Our June, 2020 Quick Question, "What assay does your lab use as an initial protein S screen?" attracted 37 answers, implying that not many of us are testing routinely for protein S deficiency. Here are the answers:
- Total protein S antigen 3% 
- Free protein S antigen 43% 
- Clot-based protein S activity: 30% 
- Chromogenic protein S activity: 8% 
- Combination of protein S antigen and activity: 5% 
- We do not screen for protein S deficiency: 11% 
Dr. Mayukh Sarkar, Wisconsin Diagnostic Laboratories, working with the ASCLS Choosing Wisely committee has proposed this Choosing Wisely recommedation: “Do not order protein S activity or total protein S antigen. Instead order free protein S antigen.” He supports the recommendation with data comparing assay precision of the clot-base protein S activity results compared to free protein S antigen. Precision BioLogic's CRYOcheck Clot S assay reports intra-assay CV% of 2.3 for normals and 7.2 for abnormals. Their inter-assay precision CV%'s are 6.7 and 7.9, respectively. HYPHEN BioMed's ZYMUTEST Free Protein S Antigen immunoassay's Intra-assay CV% is ≤ 6% for samples of normal plasma) and ≤ 8% for samples of abnormal plasma, and their inter-assay CV is ≤ 10%, thus the assays are fairly close in precision.
Few use the total protein S antigen assay, particularly not as a stand-alone assay, as a percentage of plasma protein S circulates bound to the complement control, C4B binding protein, which is an acute phase reactant. In inflammatory conditions, C4B binding protein elevation reduces the proportion of free protein S, potentially affecting its function.
I know of no successfully marketed chromogenic substrate-based protein S assay, as protein S is not an enzyme.