LDH (lactate dehydrogenase) in cats and dogs
Lactate dehydrogenase (LDH, sometimes written LD) is an enzyme found in many tissues — liver, heart, skeletal muscle and red blood cells. A high LDH says that cells somewhere released it, or that the sample itself was hemolysed, but not where it came from. On its own it rarely decides anything: vets rely on more specific tests.
What it shows
LDH converts lactate to pyruvate. It is not tissue-specific: it is found in a wide range of tissues. It exists in several forms (isoenzymes) whose mix differs between organs: in dogs and cats one form is found mostly in heart muscle and red blood cells, another in skeletal muscle and liver. Separating the forms could point to the source, but this is not routinely available, so a usual lab form shows only total LDH.
That makes LDH non-specific. Measuring it gives no additional information about muscle or liver disease beyond the tests used for them routinely — CK for muscle and ALT for the liver.
Higher than reference — what it may be a sign of
- Hemolysis of the sample — red blood cells of dogs and cats contain a lot of LDH. In dogs LDH rises in step with the hemolysis index, so a high value in a hemolysed sample may be due to hemolysis alone.
- Serum rather than plasma — cells release the enzyme during clotting, so serum shows higher activity than plasma; this is a common artifact in dogs.
- Exercise — a physiological rise.
- Muscle injury — LDH appears to increase with muscle damage in cats.
- Liver injury — some cats with liver injury have high LDH even with normal CK and no hemolysis.
- Some cancers, for example lymphoma in dogs.
Lower than reference
The sources describe no disease that lowers LDH in dogs or cats. The one known cause is technical: very marked lipemia — milky, fatty serum — can lower the measured activity. A low value is therefore not a finding in itself.
Look at together with
- CK — the routine muscle enzyme; it says more about muscle damage than LDH does.
- ALT — the routine marker of liver cell injury in dogs and cats; it says more about the liver than LDH does.
- The hemolysis note or index on the form — if the sample was hemolysed, a high LDH may tell nothing about the animal.
Units and conversion
LDH activity is reported in U/L on English forms and in МЕ/л or Ед/л on Russian ones — it is the same unit, and nothing needs converting. Values from different laboratories are read against each lab’s own interval. In the app each row keeps the unit printed on the form.
Reference range
The reference range is on your lab’s form. For LDH, how the sample was taken matters more than for most enzymes: serum gives higher values than plasma, and hemolysis raises the result. Canine serum keeps its total LDH activity for about four weeks in the fridge or freezer. LDH on its own rarely changes a decision: Merck calls it of very limited value in general clinical practice, and a vet reads a high LDH as a prompt to look at CK, ALT and the quality of the sample rather than as an answer.
Cats and dogs
Cats, like dogs, have a lot of LDH in red blood cells, so hemolysis distorts the result. Some cats with liver injury and cats with muscle injury have high LDH, but ALT and CK answer those questions more directly. In kittens LDH activity stays above adult values through about 8 weeks of age.
In dogs LDH correlates directly with the hemolysis index of the sample, and the higher activity of serum compared with plasma is a common artifact. LDH is increased in various malignancies in dogs, such as lymphoma, but high activity is also found in dogs without cancer, so its usefulness as a screen for hidden cancer is questioned.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.