D-dimer and fibrin degradation products (FDP) in cats and dogs

What it shows

After a clot has done its job, plasmin cuts it into fragments. The two tests catch different fragments:

• D-dimer forms only from fibrin that thrombin and factor XIII have cross-linked, that is, from a real clot. That makes it more specific for clot breakdown. Most laboratories now use it instead of FDP.
• FDP (fibrin and fibrinogen degradation products) — fragments of both fibrin and fibrinogen. Classic FDP tests cannot tell which of the two was broken down, so FDP can rise even without a clot.

Many D-dimer tests are latex agglutination: run undiluted they give positive or negative, run at several dilutions they give a semi-quantitative number. A number is more useful, because higher values may point more specifically to thromboembolism.

Higher than reference — what it may be a sign of

• DIC (disseminated intravascular coagulation) — the typical cause. D-dimer may rise in early DIC before PT and APTT change, and in dogs it is sensitive for DIC (75–100%). DIC is diagnosed from three or more abnormal tests together with an underlying disease; there is no single test for it.
• Thrombosis and thromboembolism — for example, a clot in the lungs. In dogs with confirmed thromboembolism D-dimer is often above 1000 ng/mL.
• Bleeding into a body cavity — in dogs D-dimer was highest in bloody effusions, for example with a bleeding tumour (hemangiosarcoma) or rat poison, and higher in the abdominal fluid than in the blood: clots form and dissolve inside the cavity.
• Liver disease — the liver clears these fragments more slowly.
• Inflammation and sepsis, wound healing, sometimes surgery.

Lower than reference

A value inside the range makes active clot breakdown less likely, but a negative FDP does not rule out DIC, thrombosis or severe internal bleeding. In cats a normal D-dimer is even less reassuring: in cats with diseases that lead to DIC it may be raised in only about half.

Look at together with

• Platelets, PT, APTT and fibrinogen — the set used to judge DIC. A markedly high D-dimer with low fibrinogen suggests clotting proteins are being consumed.
• Antithrombin — low activity is one of the more sensitive DIC tests in dogs.
• Red cell fragments (schistocytes) on the blood smear — another sign looked for in DIC.

Units and conversion

D-dimer is usually reported in ng/mL; some labs use mg/L or µg/mL. One mg/L equals one µg/mL, which equals 1000 ng/mL, so 0.25 mg/L is 250 ng/mL. The chart converts these to one unit. Some in-clinic tests give only positive or negative.

FDP is reported in µg/mL, often as a step rather than an exact number — for example “< 5”, “5–20” or “> 20” for the plasma test.

Reference range

The reference range is on your lab’s form. It depends on the method. For illustration, a teaching source gives D-dimer below 250 ng/mL for dogs and cats, while one reference coagulation laboratory lists 0–575 ng/mL for dogs and 0–250 ng/mL for cats. For the plasma FDP test used in dogs, values above 5 µg/mL are abnormal.

How the citrate sample is collected is described in the article on PT. The serum FDP test needs its own special tube with additives, supplied by the lab.

Cats and dogs

Sources

1. eClinPath (Cornell University College of Veterinary Medicine). Hemostasis — Fibrinolysis.
2. eClinPath (Cornell University College of Veterinary Medicine). Hemostasis — DIC.
3. eClinPath (Cornell University College of Veterinary Medicine). Hemostasis — Sample collection.
4. Center SA. Coagulation Tests in Hepatic Disease in Small Animals. Merck Veterinary Manual, professional version, updated November 2024.
5. McQuinn E. Coagulation Protein Disorders in Animals. Merck Veterinary Manual, professional version, updated March 2026.
6. Cornell University Animal Health Diagnostic Center, Comparative Coagulation Laboratory. Assay Reference Intervals.