A CTX result that looks like good news

A 61-year-old postmenopausal patient with a T-score of -2.6 at the lumbar spine starts an antiresorptive agent. Baseline labs are drawn fasting at 8:15 AM: beta-CTX (C-terminal telopeptide of type I collagen, a resorption marker) comes back 0.61 ng/mL. Twelve weeks later a follow-up beta-CTX is drawn non-fasting at 3:40 PM at an outside laboratory: 0.22 ng/mL. On the screen, that looks like a 64 percent drop, a strong treatment response. Whether that number can carry that conclusion by itself is what the rest of this comparison has to settle.

A laboratory bone panel can establish some things and not others. Osteoporosis is diagnosed by dual-energy X-ray absorptiometry (DXA) bone mineral density, expressed as a T-score of -2.5 or lower at the femoral neck, total hip, or lumbar spine in postmenopausal women and men 50 and older, or by a recognized low-trauma fracture site meeting the cited diagnostic criteria. No chemistry result makes that diagnosis. A T-score compares a patient's measured bone mineral density to a young-adult reference mean in standard deviations; -1.0 to -2.4 is low bone mass, and -1.0 or higher is normal.

Laboratory testing has two defined jobs in bone health, and neither of them is the DXA diagnosis. The first job is screening for secondary contributors to bone loss, conditions such as renal disease, thyroid disease, or vitamin D deficiency that mimic or worsen low bone mass. The second job is tracking bone-turnover markers, which reflect how fast bone is being built and broken down, not how much bone mineral is present. Both jobs support the clinical picture; neither replaces the scan.

The formation and resorption markers and what moves them determine whether that 0.22 ng/mL result can be called a treatment response. Before a bone-turnover result changes a clinical impression, know what the number is allowed to say.