This post is part of a series on bone markers standardization from the Committee on Bone Metabolism of the International Federation of Clinical Chemistry.
Serum acid phosphatase (ACP) isoenzymes (ACP 1 to ACP 5) can be separated by polyacrylamide gel electrophoresis. ACP 5, or tartrate-resistant acid phosphatase (TRACP), was first identified in human leukocytes and has been widely used for many years as a cytochemical marker for osteoclasts and their precursors . TRACP has since been found in lung, spleen macrophages, dendritic cells, activated macrophages and cells belonging to the osteoclast/macrophage lineage. There are 2 isoforms of TRACP-5, termed 5a and 5b. TRACP-5b is derived from osteoclasts, whereas TRACP-5a is derived from other sources, such as macrophages . The difference between TRACP-5a and 5b is characterized by the post-translational modification of each derived cell.
TRACP-5b is cleared by the liver and the concentrations are not influenced by chronic kidney disease nor food intake . TRACP-5b also presents a weak diurnal variation and a low intra-individual variability , leading to a Least Significant Change of 24% in hemodialyzed patients. TRACP-5b is very stable in serum: samples can be stored at least 2 days at room temperature, 3 days in the refrigerator and up to one month at -20°C. At -70°C, TRACP-5b is stable for years. It is however not recommended to refreeze samples after thawing TRACP-5b looses activity rapidly.
Two different immunoassays for TRACP-5b are present on the market. The first one is an ELISA immunoassay, commercialized by Nittobo Medical (Tokyo, Japan) and the second one is the IDS iSYS automated immunoassay or ELISA (Immunodiagnostic Systems, Boldon, UK). The Nittobo assay is a fragment absorbed immunocaptured enzymatic assays which uses two monoclonal antibodies (anti active TRACP-5b and anti-inactive TRACP-5b antibodies) which enables highly specific TRACP-5b measurement without cross-reactivity with TRACP-5a derived from the macrophages. The TRACP-5b assays from IDS involve the capture of both TRACP-5 forms by a biotinylated monoclonal antibody. Specificity for TRACP-5b is obtained by measuring enzyme activity using the substrate at the optimal pH, specifically for type 5b.
TRACP-5b is rather a marker of osteoclasts number than a marker of osteoclasts activity 
According to these interesting features, TRACP-5b may be useful in different clinical situations such as osteoporosis, where it is the maker recommended by the Japan Osteoporosis Society  and in monitoring bone turnover in CKD, renal transplanted and hemodialyzed patients [6,7]. In hemodialyzed patients, a TRACP-5b concentration ≤4.6 U/L was shown to be able to discriminate low form non low bone turnover with a sensitivity of 89% and a specificity of 71% .
TRACP-5b has also been used in different predictive models: bone mineral density (BMD) for up to 2 years following zoledronate administration  and hypocalcemia following Denosumab administration in hemodialysis osteoporotic patients .
Finally, TRACP-5b has shown to be elevated in bone diseases like post-menopausal osteoporosis, bone metastasis, multiple myeloma, Paget’s disease of bone and fracture risk prediction .
Due to its elegant intrinsic characteristics, TRACP-5b is a very interesting maker for bone turnover monitoring.
- Halleen JM, Ylipahkala H, Alatalo SL, Janckila AJ, Heikkinen JE, Suominen H, et al. Serum tartrate-resistant acid phosphatase 5b, but not 5a, correlates with other markers of bone turnover and bone mineral density. Calcif Tissue Int 2002;71:20–5. doi:10.1007/s00223-001-2122-7.
- Hannon RA, Clowes JA, Eagleton AC, Al Hadari A, Eastell R, Blumsohn A. Clinical performance of immunoreactive tartrate-resistant acid phosphatase isoform 5b as a marker of bone resorption. Bone 2004;34:187–94. doi:10.1016/j.bone.2003.04.002.
- Cavalier E, Delanaye P, Moranne O. Variability of new bone mineral metabolism markers in patients treated with maintenance hemodialysis: implications for clinical decision making. Am J Kidney Dis 2013;61:847–8. doi:10.1053/j.ajkd.2012.12.013.
- Chu P, Chao TY, Lin YF, Janckila AJ, Yam LT. Correlation between histomorphometric parameters of bone resorption and serum type 5b tartrate-resistant acid phosphatase in uremic patients on maintenance hemodialysis. Am J Kidney Dis 2003;41:1052–9. doi:10.1016/S0272-6386(03)00203-8.
- Nishizawa Y, Miura M, Ichimura S, Inaba M, Imanishi Y, Shiraki M, et al. Executive summary of the Japan Osteoporosis Society Guide for the Use of Bone Turnover Markers in the Diagnosis and Treatment of Osteoporosis (2018 Edition). Clin Chim Acta 2019. doi:10.1016/j.cca.2019.08.012.
- Shidara K, Inaba M, Okuno S, Yamada S, Kumeda Y, Imanishi Y, et al. Serum levels of TRAP5b, a new bone resorption marker unaffected by renal dysfunction, as a useful marker of cortical bone loss in hemodialysis patients. Calcif Tissue Int 2008;82:278–87. doi:10.1007/s00223-008-9127-4.
- Evenepoel P, Claes K, Meijers B, Laurent MR, Bammens B, Naesens M, et al. Natural history of mineral metabolism, bone turnover and bone mineral density in de novo renal transplant recipients treated with a steroid minimization immunosuppressive protocol. Nephrol Dial Transplant 2018:1–9. doi:10.1093/ndt/gfy306.
- Salam S, Gallagher O, Gossiel F, Paggiosi M, Khwaja A, Eastell R. Diagnostic Accuracy of Biomarkers and Imaging for Bone Turnover in Renal Osteodystrophy. J Am Soc Nephrol 2018:ASN.2017050584. doi:10.1681/ASN.2017050584.
- Mori Y, Kasai H, Ose A, Serada M, Ishiguro M, Shiraki M, et al. Modeling and simulation of bone mineral density in Japanese osteoporosis patients treated with zoledronic acid using tartrate-resistant acid phosphatase 5b, a bone resorption marker. Osteoporos Int 2018;29:1155–63. doi:10.1007/s00198-018-4376-1.
- Kunizawa K, Hiramatsu R, Hoshino J, Mizuno H, Ozawa Y, Sekine A, et al. Denosumab for dialysis patients with osteoporosis: A cohort study. Sci Rep 2020;10:2496. doi:10.1038/s41598-020-59143-8.