LDH (lactate dehydrogenase): what it is, what it tells you and when it’s tested

LDH is an enzyme your cells use to make energy. It is found almost everywhere: red blood cells, liver, muscles, heart, lungs, kidneys. When cells are damaged, LDH leaks into your blood and rises. It says something has broken, but not where.

When it’s tested

Doctors mostly order it when they suspect red blood cells are breaking down too fast, to understand an anaemia, and to follow some diseases of the blood, liver or lungs. Oncologists use it to follow some cancers that are already known.

It is not a routine test for people who feel well. It rises for many different reasons, often harmless, and on its own it does not say where it comes from.

That is why Rosa does not test it. Instead it measures haemoglobin and the full blood count, bilirubin and liver enzymes, which show more clearly whether the blood or the liver is under strain.

What it tells you, and what it does not

If it is high, more cells than usual have broken. Sometimes that happens in the tube, after the blood draw: it is a frequent cause of a high LDH on its own.

It does not say which organ. To find out, your doctor looks at the other tests: with anaemia and high bilirubin they think of red blood cells, with high liver enzymes of the liver, with a high CK of the muscles.

A normal value is reassuring, but on its own it does not rule out a problem.

That is why it is read together with other tests:

• Haemoglobin: With anaemia, it points to red blood cells
• Bilirubin: It rises with LDH when red blood cells break down
• ALT and AST: If they rise too, the liver or muscles are involved
• Haptoglobin: If it is low, it confirms red cells are breaking down

Normal LDH levels

The lab decides what is normal. Many lab reports give 135 to 225 U/L. Other labs use slightly different limits, because the method differs. The range on your own report is what counts.

It is higher in children and teenagers, because they are growing. In adults it changes little with age.

The blood draw has to be done well: if red cells break in the tube, LDH rises without anything being wrong with you. The report sometimes flags this as a haemolysed sample.

What your result means

Find the row for your result, in U/L. Which values need a doctor? Above 225, if it stays high or you have symptoms.

• Below 135: Low. Usually it means nothing. A very low LDH for inherited reasons is rare. Usually nothing to do.
• 135 to 225: Normal. No tissue is releasing more of the enzyme than usual. Nothing to do.
• Above 225: High. Often it is the sample or a workout. Otherwise it comes from the blood, liver, muscles, lungs or other organs. If you feel well and it is only mildly high, repeat the test rested. If it stays high or you have symptoms, talk to your doctor.

If your LDH is high or low

• High LDH: Often it is the sample or a workout. Otherwise the other tests help explain it.
• Low LDH: Usually not a problem and nothing to do.

What can skew the result

A few things raise it without any damage. It helps to know them before your blood test.

• A haemolysed sample: If red cells break in the tube, from a difficult draw or a long journey to the lab, LDH rises. It is not your problem: the test is repeated.
• Training: Weights and long efforts raise it, together with CK, for several days. Avoid hard exercise in the week before.
• Very high platelets or white cells: Very high blood count values can raise it in the sample, even without damage.

When to worry

Talk to your doctor if:

• Your LDH is above your report’s limit and stays high when you repeat the test.
• It is high together with anaemia, high bilirubin or high liver enzymes.
• You feel tired and short of breath, your skin or eyes look yellow or your urine is dark.
• You have swollen glands that do not go away, fever or night sweats for no reason, or you lose weight without trying.
• You have chest pain or sudden shortness of breath: call 112 or go to the emergency department.

Frequently asked questions

What are normal LDH levels? Many lab reports give 135 to 225 U/L. The range on your own report is what counts, because methods differ.

What is LDH in a blood test? It is an enzyme found in almost every cell. It rises when cells break. It shows there is damage, not where.

Does a high LDH mean cancer? Usually not. The most common causes are a spoiled sample, a workout, anaemia or a liver problem. In some known cancers, oncologists use it to follow the disease, but it is not a way to find them.

Do I need to fast for an LDH test? Usually not, but it is often done with other tests that need fasting. Follow the lab’s instructions.

Does Rosa test LDH? No. Rosa measures haemoglobin, bilirubin and liver enzymes, which say a lot about your blood and liver. If your doctor wants LDH, they order it separately.

Sources

• Niguarda Hospital, Milan, laboratory test catalogue: lactate dehydrogenase, reference values. Ospedale Niguarda, esami di laboratorio
• Farhana and Lappin, biochemistry, lactate dehydrogenase. StatPearls, 2023. PMID 32491468
• Barcellini and Fattizzo, clinical applications of hemolytic markers in the differential diagnosis and management of hemolytic anemia. Disease Markers, 2015. PMID 26819490
• Pettersson and colleagues, muscular exercise can cause highly pathological liver function tests in healthy men. British Journal of Clinical Pharmacology, 2008. PMID 17764474