A case of protein S-specific activity triggers detection with potential thrombosis development.

Protein S gene abnormalities are the most common congenital predisposition to thrombophilia in the Japanese population, but not in the Caucasian population. It is important to measure protein S activity, specific activity, and antigen levels in patients with thrombophilia. A 52-year-old man presented with suspected deep vein thrombosis after previously visiting an orthopedic clinic with right lower extremity swelling after his long-distance walk 5 days prior. His D-dimer level was elevated, and a thrombus was found from the femoral to the below-the-knee veins on ultrasonographic echography. Anticoagulation therapy was initiated. His parents had previously been diagnosed with deep vein thrombosis; therefore, he was examined for thrombophilic predisposition. The results showed normal protein S activity and antigen levels, but decreased protein S-specific activity. Genetic testing revealed the presence of a protein S variant (protein S Tokushima). In addition to measuring protein activity and antigen levels, protein S-specific activity measurements can reveal increased risks of thrombosis.