Diagnosis and follow-up of bone metastases in breast cancer patients usually rely on symptoms and imaging studies. Tartrate-resistant acid phosphatase 5b (TRACP 5b) is a specific marker of osteoclasts and is herein proposed as a marker of bone metastasis in breast cancer patients. An immunoassay using a monoclonal antibody, 14G6, was used to measure the activity of serum TRACP 5b at pH 6.1 in 30 early breast cancer patients without bone metastasis and in 30 aged-matched breast cancer patients with bone metastasis. Another 60 normal volunteers were recruited as controls. Bone alkaline phosphatase (BAP), a traditional marker of bone turnover, was also measured in selected cases. The overall mean TRACP 5b activity in normal women was 2.83 ± 1.1 U/l, and it increased with age. The mean TRACP 5b activity in early breast cancer patients did not differ from that of the normal group (2.93 ± 0.64 vs. 2.83 ± 1.1 U/l; p = 0.66), whereas it was significantly higher in breast cancer patients with bone metastasis (5.42 ± 2.5 vs. 2.83 ± 1.1 U/l; p < 0.0001). BAP activity was significantly higher in breast cancer patients with bone metastasis than in early breast cancer patients (p = 0.004). Serum TRACP 5b activity correlated well with BAP activity in breast cancer patients with bone metastasis (p < 0.0001), but not in normal individuals or in patients without bone metastasis. TRACP 5b activity can be considered a surrogate indicator of bone metastasis in breast cancer patients.
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