 U/L | About 1.4× ULN | Mild elevation; nonspecific. |
| 500 U/L | About 2.3× ULN | Clear elevation requiring interpretation in context. |
| 1000 U/L | About 4.5× ULN | Marked elevation; investigate tissue injury, hemolysis and other causes. |
| 2000 U/L | About 9× ULN | Marked elevation requiring prompt clinical correlation. |
These figures are examples, not diagnostic cutoffs. An LDH of 1000 does not automatically diagnose cancer, hemolysis, liver failure or any other single disease.
7. Common Causes of High LDH
| Possible Cause | Why LDH May Rise | Useful Correlated Tests |
|---|---|---|
| Hemolysis | Red blood cells release intracellular LDH. | CBC, reticulocytes, bilirubin, haptoglobin, blood smear |
| Liver injury | Damaged hepatic cells may release LDH. | ALT, AST, ALP, GGT, bilirubin |
| Muscle injury | Muscle-cell injury may release LDH. | CK and clinical assessment |
| Tissue ischemia or necrosis | Cell injury releases intracellular enzymes. | Organ-specific testing and imaging |
| Some infections | Inflammation and tissue injury may increase LDH. | CBC, CRP, cultures or disease-specific tests |
| Selected malignancies | High cell turnover or tissue injury may increase LDH. | Disease-specific hematology, pathology and imaging |
| Renal disease | LDH may rise in selected renal disorders or systemic illness. | Creatinine, eGFR, urinalysis |
| Specimen hemolysis | In-vitro rupture of RBCs causes an artificial LDH increase. | Hemolysis index and repeat specimen when appropriate |
8. LDH and Hemolysis
The relationship between LDH and hemolysis is essential for anyone interpreting laboratory results.
Red blood cells contain high intracellular LDH activity. When erythrocytes rupture, LDH is released.
This may happen:
- Inside the patient — in vivo hemolysis
- After blood collection — in vitro hemolysis
These two situations have completely different meanings.
Read the full MedLab Academy guide: Hemolysis in Clinical Chemistry: H-Index, Interference, Prevention & Interpretation .
9. In Vivo vs In Vitro Hemolysis
| Feature | In Vivo Hemolysis | In Vitro Hemolysis |
|---|---|---|
| Where does it occur? | Inside the patient's circulation. | During or after specimen collection. |
| LDH | Can be genuinely elevated. | Can be falsely elevated. |
| Clinical significance | May indicate a true pathological process. | Usually represents a preanalytical specimen problem. |
| Other clues | May include anemia, reticulocytosis, indirect bilirubin increase and low haptoglobin. | Analyzer hemolysis index may be elevated without corresponding clinical evidence of hemolysis. |
| Action | Investigate clinically. | Apply laboratory SOP and recollect when required. |
10. LDH and Hemolytic Anemia
LDH is commonly used as one component of the laboratory investigation of suspected hemolysis.
A typical hemolytic pattern may include:
- Increased LDH
- Increased indirect bilirubin
- Increased reticulocyte response
- Reduced haptoglobin, particularly with intravascular hemolysis
- Reduced hemoglobin when anemia is present
- Cause-specific peripheral blood smear abnormalities
LDH should never be used alone to diagnose hemolytic anemia.
For the hematological approach, see: Anemia: Classification, Laboratory Diagnosis & CBC Interpretation .
You can also review: Low Hemoglobin Levels: CBC, Anemia Causes & Laboratory Interpretation .
LDH and the Peripheral Blood Smear
If hemolysis is suspected, the smear may provide important additional clues.
Depending on the underlying cause, findings can include:
- Schistocytes
- Spherocytes
- Polychromasia
- Nucleated red blood cells
- Sickle cells in appropriate disorders
- Other morphology specific to the underlying disease
Review the broader hematology framework in: Complete Blood Count (CBC): Normal Ranges & Laboratory Interpretation .
11. LDH and Liver Disease
The liver contains LDH, so hepatocellular injury may contribute to an elevated serum concentration.
However, LDH is far less organ-specific than markers such as ALT. If liver disease is suspected, it should be interpreted with:
- ALT
- AST
- ALP
- GGT
- Total bilirubin
- Direct bilirubin
- Albumin
- PT/INR when clinically appropriate
A high LDH by itself should not be labeled liver disease.
For complete liver-test interpretation, read: Liver Function Tests (LFTs): Complete Laboratory Interpretation Guide .
12. LDH and Muscle Injury
Skeletal muscle contains LDH, so muscle-cell damage may increase serum LDH.
Possible clinical contexts include trauma, muscle disease or significant muscle injury.
Because LDH is nonspecific, creatine kinase (CK) and clinical findings are generally more useful when skeletal muscle injury is the primary question.
