Protein C Test: The Brake on Clotting That Warfarin Removes First

Warfarin lowers protein C. It blocks all the vitamin K–dependent proteins, and protein C is one of them, so anyone taking warfarin will have a low result whether or not they have a deficiency. Labcorp reports that protein C starts falling about six hours after the first dose and generally does not return to baseline until about two weeks after warfarin is stopped. An active or recent clot lowers it too, because protein C is used up while clotting is under way. MedlinePlus advises waiting until a person has recovered from a clot before testing.

A result of 52 percent drawn three weeks after a leg clot, on warfarin, tells you nothing about whether the deficiency is inherited. The same 52 percent on a sample taken well after recovery, two weeks off warfarin, is a different matter. Whether and when to pause an anticoagulant is the prescriber's decision. Labcorp's shorter washouts are about three days for heparin and the newer direct blood thinners.

The timing also explains a well-known warfarin precaution. Protein C has a half-life of about 8 hours, against about 60 hours for prothrombin (factor II), according to the warfarin prescribing information. In the first days of treatment the natural brake falls faster than the clotting factors, so the blood can briefly lean toward clotting.

That is why heparin is usually given alongside warfarin for the first 5 to 7 days, and why rare tissue necrosis, usually in the skin, at the start of warfarin has been linked to protein C or S deficiency. The prothrombin time and INR used to dose warfarin track the clotting factors, not protein C.

Acquired causes are more common than the inherited one, and clinicians rule them out first:

The inherited form comes from a change in the PROC gene. One altered copy is enough to cause mild deficiency, which MedlinePlus Genetics estimates affects about 1 in 500 people; many of them never have a clot. Two altered copies cause severe deficiency in newborns, about 1 in 4 million births. Separating the two forms usually takes a repeat test off anticoagulation and away from the clot, a protein C antigen test to see whether there is too little protein or protein that works poorly, and sometimes PROC gene testing.

The American Society of Hematology (ASH) 2023 thrombophilia guideline puts protein C deficiency in a median of 2.5 percent of people who present with a venous clot. In relatives of a known carrier it raises the chance of a first clot about sevenfold. In someone who has already had a clot, it roughly doubles the chance of another, a smaller effect than the word "deficiency" suggests.

Often not, according to the same guideline. It suggests against protein C testing after an unprovoked clot, because long-term anticoagulation is usually recommended anyway and a positive result would not change that plan. Its one strong recommendation is also against testing: women considering combined oral contraceptives should not be screened for thrombophilia as a group.

ASH does suggest testing in narrower situations: a clot provoked by pregnancy, the weeks after birth, combined oral contraceptives or another temporary non-surgical risk; a clot in the veins of the brain or abdominal organs when anticoagulation might otherwise stop; and relatives of someone with a known protein C, protein S or antithrombin deficiency who are weighing preventive treatment or estrogen.

Protein C activity is measured in two ways. Labcorp's test is chromogenic: an enzyme switches protein C on and the result is read from a colour change. Clot-based versions time a reaction similar to the aPTT and can read falsely low in people who carry factor V Leiden or have a high factor VIII, as happens during inflammation. A low result from a clot-based assay is worth checking against the method.

Protein C deficiency itself causes no symptoms. What people notice is a clot: swelling, pain, warmth or redness in one leg, or sudden breathlessness, chest pain or coughing blood when a clot reaches the lungs. The NHLBI lists these as reasons for urgent care, whatever any lab result says.