LDH (lactate dehydrogenase) is an enzyme found inside almost every cell in the body, and it comes out into the blood when cells are damaged or break down. So when LDH is high, you can tell it’s “coming from cells somewhere,” but LDH alone can’t tell you where. It can also read high after a hard strength-training session, or when red blood cells break apart in the blood sample after it’s drawn, so it’s important to read it together with other markers.
On the other hand, LDH doesn’t often fall below the reference range (124–222 U/L in Japan’s shared reference intervals), and there are only a few situations where a low value is a medical concern. I explain this in “What a low LDH can mean.”
What LDH reflects
LDH is an enzyme that turns pyruvate (made when sugar is broken down) into lactate, and lactate back into pyruvate. This reaction uses a coenzyme called NAD, which is made from vitamin B3 (niacin). When energy production that uses oxygen can’t keep up, during hard exercise for example, LDH turns pyruvate into lactate so that the breakdown of sugar (glycolysis) can keep going.
LDH is found throughout the body — in the heart, liver, skeletal muscle, kidneys, lungs, red blood cells, and more — and even in healthy people, a little enters the blood as cells are naturally replaced. When many cells are damaged, LDH in the blood rises to match.
The reference range
In Japan’s shared reference intervals (JCCLS), the range for LD (LDH) is 124–222 U/L. Around 2020, Japan began aligning its measurement method with the international one (the IFCC method), but the reference range stayed the same. Ranges can differ a little from lab to lab, so check the range printed on your own results, too.
The detailed tables behind the shared reference intervals put the median for healthy adults at 165 U/L. Broken down by sex and age, the median and upper limit are 168 U/L and 225 U/L for men, 158 U/L and 208 U/L for women under 45, and 178 U/L and 240 U/L for women 45 and older. Among women, values tend to run higher from 45 on, around the time of menopause. Children, especially babies and toddlers, tend to have higher values than adults, so the adult reference range doesn’t apply to them as is.
The five isoenzymes
LDH comes in five forms with slightly different structures, called isoenzymes (LDH1 through LDH5). Each is built from four parts (subunits) of two kinds — the H type, plentiful in heart muscle, and the M type, plentiful in skeletal muscle and the liver — and the mix of the two determines which of the five forms it is. Which forms are most common differs from organ to organ.
| Form | Mainly found in | Examples of when it tends to rise |
|---|---|---|
| LDH1 (H4) | Heart, red blood cells, kidneys | Hemolysis; anemia related to low vitamin B12 or folate; heart muscle damage |
| LDH2 (H3M1) | Heart, red blood cells, kidneys | Situations similar to LDH1; blood disorders |
| LDH3 (H2M2) | Lungs and elsewhere | Lung disease; blood disorders |
| LDH4 (H1M3) | Liver and elsewhere | Liver damage |
| LDH5 (M4) | Liver, skeletal muscle | Liver damage; muscle damage from intense exercise |
Sources differ slightly on where each form is mainly found. In the blood of healthy people, LDH2 is the most plentiful, followed by LDH1 and LDH3, with LDH4 and LDH5 on the low side. Isoenzymes aren’t part of a typical checkup; they’re tested at a clinic or hospital when needed.
What a high LDH can mean
“High” alone doesn’t narrow down the cause, so let’s go through the possibilities, starting with the most everyday ones.
Red blood cells broke down in the sample
Red blood cells contain far more LDH than serum does. If red blood cells in the tube break down — because the draw took a long time, or because time passed before the sample was tested, for example — LDH will read high regardless of what’s going on in your body. Even hemolysis too slight to see with the naked eye has been reported to affect LDH values. A telltale sign is that potassium and AST also tend to read high; I go into this in more detail in the article on hemolysis.
Strength training and hard exercise
When strength training or long-distance running causes tiny tears in muscle fibers, LDH, CK (creatine kinase), and AST come out of the muscles into the blood. This is generally thought to be a temporary response to exercise. Muscles are said to be more easily damaged after exercise you’re not used to, especially movements where a muscle produces force while being stretched (such as running downhill or lowering a weight slowly).
LDH can rise days after exercise rather than right after it. In a study of slow weight-lowering exercise, it peaked 3 to 5 days later, and in a study where people who don’t usually do strength training did a session, it stayed high for at least 7 days. If your urine turns dark after severe muscle soreness, please see a doctor promptly.
Red blood cells breaking down in the body, or not being made properly
In hemolytic anemia, where red blood cells break down inside the body, indirect bilirubin rises along with LDH, and haptoglobin falls. And in megaloblastic anemia, caused by a lack of vitamin B12 or folate, immature red blood cells break down in the bone marrow, which can raise LDH considerably. One clue in this case is a higher MCV (larger red blood cells).
Damage to the liver, heart, lungs, and other organs
LDH rises when liver cells are damaged, as in hepatitis, but AST, ALT, and GGT reflect how the liver is doing more closely, so LDH is read alongside them. LDH also rises with a heart attack, but today the heart is checked mainly with tests such as troponin. LDH can also go up with lung disease, infections, and more.
Tumors
Many cancer cells rely heavily on turning sugar into lactate even when there’s plenty of oxygen (the Warburg effect), and they carry a lot of LDH. When cancer has advanced and spread, and many of these cells are multiplying or breaking down, LDH in the blood is more likely to read high. In medicine, it’s used for some cancers as one guide to the outlook. But because LDH rises for so many reasons, a high value alone can’t tell you whether cancer is present.
