Lactate Dehydrogenase (LDH)

What is Lactate Dehydrogenase (LDH)?

Lactate dehydrogenase (LDH) is an enzyme almost every cell in your body uses to make energy.

LDH catalyzes one specific chemical step: it converts pyruvate to lactate (and back), the reaction cells rely on to keep making ATP when oxygen is scarce. Nearly every tissue runs this reaction, which is exactly why LDH shows up in blood at low levels all the time and why a spike doesn't point anywhere specific by itself, according to StatPearls' biochemistry review of the enzyme. That's the core difference from a marker like creatine kinase. CK concentrates heavily in skeletal muscle, heart muscle, and brain, three tissue types, so an elevated CK narrows the field fast. LDH exists in the heart, liver, kidneys, lungs, red blood cells, skeletal muscle, and most other tissue. A high LDH on its own tells you damage or high cell turnover happened somewhere. It doesn't tell you where. Labs can split total LDH into five isoenzymes, LDH-1 through LDH-5, based on which of two subunits (H and M) make up the molecule. MedlinePlus and StatPearls associate LDH-1 mainly with heart tissue and red blood cells, LDH-2 with white blood cells, LDH-3 with lung tissue, LDH-4 with kidney and pancreatic tissue, and LDH-5 with liver and skeletal muscle. Clinicians once used a "flipped" LDH-1-to-LDH-2 ratio (LDH-1 higher than LDH-2) as suggestive evidence of a recent heart attack, since LDH-1 rises 6 to 12 hours after damage and peaks at 24 to 72 hours. That pattern has since been set aside for a far more heart-specific test. Red blood cells carry their own LDH supply, which creates a well-documented lab artifact: if cells rupture during or after the blood draw, a process called hemolysis, LDH leaks out and inflates the result without reflecting anything happening in your body.

Typical reference context

Reference ranges depend on the lab's instrument, reagents, and units, so the same result can sit inside one lab's range and outside another's. Print or download whatever range your specific lab used. This table is general background, not an interpretation of any individual's result.

What changed recently

The genuinely new development isn't a changed reference range, it's a shift in how researchers think about LDH's separate isoforms in cancer biology. A March 2026 review in the Journal of Enzyme Inhibition and Medicinal Chemistry argues that most prior cancer-metabolism research fixated on one isoform, LDH-A, while a second isoform, LDH-B, turns out to matter separately: it supports mitochondrial lactate oxidation and tumor-cell survival through a mechanism that doesn't depend on lactate production at all. A third, rarer isoform, LDH-C, is drawing attention as a more selective drug target because of its distinct structure. None of this changes how a standard blood LDH test is read today, but it explains why isoenzyme-level detail on this marker is still an active research area rather than settled science.

Tests often checked alongside this

• Haptoglobin and bilirubin — ordered together with LDH when a clinician is working through possible hemolysis, since haptoglobin drops as it mops up free hemoglobin while LDH rises.
• Creatine kinase (CK) and troponin — checked alongside LDH historically for heart muscle injury; troponin has replaced LDH for that specific job.
• ALT and AST — run with LDH when liver involvement is suspected.
• Complete blood count (CBC) — ordered alongside LDH when anemia or a blood disorder is on the differential.
• Uric acid, potassium, phosphorus, and calcium — monitored together with LDH in people undergoing cancer treatment, as part of tumor lysis syndrome risk criteria.

What can affect this marker

• Hemolysis during the blood draw: red blood cells store their own LDH, so a rough draw or a delay in processing the sample can rupture cells and push the result up without any change in the body.
• Recent strenuous exercise: muscle tissue releases LDH after heavy exertion, a documented but temporary elevation unrelated to organ damage.
• Certain medications: aspirin, anesthetics, and some narcotics have been associated with transient LDH increases, per Cleveland Clinic's patient reference.
• High-dose vitamin C or E intake: associated with falsely low readings in some lab methods.
• Cell turnover from treatment or illness: rapid destruction of cancer cells during chemotherapy, or red cell breakdown from any cause, both release intracellular LDH into blood.

What to track over time

• The date and reference range printed on each report, since ranges shift between labs and assay methods.
• Whether the result was flagged alongside notes on how the sample was drawn or handled, given how sensitive LDH is to hemolysis.
• Any related results ordered the same day, such as haptoglobin, CK, or liver enzymes.
• How your own LDH values move across multiple visits, viewed on one chart instead of scattered across separate PDFs.

What to ask your doctor

• What made you order an LDH test for me specifically, and what are you ruling in or out?
• Could hemolysis during collection explain this result, and would a repeat draw help clarify that?
• Do you want the five-isoenzyme breakdown, or does the total LDH answer your question?
• Which other results from today's panel are you reading together with this one?
• Is this a value worth tracking at future visits, or a one-time check?

This glossary entry is for informational purposes only and is not a substitute for professional medical advice. Read our Terms of Use.

Frequently asked questions