Which statement about alkaline phosphatase is accurate in the context of bone turnover?

• A. It directly measures bone mineral density.
• B. It is a marker of bone resorption.
• C. It is a marker of bone formation; elevated levels indicate increased bone turnover or osteomalacia in the proper clinical context.
• D. It only reflects kidney function.

Answer: (C)

Alkaline phosphatase reflects osteoblast activity during bone formation, so it serves as a marker of bone formation rather than bone mineral density or resorption. When bone turnover is high, osteoblasts are more active and release more ALP into the blood. In conditions like osteomalacia, mineralization is defective, and osteoblasts work harder to lay down bone matrix, leading to elevated ALP in the right clinical context. It doesn’t directly measure bone density—that’s measured by DXA—and it isn’t a marker of bone resorption (that role is filled by markers such as CTX or NTX). ALP can originate from bone or liver, so clinical context or isoenzyme testing helps determine the source.