Macro-LDH
Some people have LDH that stays high even without any cell damage, because of “macro-LDH,” in which LDH is bound to antibodies (immunoglobulins) in the blood. It’s often harmless in itself, but since it can show up alongside other illnesses, it’s worth having a doctor check.
What a low LDH can mean
LDH doesn’t often fall below the reference range, and there are only a few situations where a low value is a medical concern. One known cause of values that stay clearly low is an inherited trait in which one of the subunits that make up LDH (the H type or the M type) is missing from birth. In a case of M-subunit deficiency reported by researchers in Japan in 1980, the person was recorded as having dark urine after exercise (from muscle breaking down and releasing myoglobin into the urine) and as tiring easily. H-subunit deficiency tends not to cause anything in particular that you’d notice and is mostly found through testing (there is also a report of a family in which hemolysis was seen).
On the other hand, you may come across explanations that link low LDH to “a sign that energy metabolism has slowed down.” But LDH on a blood test is the amount of the enzyme that has come out of cells; it doesn’t measure how much work the enzyme is doing inside them. So far, there isn’t enough evidence to say that energy metabolism is sluggish just because LDH is on the low side within the reference range.
How I read the numbers
Where you sit within the reference range
From a precision nutrition perspective, I look not only at whether a value falls within the reference range, but also at which end of the range it leans toward. For example, an LDH of 215 U/L is within the shared reference range, but compared with the overall median of 165 U/L, it leans well toward the top. Since the median differs a little by sex and age, I also compare it with the group closest to you. For a woman under 45, 215 U/L sits just above her group’s upper limit of 208 U/L. That alone doesn’t mean anything is wrong. I use it as a prompt to look at other markers for what might be pushing LDH up. That’s my own way of reading it, separate from medical standards.
For LDH, the precision nutrition optimal range I use in my counseling is 180–200 U/L (as measured by the current IFCC method). This is a precision nutrition guide, not a medical reference range. An LDH of 215 U/L sits above this range as well.
It’s also important to compare with the same person’s previous results. If someone who usually sits around 180 U/L comes in at 210 U/L, I read that as a change, even though it’s still within the reference range.
Reading it alongside other markers
Markers that move together with LDH can give clues to where it’s coming from.
| What moves with it | Possible background (examples) |
|---|---|
| CK is clearly high and AST is up (ALT too, after strength training), but GGT hasn’t moved | It may be coming from muscle (after exercise, for example) |
| AST and ALT are high, GGT is high too, and CK isn’t | It may be coming from the liver |
| Indirect bilirubin is high and haptoglobin is low | Red blood cells may be breaking down inside the body |
| Potassium and AST are also slightly high, with no other explanation | Red blood cells may have broken down in the tube (check with a retest) |
| MCV is high (large red blood cells), and LDH is clearly high | A lack of vitamin B12 or folate may be involved |
These are clues for reading the results, not a way to pin down the cause. Checking your exercise and the circumstances of your blood draw first can sometimes explain things. If you see a combination involving the liver or red blood cells, or if exercise and the blood draw don’t explain it, please have a doctor check.
As for low values, even when LDH falls below 180 U/L, the bottom of the optimal range, I don’t judge nutritional status from LDH alone. I look at how much you eat and at protein markers (such as albumin and BUN (blood urea nitrogen)) as separate items, without tying them to a low LDH.
What you can revisit in everyday life
The LDH reaction uses NAD, which is made from niacin. But blood tests measure LDH activity with NAD already added to the reagent, so the amount of niacin in your body isn’t directly reflected in your LDH value. There’s also no evidence so far that taking niacin can bring your LDH value into balance. What you can revisit, rather, is “how you’re tested” and “what may be pushing LDH up.”
- For at least 3 days before your test, hold off on hard strength training and long workouts. After exercises you’re not used to, or your first strength training in a while, leaving about a week gives you more peace of mind. If that’s hard to do, note when and what kind of exercise you did, and look back on it together with your results.
- When you compare with last time, keep things as similar as possible: the time of day, meals, how long it’s been since you exercised, and so on.
- Check whether your report includes a comment about “hemolysis.” If LDH, potassium, and AST are all slightly high together, hemolysis after the blood draw is a possibility, so talk with your doctor about a retest.
- Don’t ramp up your training too quickly, and make time for sleep and rest days.
- Look back on whether meat, fish, eggs, and dairy (which contain vitamin B12) or green vegetables and beans (which contain folate) have dropped out of your diet in a big way. People on a vegan diet with no animal foods, people who have had stomach or intestinal surgery, older adults, and people who have taken metformin or acid-reducing medicines for a long time are said to be more likely to run low on B12, and people who drink a lot of alcohol are more likely to run low on folate. Don’t stop any medicine on your own; talk with your doctor.
If you’re concerned
If your LDH is far above the reference range, stays high on repeat tests, or can’t be explained by exercise or the circumstances of your blood draw, please talk with a doctor.
If you notice shortness of breath or palpitations, yellowing of your skin or the whites of your eyes, dark urine, an unexplained fever or weight loss that won’t go away, or brown urine after severe muscle soreness, please see a doctor soon. If you’re already under a doctor’s care, please don’t stop any medicine on your own, and work things through together with your doctor.