Clinical significance of immunoassays for type-5 tartrate-resistant acid phosphatase

Background: Tartrate-resistant acid phosphatase (TRAP; EC 3.1.3.2) is a pro duct of osteoclasts and a biochemical marker of bone resorption rate. Howev er, erythrocytes and platelets contribute to total TRAP activity in serum, reducing the specificity of direct biochemical assays in serum. Osteoclast TRAP is also known as type-5 TRAP and is antigenically unique. Immunoassays are sought to improve the specificity and sensitivity of TRAP as a bone ma rker. Methods: We developed two colorimetric microplate assays for typed TRAP: an enzyme capture immunoassay to measure antibody-bound enzymatic activity, a nd a two-site immunoassay to measure bound enzyme protein. Both use the sam e monoclonal antibody (14G6) to capture type-5 TRAP, which permits determin ation of specific activity of serum TRAP in health and disease. Results: Both TRAP assays were linear from one-tenth to fivefold the mean v alue in 18 healthy subjects. In these subjects, the mean (SD) TRAP activity was 3.2 (0.54) U/L for the enzyme capture assay and 37 (13) mu g/L for the two-site assay. Mean TRAP activity was not significantly increased in 64 p atients with endstage renal disease requiring hemodialysis (HD) or 99 unsel ected patients with rheumatic diseases. By contrast, TRAP protein was incre ased in both the HD and rheumatic disease groups. The specific activity of TRAP in the 17 of 64 HD sera that had increased TRAP activity (0.088 U/mu g ) was similar to that in healthy subjects (0.091 U/mu g). By contrast, the specific activity of TRAP in the 31 of 99 rheumatic sera with increased TRA P protein (0.035 U/mu g) was significantly decreased. Conclusions: Wide sample distributions for TRAP activity in HD patients and TRAP protein in rheumatic disease patients suggest the presence of subpopu lations of HD patients with increased TRAP activity and of rheumatic patien ts with increased TRAP protein. Each assay for TRAP activity and protein ma y have its own biological significance and clinical applications in specifi c groups of patients. (C) 1999 American Association for Clinical Chemistry.