High LDH (lactate dehydrogenase) on a blood test: causes and what happens next

Lab Tests Online UK describes lactate dehydrogenase (LDH, also written LD) as an enzyme found in almost all body tissues. The concentration in the blood is usually low because LDH stays contained inside the cells. When cells are damaged or destroyed they release LDH into the bloodstream and the blood level rises. For this reason LDH is used as a general marker of cell injury.

A total LDH is an overall measurement of five isoenzymes, slightly different forms of the same enzyme concentrated in different tissues. The site gives the general distribution: LDH-1 and LDH-2 mainly in the heart, red blood cells and kidney, LDH-3 in the lungs and other tissues, LDH-4 in white blood cells, lymph nodes, muscle and liver, and LDH-5 in the liver and muscle. It notes that measurement of individual isoenzymes is rarely available now.

According to Lab Tests Online UK, LDH is requested alongside other tests when a doctor suspects an acute or chronic condition causing tissue or cell destruction, and it may be repeated to monitor a known condition. It may occasionally be used to monitor damage from muscle injury and to help identify haemolytic anaemia, in which red blood cells break down faster than usual.

It was historically used with other tests to diagnose heart attacks, but the site says troponin has almost entirely replaced it for this, because troponin is more sensitive and specific for heart tissue injury. The NHS blood tests page explains in general terms that blood tests are done to check general health, to find out whether symptoms are caused by certain conditions or to see how well a condition is being managed.

Lab Tests Online UK lists the conditions in which LDH may be elevated. They fall into several groups, and because the enzyme is so widely distributed the list is long.

Lab Tests Online UK notes that many things can affect LDH results without being a cause for concern. Strenuous exercise can cause temporary elevations. Haemolysis of the blood specimen, meaning red cells broken in the tube because the sample was handled roughly, stored in extreme temperatures or difficult to collect, can give falsely high results. The site also notes that with some chronic and progressive conditions and some drugs, moderately elevated LDH levels may persist, and that low or normal levels do not usually indicate a problem.

This is why a single mildly raised LDH on a routine panel is usually repeated, often with other tests, before anything further is concluded. If you have just done a hard gym session or had a sample that was slow to reach the lab, tell the clinician.

Lab Tests Online UK says that if total LDH is raised the doctor may request LDH isoenzymes, or more commonly other tests such as ALT, AST or ALP, to help diagnose the condition and which organs are involved. In practice a GP will look at the rest of your blood results, ask about symptoms and medicines and often arrange a full blood count and liver and kidney tests.

Typical associated tests include a full blood count and blood film for anaemia or haemolysis, liver function tests, creatine kinase for muscle injury, and kidney function tests. If the picture suggests a blood or lymph node disorder, the NHS pathway involves referral for further assessment, including imaging or biopsy as decided by a specialist. The NHS describes a blood test as usually taking about 10 minutes and being done at a GP surgery or hospital.

Lymphoma and other cancers appear in the list of conditions that can raise LDH, and in specialist care LDH is sometimes used as one of the measurements to monitor some cancers. That does not make a raised LDH a cancer marker in everyday practice. Lab Tests Online UK is clear that a high total LDH by itself cannot be used to identify the underlying cause or its location, and most raised results turn out to have other explanations.

If you have a raised LDH together with symptoms such as unexplained weight loss, drenching night sweats, persistent fever or lumps in the neck, armpit or groin, see your GP promptly so these can be assessed.

A result alone is rarely an emergency, but symptoms are. Call 999 or go to A&E for chest pain, sudden breathlessness, signs of stroke such as face drooping, arm weakness or slurred speech, severe abdominal pain, or confusion. Contact your GP or NHS 111 promptly if a raised LDH comes with new yellowing of the skin or eyes, dark urine, extreme tiredness and pallor, or muscle pain with very dark urine.

LDH is part of some NHS hospital and GP biochemistry requests rather than a routine screening test. It is also included in some private health-check panels. Whichever route, interpret the figure against the reference range printed on your own report, because ranges differ between laboratories, and share an unexpected result with your GP so it can be put in context with your history.