Platform
Company
Coagulation
Review status
Currently under review
Pending specialist review and validation.
Protein S is a natural anticoagulant protein that helps keep clotting in balance. It works with activated protein C to turn off clotting factors so that unwanted clots do not form. In the bloodstream, some protein S is bound to another protein, and some is unbound or free. Only the free form is biologically active.
The Protein S Free test measures the amount of the free, active form in your plasma. It is often ordered along with tests for protein C, antithrombin, and other evaluations when your care team is assessing clotting risk or investigating why a clot occurred.
Having too little free protein S can increase the tendency to form blood clots in the veins, which can affect the legs, lungs, or other sites. The test helps your clinician evaluate whether a deficiency may be contributing to a past clot, a family history of clotting, or pregnancy‑related complications. It can also help differentiate inherited causes from temporary or acquired conditions that lower protein S.
Results may guide decisions about the duration of anticoagulation, the safety of estrogen‑containing medicines, and whether family members should consider evaluation. The test is also useful when liver function, vitamin K status, or other medical conditions could be affecting your clotting balance.
Your result is interpreted in context with your history, examination, and other laboratory findings. A value lower than expected may suggest a protein S deficiency, but temporary reductions can occur with an active clot, inflammation, pregnancy, liver disease, vitamin K deficiency, or certain medicines. In such situations, your clinician may repeat testing when you are well and off interfering treatments to confirm a persistent finding.
If your level is persistently low, additional tests such as total protein S, protein S activity, and sometimes genetic testing can help determine the cause and guide care. Higher than expected values are usually not clinically significant. Always discuss results and next steps with your clinician, who can decide whether any treatment changes or family counseling are appropriate.
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.
Vitamin K antagonists like warfarin lower protein S production, and vitamin K deficiency can do the same. Your clinician may defer testing or interpret results cautiously if you use these medicines or have poor vitamin K intake.
Free protein S naturally decreases during pregnancy and with estrogen‑containing contraceptives or hormone therapy. Testing is often postponed or repeated after pregnancy or after stopping estrogen, to avoid a misleading low result.
Active thrombosis, recent surgery, or systemic inflammation can transiently lower free protein S. Testing is ideally done when you are clinically stable and recovered, to reflect your baseline.
Protein S is made in the liver. Liver conditions can reduce production and lead to lower values, so liver function is considered when interpreting results.
Improper tube fill, delayed processing, or a clotted or hemolyzed specimen can affect results. Using the correct citrate tube and prompt centrifugation helps ensure accuracy.
Kidney disorders that cause protein loss in the urine can lower circulating protein S levels. Your clinician may evaluate kidney status if results are unexpectedly low.
Protein S levels can vary with age and hormonal state. Laboratories use method‑specific reference intervals, and your clinical context helps determine significance.
References