High LDH (Lactate Dehydrogenase) Explained
Why LDH is the most non-specific marker in your blood test, why the first step is usually just repeating the sample, and when a high result actually matters — in plain English.
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The Quick Answer
LDH (lactate dehydrogenase) is an enzyme found inside virtually every cell in your body. Its job is to help convert sugar into energy. When cells are damaged or die — for any reason, in any organ — they release LDH into the bloodstream, raising the measured level.
The normal range in most Australian pathology labs is approximately 120-250 U/L for adults. An elevated LDH tells you that cell damage has occurred somewhere — but because LDH is in every cell, it is deliberately non-specific. Think of it as a general alarm bell, not a diagnosis.
The single most important thing to know: a haemolysed blood sample (where red cells break open during collection) is the most common cause of an unexpected elevated LDH, and it looks identical to a genuine disease result. Repeating the test with a fresh, clean sample is almost always step one.
Why LDH Is Deliberately Non-Specific
LDH catalyses a reversible chemical reaction that is central to energy metabolism in every cell. Because of this universal role, LDH is present in high concentrations in the heart, liver, skeletal muscle, kidneys, lungs, and red blood cells. When any of these tissues is damaged, LDH leaks out in proportion to the amount of cell death.
This universality is both LDH's value and its limitation. It is a sensitive indicator that something is wrong, but it does not tell you what or where. In clinical practice, an isolated mildly elevated LDH without symptoms and without other abnormalities on the blood test is often a collection artefact or a transient effect of recent exercise.
An LDH that is significantly elevated, or that is elevated alongside other abnormal results, is a different matter. The art is in reading the full picture — including symptoms, the rest of the blood count, liver function, and kidney function — rather than treating the LDH number in isolation. Uploading your complete blood test to SmarterBlood lets the AI flag other markers on the same report that may explain the elevated LDH.
The Haemolysed Sample Artefact — The Most Common Cause
Red blood cells contain approximately 100 times more LDH than blood serum. This means that even very mild haemolysis — the rupture of just a small fraction of red cells in the blood tube during or after collection — can raise the apparent LDH dramatically, sometimes to three or four times the upper limit of normal.
Haemolysis during collection can happen from a difficult vein, excessive suction on the syringe, vigorous shaking of the tube, or a delay between collection and processing. The lab may or may not flag the sample as visually haemolysed (pink or red serum) — mild haemolysis is not always visible.
Causes of High LDH by Organ
Causes are grouped by the organ or process involved. Always exclude the haemolysed sample artefact before attributing an elevated LDH to any disease process.
Haemolysed blood sample (collection artefact)
Red cells contain extremely high LDH. Even mild haemolysis during blood collection causes a significant false elevation. Repeat the test with a fresh, clean sample before investigating further. The lab may or may not flag the sample as haemolysed.
Haemolytic anaemia (red cell destruction in the body)
When red cells are destroyed within the circulation (haemolysis), they release LDH in the same way as a haemolysed sample - but this time it is happening inside you. Associated with elevated bilirubin, low haptoglobin, and high reticulocyte count. Causes include autoimmune haemolytic anaemia, haemoglobin disorders, and some medications.
Liver disease (hepatitis, cirrhosis, fatty liver)
The liver is rich in LDH-4 and LDH-5. Hepatitis (especially viral), severe fatty liver disease, and cirrhosis all release LDH into the blood. Usually accompanied by elevated ALT, AST, and GGT. LDH alone is not a sensitive liver marker - dedicated liver function tests are more informative.
Muscle injury or intense exercise
Strenuous unaccustomed exercise, muscle trauma, or inflammatory muscle disease (myositis) releases LDH from muscle cells. Creatine kinase (CK) is a more sensitive and specific marker for muscle damage. If both are elevated, muscle injury is likely.
Pulmonary embolism (PE)
Pulmonary embolism causes lung cell ischaemia and death, releasing LDH (predominantly LDH-3). LDH is not sensitive enough to diagnose PE, but a high LDH in the context of chest pain, breathlessness, and risk factors adds to clinical concern. D-dimer and CT pulmonary angiogram are the diagnostic tests.
Lymphoma and leukaemia
LDH is a useful tumour marker for lymphoma and leukaemia. Rapidly dividing lymphoma cells have high metabolic activity and release LDH. A high LDH in someone with lymphadenopathy, night sweats, or unexplained weight loss should prompt urgent investigation. LDH level also correlates with prognosis and treatment response in lymphoma.
