Common Misconceptions in Pre-Analytical Variables and Result Interpretation of Protein S Activity Assay Kits (Clotting Method)

Key Impact of Specimen Collection and Processing on Coagulation Assay Results

The Protein S Activity Assay Kit (Coagulation Method) is highly sensitive to sample quality. The preferred blood collection tube is a vacuum tube containing 3.2% sodium citrate, and the ratio of blood to anticoagulant should be strictly 9:1. After blood collection, the tube should be gently inverted to mix, avoiding the generation of microbubbles, and plasma should be separated by centrifugation within 1 hour (centrifugation at 4°C for 15 minutes at 3000g or above). If testing cannot be performed promptly, plasma should be stored at -80°C and avoid repeated freeze-thaw cycles. It is noteworthy that the coagulation method detects the activity state; if improper specimen storage leads to consumption or activation of coagulation factors, it will alter the clotting time, thereby affecting the calculation of protein S activity. In addition, heparin contamination (e.g., from an indwelling needle) can inhibit the coagulation reaction, leading to prolonged clotting time and falsely elevated protein S activity; lipemic plasma can interfere with optical endpoint detection, and mechanical methods or optical correction modes may be used. Laboratories should strictly review samples upon receipt, send unsuitable specimens for rejection, and record the reasons for rejection. Standardized specimen processing is a prerequisite for obtaining reliable results.

Activity Fluctuations Under Physiological and Pathological Conditions

Protein S activity is not constant. In healthy populations, free protein S activity decreases slightly with age, is lower in women than in men, and in women during pregnancy (especially in late pregnancy) it can decrease by 35%–50%; oral contraceptives can also cause a decrease. In addition, acute inflammatory responses can temporarily reduce protein S activity through decreased hepatic synthesis and increased consumption. Anticoagulants such as warfarin markedly reduce protein S activity by inhibiting the synthesis of vitamin K-dependent factors, interfering with the diagnosis of congenital deficiency. Therefore, testing should be performed whenever possible when the patient is not on anticoagulation or 2 weeks after discontinuation of warfarin. If anticoagulation has already been administered in an emergency, factor II or X activity should also be measured to aid judgment. When interpreting results, it is necessary to combine them with protein S antigen testing (free protein S or total protein S): if activity is low but antigen is normal, it suggests a functional defect; if both are low, it suggests quantitative deficiency. For individuals with repeatedly low activity but no history of thrombosis, temporary interfering factors should be excluded before making a diagnosis.

Differences Among Methodologies and Consistency of Clinical Results

The coagulation method is currently the most commonly used method for protein S activity testing in clinical practice, but kits from different manufacturers may use different activators (such as snake venom or phospholipids) and APC sources, resulting in limited comparability of results. For example, the selectivity of some kits for protein S may be interfered with by protein C resistance phenomena (such as the FV Leiden mutation), making them insensitive to APC, with insufficient prolongation of clotting time, leading to falsely low protein S activity. For individuals with a known FV Leiden mutation, modified reagents that are insensitive to APC resistance should be selected, or the chromogenic substrate method should be used instead. In addition, laboratories should establish their own reference intervals, with particular attention to regional and ethnic differences. When using the reference range provided by the manufacturer, small-sample validation (at least 20 healthy adults) is required. When clinicians judge protein S deficiency, they should recommend repeat testing 2–3 times to exclude temporary interference, and simultaneously measure protein C, antithrombin, lupus anticoagulant, and factor V Leiden genetic testing, etc., to clarify the cause of thrombophilia. Through combined analysis of multiple items, the specificity and accuracy of thrombophilia diagnosis can be improved.