Platform
Company
Coagulation
Review status
Currently under review
Pending specialist review and validation.
Protein S is a natural anticoagulant protein that works with activated protein C to help keep blood clotting in balance. In the bloodstream, much of protein S is bound to another protein, while the unbound or free portion is the form that is biologically active.
The Protein S Free Preliminary test measures the free, active fraction in your plasma as an initial screen. It helps your care team assess whether a low level of free protein S might be present, which can be inherited or acquired due to other health conditions or treatments.
Low free protein S can reduce your body’s ability to control clot formation and may be linked with a tendency to develop blood clots in the veins. Clinicians may order this test if you have had a clot without a clear trigger, a strong family history of clots, or when evaluating conditions that can affect clotting balance.
This test is often part of a thrombophilia workup alongside tests for protein C, antithrombin, and antiphospholipid antibodies. Results can also inform planning around pregnancy, surgery, or hormone therapies when clotting risk needs careful consideration.
Your clinician will interpret your result in the context of your health history, medications, and timing of testing. If your result is low, they may look for temporary influences such as a recent clot, acute illness, pregnancy, or certain medicines, and may repeat testing when you are well and off interfering therapies if safe to do so.
Follow-up testing can include confirmatory assays, such as total protein S antigen or functional activity, to help determine the cause and classification. A normal result makes a significant inherited deficiency less likely, but decisions about prevention, treatment, or family testing depend on your overall risk and clinical situation.
Reference intervals vary by laboratory, analyzer, methodology, population, and units. The ranges shown here are for education only. Always interpret your results against the reference interval printed on your own lab report.
Recent venous thrombosis, surgery, or acute illness can temporarily lower free protein S. Testing is often deferred or repeated after recovery to avoid misclassification.
Warfarin and vitamin K deficiency can lower protein S levels, and some anticoagulants can affect functional assays. Your team may adjust or time testing around medications when feasible.
Pregnancy, the postpartum period, and estrogen-containing therapies can reduce free protein S. Results should be interpreted with awareness of these physiologic changes.
Liver disease and significant inflammation can decrease protein S production or alter binding proteins, which may lower the measured free fraction.
Proper citrate tube fill, prompt processing, and avoiding hemolysis are important. Pre-analytic issues can lead to inaccurate results or require repeat sampling.
Inherited protein S deficiency can be quantitative or qualitative. Family history and confirmatory assays help clarify whether a genetic cause is likely.
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