Lactate Dehydrogenase (LDH) Isoenzymes Test

The Lactate Dehydrogenase (LDH) Isoenzymes Test is a specialised blood investigation that goes beyond a simple total LDH measurement to identify which tissues are releasing this enzyme. LDH is found in almost every cell of the body, so a raised total LDH is a sensitive but non-specific signal. By separating LDH into its five distinct isoenzyme forms, this test helps clinicians localise the likely source of cellular injury or turnover, whether that is the heart, liver, skeletal muscle, red blood cells, or other tissues.

This test is typically requested when a total LDH result is elevated and the underlying cause is unclear. It is useful in evaluating suspected myocardial damage, haemolysis, liver disease, muscle injury, and certain cancers or lymphomas. It can also help monitor disease progression or response to treatment where LDH is known to be raised.

The test is performed using a simple blood sample, with isoenzyme fractions separated by electrophoresis and quantified to give a clear percentage and absolute value for each fraction.

LDH Isoenzyme Fractions
- Lactate dehydrogenase (LDH), total
- LDH-1 (H4): predominantly found in heart muscle and red blood cells; raised in myocardial injury and haemolysis
- LDH-2 (H3M): the main fraction in healthy serum; useful as a reference point
- LDH-3 (H2M2): found in lung tissue and lymphoid cells; may rise in pulmonary or lymphoproliferative conditions
- LDH-4 (HM3): present in liver, skeletal muscle, and kidney; raised in hepatic and muscular injury
- LDH-5 (M4): predominantly liver and skeletal muscle; the most sensitive fraction for hepatocellular damage

Why it matters
A raised total LDH tells you that cells are being damaged or turned over somewhere in the body, but it does not tell you where. The isoenzyme pattern adds that crucial layer of detail. For example, a rise in LDH-1 and LDH-2 with an inverted LDH-1 to LDH-2 ratio can point toward cardiac injury or haemolysis, while a predominant LDH-5 rise suggests liver involvement. This targeted information helps your doctor narrow the differential diagnosis, choose the right follow-up investigations, and avoid unnecessary tests. In short, LDH isoenzymes turn a vague signal into actionable clinical insight.

• Lactate dehydrogenase (LDH), total
• LDH-1 (H4)
• LDH-2 (H3M)
• LDH-3 (H2M2)
• LDH-4 (HM3)
• LDH-5 (M4)

No special preparation required.

For blood tests, we recommend drinking plenty of water beforehand to ensure smooth sample collection. If you are taking any regular medication, continue to do so unless advised otherwise by your doctor.