Recent strenuous exercise should also be considered when interpreting an unexpected enzyme elevation.
13. High LDH and Cancer: What Does It Really Mean?
One of the most common internet questions is: “Does high LDH mean cancer?”
The answer is no.
LDH can be elevated in selected malignancies because of increased cell turnover, tumor burden, tissue injury or associated systemic effects. In some established cancers, LDH may contribute to prognosis or disease monitoring.
Examples in which LDH may have clinical relevance include selected:
- Lymphomas
- Leukemias
- Multiple myeloma
- Melanoma
- Testicular cancer
- Other malignancies depending on the clinical protocol
However:
High LDH is not a cancer screening test. A high LDH result cannot diagnose cancer, determine its type, or prove that malignancy is present. The same elevation may occur with hemolysis, infection, liver injury, muscle injury and many other conditions.
14. LDH and Infection or Tissue Injury
Some severe infectious or inflammatory conditions may increase LDH because of associated cellular injury.
LDH cannot determine whether an infection is bacterial or viral, and it cannot identify the infectious organism.
When infection or systemic inflammation is being investigated, other tests may include:
- CBC and differential
- CRP
- Procalcitonin when appropriate
- Blood cultures
- Urine or respiratory microbiology
- Disease-specific molecular or serological tests
For inflammatory-marker interpretation, see: High CRP Levels: Causes, Normal Range & Laboratory Interpretation .
15. LDH and Heart Disease
Historically, LDH and LDH isoenzymes were used in the investigation of myocardial infarction.
Modern cardiac assessment relies on far more specific biomarkers, especially cardiac troponin.
Therefore, a high LDH level should not be interpreted as evidence of a heart attack.
For modern cardiac-marker interpretation, read: High Troponin Levels: ACS, Non-ACS Causes & Delta Interpretation .
Also see: Cardiac Biomarkers: Troponin, CK-MB, Myoglobin & BNP .
16. LDH and Kidney Disease
Kidney tissue contains LDH, and LDH may increase in selected renal or systemic conditions involving tissue injury.
However, serum LDH is not a routine standalone marker of kidney filtration.
Renal assessment is more appropriately centered on:
- Serum creatinine
- eGFR
- Urine albumin-to-creatinine ratio
- Urinalysis
- Electrolytes
- Clinical context
See: High Creatinine Levels: eGFR, BUN & Kidney Function .
17. What Does a Low LDH Level Mean?
Low LDH concentrations are much less commonly clinically significant than high LDH values.
A low result usually does not indicate a serious medical disorder by itself.
Rare inherited abnormalities affecting LDH metabolism exist, and some analytical or biological factors may also influence measurements. However, these situations are uncommon.
An unexpectedly low value should be interpreted according to the laboratory method and patient context rather than through a generic internet cutoff.
18. What Are LDH Isoenzymes?
LDH exists as five major isoenzyme forms, called LDH-1 through LDH-5.
| Isoenzyme | Important Tissue Associations |
|---|---|
| LDH-1 | Heart and red blood cells |
| LDH-2 | White blood cells and other tissues |
| LDH-3 | Lungs and other tissues |
| LDH-4 | Kidneys, pancreas and placenta |
| LDH-5 | Liver and skeletal muscle |
Isoenzyme testing can provide clues about the likely tissue source of an LDH elevation, but individual isoenzymes are not completely organ-specific.
More specific laboratory biomarkers are often used instead when the clinical question can be answered more accurately by another test.
19. LDH Sample Requirements
LDH is commonly measured using a venous blood specimen. The acceptable specimen depends on the analytical method and manufacturer instructions.
| Specimen | Serum or validated plasma according to the laboratory method |
|---|---|
| Collection | Routine venipuncture |
| Reporting | Usually U/L or IU/L |
| Major Preanalytical Concern | Hemolysis |
| Method | Automated enzymatic measurement; exact methodology is platform specific |
| Reference Interval | Laboratory and method specific |
20. Do You Need to Fast for an LDH Test?
Routine LDH measurement generally does not require special preparation unless other tests being collected at the same time require it.
Patients should follow the instructions given by their healthcare provider or laboratory.
Medication and supplement information may be clinically relevant. Patients should not discontinue prescribed medication solely because of an abnormal LDH result unless advised by their healthcare professional.
21. Preanalytical Factors That Can Affect LDH
For LDH, the preanalytical phase is particularly important because even specimen damage can materially alter the result.
Review:
- Patient identification
- Correct tube
- Collection technique
- Needle and collection system
- Difficult or traumatic venipuncture
- Excessive mechanical agitation
- Transport conditions
- Delayed processing
- Storage conditions
- Visible hemolysis
- Analyzer-generated H-index
Why Is Hemolysis So Important for LDH?
