High amounts of tartrate-resistant acid phosphatase (TRACP) are expressed by bone-resorbing osteoclasts and
activated macrophages. Two forms of TRACP circulate in blood, known as TRACP 5a and TRACP 5b.
TRACP 5b is derived from osteoclasts and TRACP 5a from inflammatory macrophages. Osteoclasts secrete TRACP 5b into the blood circulation as an active enzyme that is inactivated and degraded to fragments before it is removed from the circulation. Thus, TRACP 5b activity does not accumulate into the circulation in renal or hepatic failure. Diurnal variability of serum TRACP 5b is low and the levels are not affected by feeding, allowing sample collection at any time of day.
Recent studies have shown that secreted TRACP 5b indicates the number of osteoclasts rather than their activity.