Solid tumours (melanoma, testicular cancer, others)
LDH is elevated in many solid tumours and is particularly used as a prognostic marker in melanoma (stage III-IV) and in the International Germ Cell Tumour Classification for testicular cancer. In someone with a known diagnosis, a rising LDH often signals disease progression.
Tumour lysis syndrome
When large numbers of cancer cells are destroyed rapidly by chemotherapy, they release massive amounts of intracellular contents including LDH, uric acid, potassium, and phosphate. This is a medical emergency managed in hospital.
Myocardial infarction (heart attack)
LDH was historically used to diagnose heart attacks (LDH-1 and LDH-2 pattern). Troponin I and T are now far more sensitive and specific for cardiac injury and have replaced LDH in acute cardiac care. Elevated LDH after a heart attack peaks at 48-72 hours and returns to normal over 7-10 days.
Megaloblastic anaemia (B12 or folate deficiency)
Abnormal red cell precursors (megaloblasts) in the bone marrow are destroyed before they mature - a process called ineffective erythropoiesis - releasing large amounts of LDH. A high LDH alongside a high MCV should always prompt B12 and folate testing.
Red Flags — When a High LDH Is Genuinely Concerning
An isolated mildly elevated LDH in a well person is usually an artefact or benign. These combinations should prompt prompt GP review:
Very high LDH (above 500 U/L) with no obvious cause
A markedly elevated LDH with no history of strenuous exercise, recent illness, or haemolysed sample is a serious finding that should be investigated promptly. Lymphoma, haemolytic anaemia, and liver failure are all possibilities.
High LDH with night sweats, weight loss, or lymph node swelling
The combination of these B-symptoms with a high LDH is a classic presentation of lymphoma. Urgent GP review and likely haematology referral is needed.
High LDH with rising uric acid and kidney impairment
This pattern can indicate spontaneous tumour lysis syndrome from a rapidly proliferating haematological malignancy, which is a medical emergency.
High LDH alongside low haemoglobin and elevated bilirubin
This triad strongly suggests haemolytic anaemia - the body is destroying red cells faster than it can replace them. Autoimmune haemolytic anaemia needs treatment promptly.
Significantly elevated LDH in someone with chest pain and breathlessness
While LDH is not diagnostic for pulmonary embolism or myocardial infarction, these serious conditions must be excluded. Troponin, D-dimer, and ECG are more specific but LDH elevation adds to the overall clinical picture.
LDH Isoenzymes — Narrowing Down the Source
LDH exists as five different molecular forms called isoenzymes, each containing different combinations of two protein subunits (H and M). Because each organ expresses a characteristic isoenzyme pattern, measuring them can help pinpoint the source of the elevation:
LDH-1 and LDH-2
Heart and red blood cellsThe dominant form in cardiac muscle. Historically used for heart attack diagnosis before troponin became available. Also elevated in haemolysis.
LDH-3
Lung and plateletsElevated in pulmonary embolism, pneumonia, and platelet destruction. Less commonly measured in routine practice.
LDH-4 and LDH-5
Liver and skeletal muscleElevated in liver disease (hepatitis, cirrhosis), muscle injury, and myositis. LDH-5 is the most liver-specific isoenzyme.
LDH isoenzymes are less commonly ordered today because troponin, CK, and liver function tests provide more organ-specific information. However, they remain useful when the source of elevated LDH is unclear after first-line tests.
The Workup — What Your GP Will Investigate
Investigation is guided entirely by the clinical context. The steps below represent a typical logical sequence for an unexplained elevated LDH.
Repeat with a fresh sample - exclude haemolysis artefact
Before any investigation, a fresh, carefully collected venous sample should be repeated. If the repeat is normal, the original elevation was almost certainly a collection artefact. This single step saves many patients unnecessary investigations.
Check the full blood count and blood film
A low haemoglobin with raised LDH and reticulocytes suggests haemolytic anaemia. A blood film looking for fragmented red cells (schistocytes), spherocytes, or sickle cells can identify the cause. A high white cell count may raise concern for leukaemia or lymphoma.
Liver function tests
If liver disease is suspected, ALT, AST, ALP, GGT, bilirubin, and albumin provide a far more sensitive and specific picture than LDH alone. LDH elevation in the context of abnormal liver enzymes confirms hepatic origin.
