Internal Medicine II Practice Test

Osteoblasts are the cells that form new bone, and they produce osteocalcin, a non-collagenous protein that is released into the bloodstream during bone formation. Because osteocalcin production directly tracks osteoblast activity, its level in serum is a reliable marker of bone formation and osteoblast function. By contrast, compounds measured in urine like pyridinoline cross-links and hydroxyproline come from collagen breakdown and mainly reflect bone resorption rather than formation. Serum phosphatase (if meaning serum phosphate or non-specific alkaline phosphatase) is less specific for osteoblast activity, whereas osteocalcin is more directly tied to osteoblast-driven bone formation. Thus serum osteocalcin best indicates osteoblast activity.