Multicenter evaluation of three commercial methods for measuring protein S antigen

Area of Science:

• Clinical Chemistry
• Immunology
• Coagulation Diagnostics

Background:

• Protein S (PS) is a critical vitamin K-dependent plasma protein involved in regulating coagulation.
• Accurate measurement of PS antigen is essential for diagnosing thrombotic disorders and managing anticoagulant therapy.
• Commercial immunoassays are widely used for PS quantification, but their performance characteristics require thorough evaluation.

Purpose of the Study:

• To evaluate the performance of commercial methods for protein S (PS) antigen measurement.
• To assess interlaboratory reproducibility and identify factors affecting assay accuracy.
• To compare the sensitivity and specificity of different immunoassay platforms.

Main Methods:

• A collaborative study involving five laboratories analyzing 25 plasma samples.
• Blind testing of local methods alongside three commercial assays: Asserachrom-PS (ELISA), Asseraplate-PS (EIA), and Rellplate-S (EIA).
• Evaluation of reproducibility (interlaboratory CVs), specificity (influence of C4b-binding protein), and sensitivity (lower detection limits).

Main Results:

• Interlaboratory coefficients of variation ranged from 15.4% to 25.3% across methods.
• High C4b-binding protein levels led to underestimation of PS antigen, particularly with ELISA, which was mitigated by overnight incubation.
• ELISA demonstrated higher sensitivity (detecting 4 U/dl) compared to EIAs (14 U/dl), though all methods correctly identified a PS-deficient sample.

Conclusions:

• Commercial immunoassays for protein S antigen exhibit significant interlaboratory variability.
• Standardization of PS immunoassays is necessary to improve accuracy and reduce variability.
• Further standardization efforts are required before a candidate plasma standard can be reliably calibrated.