Creatine kinase (CK) for muscle injury
CK is the gold-standard marker for skeletal muscle damage. If LDH is elevated alongside a high CK, muscle injury or inflammatory myopathy (such as polymyositis) is the likely source. A detailed exercise and medication history is important here.
Haemolysis screen if red cell destruction is suspected
A haemolysis screen includes haptoglobin (low in haemolysis), bilirubin (elevated), reticulocyte count (elevated), direct antiglobulin test (DAT/Coombs test for autoimmune causes), and blood film. Urine for haemoglobin may also be requested.
LDH isoenzymes if the source remains unclear
Fractionating LDH into its five isoenzymes can localise the source to heart, lung, liver, or muscle. This is less commonly ordered now that troponin and CK provide more specific answers, but remains useful when the picture is unclear.
Specialist workup if lymphoma or malignancy is suspected
If a haematological malignancy or solid tumour is suspected (based on symptoms, lymphadenopathy, or other abnormalities), urgent referral is warranted. LDH helps stage the extent of disease once a diagnosis is established.
High LDH - Frequently Asked Questions
What does a high LDH mean on a blood test?
LDH (lactate dehydrogenase) is an enzyme found in almost every cell in your body. When cells are damaged or destroyed, LDH is released into the blood, raising the measured level. The normal range in most Australian labs is approximately 120-250 U/L. A high LDH on its own is non-specific - it tells you that cell damage has occurred somewhere, but not where or why. The clinical context, symptoms, and other blood results are essential to interpret it.
Can a haemolysed blood sample cause a falsely high LDH?
Yes, and this is the most important thing to know about a single elevated LDH. Red blood cells contain very high concentrations of LDH. If the blood sample haemolyses during collection - for example from a difficult venepuncture, excessive needle suction, or rough handling - red cells rupture and flood the sample with LDH, producing a falsely elevated result. The lab usually reports a haemolysed sample, but not always. Repeating the test with a clean draw is often the first step before any further investigation.
What is the normal LDH range in Australia?
Most Australian pathology labs report a reference range of approximately 120-250 U/L for adults, though this varies between laboratories. Children have higher normal ranges. The LDH assay method also varies between labs, so comparisons between different labs can be misleading. Always compare your result to the reference range printed on your own report.
Can exercise cause high LDH?
Yes. Strenuous or unaccustomed exercise causes muscle cell breakdown (rhabdomyolysis in severe cases), releasing LDH and also creatine kinase (CK) into the blood. Even moderate exercise in the 24-48 hours before a blood test can elevate LDH by 50-100 U/L above the reference range. If you exercised vigorously before your test, repeat it after 48-72 hours of rest.
Is a high LDH a sign of cancer?
LDH is used as a tumour marker for some cancers - particularly lymphoma, leukaemia, melanoma, testicular cancer, and some solid tumours. However, high LDH alone is not diagnostic of cancer. It is non-specific and rises with many benign conditions. In someone with a known cancer diagnosis, LDH is used to track disease activity and response to treatment. If your LDH is high with no obvious cause, your GP will investigate the most common causes first before considering cancer.
What tests come after a high LDH?
Your GP will consider the clinical context. A common first step is to repeat the LDH with a fresh sample to exclude a haemolysed specimen. Depending on the likely cause, they may order liver function tests, full blood count, creatine kinase (for muscle injury), uric acid and kidney function (for haemolysis or tumour lysis), a reticulocyte count and blood film, and possibly LDH isoenzymes to narrow down which organ is the source.
What are LDH isoenzymes and why do they matter?
LDH exists as five different isoenzymes (LDH-1 through LDH-5), each found in highest concentrations in different organs: LDH-1 and LDH-2 in the heart and red blood cells; LDH-3 in the lungs; LDH-4 and LDH-5 in the liver and skeletal muscle. Measuring the isoenzyme pattern can help identify the source of the elevation. LDH isoenzymes are less commonly ordered now that troponin (for heart damage) and CK (for muscle damage) provide more specific information, but they remain useful in some diagnostic workups.
Related Reading
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This page provides general educational information about elevated LDH and lactate dehydrogenase. It is not a substitute for professional medical advice, diagnosis, or treatment. Always discuss abnormal blood test results with your GP — they have access to your full medical history and can interpret results in context. SmarterBlood does not provide medical care.