Protein C activity in cats and dogs

What it shows

Clotting needs both an accelerator and brakes. When thrombin binds to a receptor (thrombomodulin) on the inner lining of vessels, it activates protein C. Activated protein C, with its helper protein S, switches off two clotting factors, Va and VIIIa, and keeps the clot from spreading. It also has anti-inflammatory and cell-protecting effects.

Blood from the intestines normally reaches the liver through the portal vein. In a portosystemic shunt part of this blood bypasses the liver, the liver makes less protein C, and activity falls — so the test works as an indirect, non-invasive measure of portal blood flow. It is a functional test: protein C in the plasma is activated, and the rate of a colour reaction shows how active it is.

Higher than reference — what it may be a sign of

After a congenital shunt is closed surgically, a rise in protein C suggests that portal blood flow to the liver is restored, even when bile acids stay high.

Lower than reference

• Portosystemic shunt — activity below 70% is typical of dogs with signs of shunting. In one study, 88% of dogs with congenital or acquired shunts were below 70%.
• Microvascular dysplasia (MVD), now also called portal vein hypoplasia: activity is usually 70% or more (30 of 35 dogs in the same study), so protein C helps tell MVD from a shunt when bile acids are high in both.
• Severe liver disease — below 50% indicates severe liver injury; activity is lowest in liver failure and can drop early after exposure to liver toxins.
• Vitamin K deficiency or antagonism — blocked bile flow, anticoagulant rodenticide poisoning. Because protein C lasts only hours, its activity can fall before routine clotting tests change.
• Sepsis, inflammation and DIC — protein C is used up and activated less; some dogs with bacterial sepsis have low activity.

Look at together with

• Bile acids — high with both shunts and MVD; protein C helps tell them apart.
• Ammonia — part of the same liver blood-flow work-up as bile acids.
• Cholesterol and MCV — low values together with protein C below 70% further support a shunt.
• Antithrombin and fibrinogen — other liver-made clotting proteins; low values of all three together are linked to a more severe course.
• PT and aPTT — clotting tests that change later with vitamin K problems.

Units and conversion

Protein C activity is reported in percent (%) of a standard plasma pool from healthy animals of the same species, taken as 100%. Forms may write “protein C”, PC or “protein C activity”. There is nothing to convert; in the app each row keeps the unit from the form.

Reference range

The reference range is on your lab’s form; the 70% and 50% points above are guides from the Cornell coagulation laboratory for reading a low value in dogs, not a replacement for the lab’s interval. A single result is read together with bile acids, imaging and the clinical picture.

Only citrated plasma is valid; serum is not. Hemolysis suggests a sampling problem, and the lab may ask for a new sample. Frozen plasma kept its activity for 7 days but lost some by 28 days, so delays matter.

Cats and dogs

Sources

1. eClinPath (Cornell University College of Veterinary Medicine). Hemostasis — Protein C.
2. Cornell University Animal Health Diagnostic Center, Comparative Coagulation Laboratory. Protein C (clinical topic).
3. Center SA. Coagulation Tests in Hepatic Disease in Small Animals. Merck Veterinary Manual, full review August 2023.
4. Cornell University Animal Health Diagnostic Center, Comparative Coagulation Laboratory. Sampling Instructions.