LDH (lactate dehydrogenase) blood test: what it measures, normal levels, and what a high LDH means

Lactate dehydrogenase converts lactate to pyruvate and back, a step in how cells produce energy without oxygen. Because nearly every tissue uses it, LDH is present in red cells, liver, heart, kidney, muscle, lung, lymphoid tissue and brain. Inside cells its concentration is hundreds of times higher than in blood, so even modest cell damage raises the blood level measurably. The test reports total LDH activity in units per liter (U/L).

There are five isoenzymes, numbered LDH-1 to LDH-5, distributed differently by tissue: LDH-1 and LDH-2 predominate in heart and red cells, LDH-3 in lung and lymphoid tissue, LDH-4 and LDH-5 in liver and skeletal muscle. Isoenzyme analysis can sometimes suggest the source of a raised total, but in practice clinicians usually locate the source with more specific tests — a CBC and hemolysis markers, liver enzymes, CK, troponin — and reserve LDH for the roles where it adds something: monitoring hemolysis, and staging and following certain cancers.

LDH is a marker of damage, not of function. A normal LDH does not mean a tissue is healthy, and a high LDH does not mean any particular organ is failing.

Representative adult reference ranges run about 122–222 U/L, though laboratories differ in method and cutoffs — some report up to 250 U/L — and children and adolescents have higher normal values. Read your result against the range on the report, and compare repeats only within the same laboratory.

There is no meaningful 'low LDH'. Values at or below the range have no clinical significance and are occasionally seen with high-dose vitamin C or in rare inherited enzyme variants. The test is interpreted at its upper end only.

Because so many tissues contain LDH, the list of causes is long. Ordered from most to least common in outpatients who get tested:

LDH's main modern role is in oncology, where it serves as a general index of how much rapidly dividing tissue is present. In lymphoma it is one of the five factors in the International Prognostic Index for diffuse large B-cell lymphoma and part of the staging work-up for Hodgkin and other lymphomas; a high LDH at diagnosis is associated with more aggressive disease. In testicular and other germ-cell tumors it is one of the three serum markers (with AFP and beta-hCG) that define the 'S' stage and guide treatment intensity and follow-up. In melanoma an elevated LDH upgrades stage IV disease and predicts a poorer response to some therapies.

In treatment, a rising LDH can signal tumor growth or recurrence and a falling one can reflect response; in very rapidly growing cancers or at the start of chemotherapy it also warns of tumor lysis syndrome, when large numbers of cancer cells die at once. It is never a screening test — the same rise occurs with hemolysis, exercise or a viral illness — and an isolated mildly raised LDH in someone who feels well is not a reason to fear cancer.

Start by asking whether the sample could have hemolyzed and whether anything obvious happened in the preceding days — a hard workout, an illness, an injury. If so, repeat the test in one to two weeks with a clean, rested draw; most mild elevations resolve. If it persists, the work-up follows the tissues that make LDH: a CBC with reticulocyte count, haptoglobin and bilirubin for hemolysis; a hepatic panel for liver disease; a CK for muscle; a review of symptoms such as fever, night sweats, weight loss, lumps or bone pain that would prompt imaging or a hematology referral.

In someone with a known condition — hemolytic anemia, lymphoma, a germ-cell tumor — LDH is followed as a trend, on the same assay, with the treating clinician setting what change matters. Day-to-day biological variation of LDH is small, so a sustained change of more than about 15–20% between well-collected samples is usually real.

LDH is a routine chemistry test at Quest and Labcorp, orderable through a clinician or directly through TestWell with physician review; results post within a few business days. Fasting is not required. To avoid a misleading result, skip strenuous exercise for 48 hours beforehand, do not test during an acute illness, and if the result is unexpectedly high, repeat it before drawing conclusions — hemolysis of the sample is the most common explanation for an isolated elevation. Because LDH rarely answers a question by itself, ordering it with a CBC and a hepatic function panel gives the context needed to interpret it.