[Congenital protein S deficiencies; diagnostic difficulties]

Area of Science:

• Hematology
• Biochemistry
• Immunology

Context:

• Protein S (PS) is a vital anticoagulant cofactor for activated protein C (APC), primarily inhibiting factors Va and VIII.
• While free PS is essential for APC cofactor activity, a significant portion binds to C4b-binding protein (C4bBP).
• Existing diagnostic methods for Protein S deficiency include immunologic assays for free and total PS and functional assays measuring APC cofactor activity.

Purpose:

• To evaluate the limitations of current diagnostic assays for Protein S deficiency.
• To explore the direct anticoagulant activities of free and bound Protein S, independent of APC.
• To address the diagnostic challenges posed by overlapping normal and pathological values and interferences in functional assays.

Summary:

• Functional assays for Protein S deficiency may underestimate its anticoagulant capacity, as they primarily assess APC cofactor activity and can be affected by various interferences.
• Free Protein S exhibits direct anticoagulant properties independent of APC, and even Protein S bound to C4bBP may possess anticoagulant effects.
• Immunologic assays are generally more reliable but may fail to detect rare qualitative Protein S deficiencies. Familial studies are crucial for confirming hereditary origins due to acquired deficiencies.

Impact:

• Recommends measuring free Protein S, ideally with APC cofactor activity, as the primary diagnostic approach.
• Highlights the need for improved diagnostic strategies that account for the multifaceted anticoagulant roles of Protein S.
• Emphasizes the clinical significance of understanding both free and bound Protein S anticoagulant functions for accurate diagnosis and management of thrombophilia.