Red blood cells have substantially higher intracellular LDH activity than the surrounding plasma or serum.
When red cells rupture after collection, LDH enters the liquid portion of the specimen and can create an artificial increase.
22. Analytical Limitations and Quality Control
Before reporting an unexpected or clinically significant LDH result, laboratory professionals should consider analytical validity.
Review may include:
- Internal quality-control status
- Calibration status when applicable
- Reagent integrity
- Analyzer maintenance
- Reaction flags
- Analytical measurement range
- Dilution requirements
- Hemolysis index
- Lipemia and icterus interference where method relevant
- Previous results and delta checks
Interference limits must be based on the exact analyzer/reagent system and the laboratory's validated procedures.
For broader quality-management concepts, see: Laboratory Quality Indicators: KPIs & ISO 15189 .
23. Step-by-Step LDH Interpretation Algorithm
Step 1 — Confirm That LDH Is Truly Elevated
Compare the result with the laboratory-specific reference interval.
Step 2 — Check the Hemolysis Index
Determine whether the specimen itself could explain the result.
Step 3 — Review the CBC
Look for anemia, thrombocytopenia, abnormal WBC counts or other hematological clues.
Step 4 — Consider True Hemolysis
If clinically appropriate, review reticulocytes, indirect bilirubin, haptoglobin and peripheral smear morphology.
Step 5 — Review Liver Markers
Evaluate ALT, AST, ALP, GGT and bilirubin when liver disease is possible.
Step 6 — Review Muscle Markers
Consider CK and clinical history when muscle injury is suspected.
Step 7 — Review Renal Function
Creatinine, eGFR and urinalysis may provide important context.
Step 8 — Review Inflammation and Infection
Consider CRP, CBC, cultures or other disease-specific tests.
Step 9 — Consider Malignancy Only in Context
Do not interpret isolated LDH elevation as evidence of cancer.
Step 10 — Compare With Previous Results
A trend may be more informative than one isolated measurement.
24. When Should High LDH Be Concerning?
There is no universal LDH number above which every patient is in danger.
Urgency depends much more on the clinical condition and associated abnormalities.
Prompt medical evaluation is appropriate when an abnormal LDH occurs with concerning features such as:
- Severe shortness of breath
- Chest pain
- Syncope or severe weakness
- Marked jaundice
- Dark urine with suspected hemolysis
- Severe abdominal pain
- Significant unexplained anemia
- Neurological symptoms
- Persistent high fever or serious systemic illness
- Rapid clinical deterioration
25. When Should LDH Be Repeated?
Repeat testing depends on the reason for the original measurement.
Repetition may be appropriate when:
- The specimen is significantly hemolyzed.
- The result is inconsistent with other findings.
- An analytical problem is suspected.
- The result differs unexpectedly from previous measurements.
- Serial LDH has a defined monitoring role for the patient's condition.
- The clinician needs a trend as part of an established management plan.
Routine repetitive LDH measurement without a clinical question provides limited value.
26. Clinical Case Examples
Case 1 — High LDH in a Hemolyzed Specimen
A patient has:
- LDH: 780 U/L
- Potassium: unexpectedly high
- AST: mildly elevated
- Analyzer H-index: markedly elevated
Case 2 — LDH Elevation With Hemolytic Pattern
A patient has:
- Low hemoglobin
- High reticulocyte count
- High indirect bilirubin
- Low haptoglobin
- High LDH
Case 3 — LDH 500 U/L With Elevated Liver Enzymes
A patient has:
- LDH: 500 U/L
- ALT: markedly elevated
- AST: elevated
- Bilirubin: elevated
Case 4 — Isolated LDH Elevation
A patient has:
- LDH: mildly elevated
- CBC: normal
- ALT/AST: normal
- Creatinine: normal
- CRP: normal
- No significant symptoms
Case 5 — High LDH in a Patient With Known Lymphoma
A patient already diagnosed with lymphoma has an elevated LDH as part of a specialist evaluation.
Case 6 — High LDH and Thrombocytopenia
A patient has:
- Low platelet count
- Anemia
- Marked LDH elevation
- Schistocytes on peripheral smear
For platelet interpretation, see: Thrombocytopenia: Low Platelet Counts, Smear Findings & Diagnostic Clues .
27. Lab Professional Insight
From the Laboratory Bench
LDH is one of the tests where specimen quality can completely change the interpretation.
When an LDH result is unexpectedly high, my practical laboratory sequence is:
- Check the result against the laboratory reference interval.
- Review the hemolysis index.
- Check potassium and AST for a compatible specimen-hemolysis pattern.
- Compare with previous LDH results.
- Review CBC and bilirubin if true hemolysis is possible.
- Check liver, renal and muscle markers according to the clinical context.
