Case report: a novel form of free protein S deficiency in an HIV-positive patient on hemodialysis.

Investigation of repetitive thrombotic episodes in an HIV-positive patient on maintenance hemodialysis revealed extraordinarily high levels of total (free plus protein bound) protein S antigen but severe reductions in free (interactive) protein S antigen. Patients with the nephrotic syndrome or chronic renal failure have shown elevations of total protein S antigen, associated with increased levels of the specific C4b-binding protein, yet have had normal free protein S antigen levels. Based on the fall in total and free protein S levels during hemodialysis treatments, the presence of normal levels of C4b-binding protein, and the coexistence of a polyclonal gammopathy, the authors infer the presence of a unique protein that binds protein S in this patient and that is not apparent in normal plasma. The anomalous association of protein S with this binding protein resulted in levels of free protein S antigen in the range of those reported in hereditary deficiencies of the anticoagulant.