Short answer
Protein C, protein S, and antithrombin help regulate clotting. Low activity or low levels can be inherited or acquired and may increase venous clot risk. These tests are not simple screening tests: acute clots, pregnancy, liver disease, inflammation, warfarin, heparin, direct oral anticoagulants, and timing can all affect results.
What each test measures
| Test | Why it is ordered | Interpretation caution |
|---|---|---|
| Protein C | Checks one natural anticoagulant pathway and may be used as an activity or antigen test. | Warfarin, acute thrombosis, and liver disease can lower results. |
| Protein S | Often paired with protein C in inherited thrombophilia workups. | Pregnancy, estrogen, inflammation, liver disease, vitamin K problems, and medicines can change results. |
| Antithrombin | Evaluates another natural anticoagulant that interacts with heparin biology. | Heparin exposure, acute thrombosis, liver disease, kidney protein loss, and other factors can affect results. |
When testing helps
Testing is most useful when it can change a real decision. That may include whether to continue anticoagulation, how to think about pregnancy, whether estrogen-containing contraception is a bad fit, or whether a known family variant should be checked in relatives.
- Unprovoked, recurrent, or unusual-site venous clotting
- Strong family history with a known inherited deficiency
- Pregnancy planning or postpartum counseling
- Evaluating a low result that may need repeat confirmation
What can distort results
If testing is done during an acute clot, during pregnancy, or while taking anticoagulants, abnormal results may reflect the situation rather than an inherited deficiency. Many clinicians delay or repeat testing when the result would change management. Genetic testing may be considered after abnormal functional testing or when a known family variant exists.
- Warfarin can lower protein C and protein S
- Heparin and acute thrombosis can complicate antithrombin interpretation
- Pregnancy, estrogen, inflammation, and liver disease can change protein S more than many people expect
- Kidney protein loss and vitamin K deficiency can also matter
What a low result can mean
A low result may point to an inherited deficiency, but it can also be acquired. That is why the answer is usually not just "positive or negative." It is "Does this result fit the clot history, the medicines, the timing, and the family pattern?"
If the result is low, clinicians often ask whether the test was done at the right time, whether the assay was functional or antigen-based, and whether repeat testing or family testing is the next step.
Questions to ask
- Was testing timed after recovery from an acute clot and with medication effects considered?
- Are these activity tests, antigen tests, genetic tests, or a mix?
- Could pregnancy, estrogen therapy, liver disease, kidney disease, warfarin, heparin, or DOACs affect my result?
- Would the result change anticoagulation duration, pregnancy planning, hormone decisions, or family testing?
Related guides: hereditary thrombophilia testing, Factor V Leiden testing, prothrombin G20210A testing, and PT/INR blood test.
FAQ
Are these genetic tests or blood activity tests?
They can be either, depending on the lab order. Some clinicians start with activity or antigen testing and then move to genetic testing if needed.
Can warfarin or heparin make the result look low?
Yes. Warfarin can lower protein C and protein S, and heparin can complicate antithrombin interpretation.
Should these tests be done right after a clot?
Usually not. Acute thrombosis can distort the picture, so timing matters and repeat testing is common.
Does a low protein S or antithrombin result always mean inherited disease?
No. Pregnancy, liver disease, kidney disease, inflammation, vitamin K issues, and medications can all create acquired low results.
Do normal results rule out clot risk?
No. They only rule out this specific pathway problem, not every inherited or acquired clotting risk.
When should family testing be considered?
Family testing is most useful when a known familial variant exists and the result would change decisions for relatives.
Related guides: hereditary thrombophilia testing, Factor V Leiden testing, Prothrombin G20210A testing, and antiphospholipid syndrome antibody testing.