- Only then interpret the LDH biologically.
This approach prevents a preanalytical error from being mistaken for a disease process.
28. Related MedLab Academy Guides
The following are real MedLab Academy articles that provide useful context for LDH interpretation:
- Hemolysis in Clinical Chemistry: Complete Guide
- Complete Blood Count (CBC): Normal Ranges & Interpretation
- Anemia: Classification & Laboratory Diagnosis
- Low Hemoglobin Levels: CBC & Anemia Interpretation
- Liver Function Tests (LFTs): Complete Guide
- High Creatinine Levels, eGFR & Kidney Function
- High CRP Levels: Causes & Laboratory Interpretation
- High Ferritin Levels: Causes & Interpretation
- High D-Dimer Levels: Causes, DVT & PE Interpretation
- Thrombocytopenia: Low Platelet Count & Smear Findings
- High Troponin Levels: ACS vs Non-ACS
- Cardiac Biomarkers: Troponin, CK-MB, Myoglobin & BNP
- Pancytopenia: Laboratory Approach to Low RBCs, WBCs & Platelets
29. Frequently Asked Questions About LDH
What is LDH in a blood test?
LDH is lactate dehydrogenase, an enzyme found in many tissues. Blood LDH may increase when cells are damaged or destroyed.
What is the normal range for LDH?
Reference intervals vary by laboratory and method. MedlinePlus provides approximately 125–220 IU/L as one example, but the range printed on the patient's own laboratory report should be used.
Is LDH 300 high?
It may be mildly elevated depending on the laboratory's upper reference limit. The result cannot be interpreted correctly without the laboratory-specific range.
Is LDH 500 high?
A result of 500 U/L is above the normal range in many laboratories, but its significance depends on the reference interval and the clinical context.
What does LDH 1000 mean?
LDH around 1000 U/L represents a marked elevation relative to many reference intervals. Possible causes include significant tissue injury, hemolysis and other disorders, but the value alone cannot identify the cause.
Does high LDH mean cancer?
No. Some cancers may increase LDH, but many non-cancer conditions can also cause high LDH. LDH is not a stand-alone cancer diagnostic test.
Can hemolysis cause high LDH?
Yes. Both true in-vivo red-cell destruction and in-vitro specimen hemolysis can increase LDH. In-vitro hemolysis may create a falsely high patient result.
Why is LDH high in hemolytic anemia?
Red blood cells contain LDH. When circulating red cells are destroyed, intracellular LDH is released into the bloodstream.
Can a hemolyzed blood sample falsely increase LDH?
Yes. LDH is particularly sensitive to specimen hemolysis, which is why the hemolysis index should be checked before interpreting an unexpected increase.
Does high LDH mean liver disease?
Not necessarily. Liver disease can increase LDH, but so can red-cell destruction, muscle injury, infection, tissue ischemia and other conditions.
Is LDH a liver function test?
LDH may be included in some biochemical assessments, but it is not specific for the liver and should not be interpreted as a direct measure of liver function.
Can exercise increase LDH?
Significant muscle activity or injury may contribute to an LDH increase. Recent strenuous exercise can therefore be relevant when an elevation is otherwise unexplained.
Can high LDH indicate hemolytic anemia?
It can support the diagnosis when combined with other findings such as anemia, increased reticulocytes, increased indirect bilirubin, reduced haptoglobin and appropriate blood-smear findings.
What are LDH isoenzymes?
LDH has five major isoenzyme forms, LDH-1 through LDH-5. Their distribution differs among tissues and may provide clues about the source of tissue injury, although they are not completely organ-specific.
Does low LDH mean anything serious?
Low LDH is uncommon and usually has much less clinical significance than high LDH. Interpretation should still use the laboratory-specific reference interval.
When should high LDH be investigated?
Persistent, marked or unexplained LDH elevation should be interpreted by a healthcare professional, particularly when accompanied by abnormal CBC, bilirubin, liver enzymes, kidney tests or significant symptoms.
30. Authoritative & Professional References
- MedlinePlus — Lactate Dehydrogenase (LDH) Test
- MedlinePlus Medical Encyclopedia — Lactate Dehydrogenase Test
- MedlinePlus — LDH Isoenzymes Test
- National Cancer Institute — LDH Definition
- NCBI Bookshelf — Biochemistry, Lactate Dehydrogenase
Official Information Check
Last verified: September 12, 2026
Information checked: LDH biological role, tissue distribution, reference-range variability, common causes of elevation, specimen hemolysis, LDH isoenzymes and limitations of total LDH interpretation.
Primary official source: U.S. National Library of Medicine — MedlinePlus
Cancer terminology source: National Cancer Institute
Prepared for MedLab Academy • Last Updated September 12, 2026