LDH

Every cell in your body makes energy, and when there is briefly too little oxygen, it switches to an emergency route in which LDH, lactate dehydrogenase, converts pyruvate into lactate. That is why the enzyme sits in almost every tissue: muscles, heart, liver, kidneys, lungs, and in large amounts in your red blood cells. If cells break down somewhere, LDH leaks into your blood, and the value rises. That makes LDH a broad alarm that points at no organ. There are five variants of the enzyme that differ per tissue, and those can be determined separately, but what stands on an ordinary report is the sum. The meaning comes from the company: high AST and CK alongside is muscle, high bilirubin and many reticulocytes alongside is breakdown of red cells, high ALT alongside is the liver. And one practical point colours everything: a red blood cell contains roughly fifty times as much LDH as the blood plasma. If cells break in the tube, with a difficult draw, you mainly measure that. Your report shows LDH in U/l. The national reference value runs up to 225 U/l, and the upper limit of your own lab usually sits slightly higher, around 250, and rises with age at some labs. A slightly raised value with an otherwise normal blood count is rarely the trail that takes you further.

For anyone who does sport, LDH is first of all a training marker. After a hard strength session, a long run or a race, LDH, CK and AST sit above the band for a few days, because muscle fibres break down and repair with exertion. That is not damage to worry about, it is the reason you get stronger. Anyone who knows that has blood drawn on a rest day; anyone who does not reads a liver work-up that leads nowhere. The second reason is breakdown of red blood cells. In haemolysis, from antibodies, an inherited cell shape or a faulty heart valve, LDH rises together with bilirubin and the reticulocytes, while haemoglobin falls and haptoglobin disappears. That is a picture LDH makes visible first, and in runners it happens in a small way with every footstrike. LDH also rises with a deficiency of vitamin B12 or folate, because the marrow then makes oversized cells that already break down in the marrow. The third reason is what LDH is not. It is used in some forms of cancer to follow the course, because fast-growing tissue releases a lot of LDH, and that makes people afraid of a raised value. But in someone without a diagnosis a raised LDH is almost never cancer and almost always muscle, draw or red cells. It is a measure of course beside a diagnosis that is already established, not a test to find one.

A high LDH means cells have broken down somewhere, and the first question is whether that happened in the tube. If the draw was difficult, red cells have broken and you measure their contents; usually 'haemolysis' is then noted beside the result, and potassium is falsely high too. After that, in order of how often it occurs: muscle damage after exertion, liver damage, breakdown of red blood cells, and rarely tissue damage elsewhere, such as a heart attack, a pulmonary embolism or fast-growing tumour tissue. AST and CK point at muscle, ALT at the liver, bilirubin and reticulocytes at red cells. A low LDH means nothing; it is simply little cell breakdown, and nothing belongs with it. Never read LDH alone, but always beside AST, CK, bilirubin, the reticulocytes and your haemoglobin. Have blood drawn on a rest day, and compare over time at the same lab, because the upper limits differ.