LDH (lactate dehydrogenase)

LDH (lactate dehydrogenase) is an enzyme found in the cytoplasm of almost all body cells. Red blood cells, heart and skeletal muscle, liver, kidneys and lungs contain particularly large amounts. There are five subtypes (isoenzymes), distributed differently depending on the tissue. Because LDH is so widespread, the value indicates cell breakdown in general and is interpreted together with other values. Total LDH is measured in serum together with the other liver values; the blood sample is taken in a practice.

What LDH does in the body

LDH converts pyruvate, the end product of sugar breakdown, into lactate and back again. In the process the helper substance NADH is converted back to NAD⁺. This is crucial when little oxygen is available – for example in hard-working muscle or in red blood cells, which have no mitochondria: only when NAD⁺ is replenished can sugar breakdown continue and supply energy. In the liver and heart the reaction preferably runs the other way, so that lactate becomes pyruvate again and can be used for energy production or for making new glucose.

Low LDH – what happens in the body?

A low LDH value shows that few cells are being lost and generally has no disease meaning. Very rarely there is an inherited deficiency of one of the two LDH subunits. If the muscle form is missing, the muscle cannot replenish enough NAD⁺ during heavy exertion; sugar breakdown stalls, leading to reduced exercise tolerance, muscle pain and sometimes breakdown of muscle fibres. In people without such complaints, a low value speaks against increased cell damage.

Raised LDH – what happens in the body?

Because almost every cell contains LDH, the value rises whenever more cells are lost – it therefore shows cell breakdown, but not where. Common sources are increased breakdown of red blood cells (haemolysis), muscle damage after intense exertion or injury, liver inflammation and damage to the heart muscle or lungs, for example with infections. With a marked vitamin B12 or folate deficiency, immature red blood cells are already lost in the bone marrow, which can raise LDH considerably. Diseases with high cell turnover also raise it. If the blood sample is stored too long or shaken hard, red blood cells break down in the tube and mimic a raised value.

The interpretation is given in your report.