Biochemical markers of bone remodeling and diseases

Bone Turnover Markers Overview

Biochemical markers of bone turnover (also called bone remodeling markers) are substances released into the blood or excreted in the urine during the process of bone formation (by osteoblasts) and bone resorption (by osteoclasts). Bone is a dynamic tissue constantly undergoing this remodeling cycle to maintain skeletal integrity and mineral homeostasis.

Measuring these markers provides a snapshot of the current rate of bone remodeling throughout the skeleton. This information complements bone mineral density (BMD) measurements by reflecting the dynamic activity of bone cells, which can be useful in understanding the pathogenesis of skeletal diseases, assessing fracture risk, and monitoring response to therapy, particularly for conditions like osteoporosis and Paget's disease.

These markers are broadly categorized into markers reflecting bone formation (osteoblast activity) and markers reflecting bone resorption (osteoclast activity).

The Bone Remodeling Process

Bone remodeling is a continuous cycle involving the removal of old or damaged bone by osteoclasts and the subsequent formation of new bone by osteoblasts at the same site. This process repairs microdamage, adapts bone structure to mechanical stress, and regulates calcium homeostasis. The cycle includes activation of remodeling sites, resorption of bone by osteoclasts, a reversal phase, formation of new bone matrix (osteoid) by osteoblasts, and finally mineralization of the osteoid.

Clinical Uses of Bone Turnover Markers

Bone turnover markers (BTMs) have several clinical applications:

• Monitoring Osteoporosis Treatment: BTMs change much more rapidly than BMD in response to therapy. A significant decrease in resorption markers (e.g., CTX, NTX) within 3-6 months confirms adherence and effectiveness of anti-resorptive drugs (like bisphosphonates). An increase in formation markers (e.g., PINP) shows response to anabolic drugs (like teriparatide).
• Fracture Risk Assessment: High levels of resorption markers can indicate increased fracture risk, sometimes independently of BMD, helping to guide treatment decisions.
• Diagnosis and Monitoring of Paget's Disease: Markedly elevated formation (BSAP) and resorption (NTX/CTX) markers reflect the high turnover characteristic of this disease and are used to monitor treatment response.
• Evaluating Other Metabolic Bone Diseases: Can provide insights into conditions like hyperparathyroidism, osteomalacia, renal osteodystrophy, and bone metastases.
• Research: Studying bone physiology and the effects of new drugs.

BTMs are generally not used for the initial diagnosis of osteoporosis.

Specific Bone Turnover Markers

Bone Formation Markers

These reflect osteoblast activity.

Clinical Significance (Formation Markers):

• BSAP: Increased during growth, fracture healing, Paget's disease, hyperparathyroidism, osteomalacia, some bone tumors/metastases, rickets. Need to distinguish from liver ALP.
• Osteocalcin (OC): Increased in high turnover states (Paget's, hyperthyroidism, chronic renal failure due to reduced clearance). Decreased with glucocorticoids, hypothyroidism, hypoparathyroidism. Levels influenced by age, renal function, circadian rhythm.

Factors Influencing Osteocalcin Levels

Increase OC	Decrease OC
Age (Postmenopausal women 50-60 yrs) Puberty / Growth Spurt Menstrual cycle (luteal phase) Lactation Nighttime Paget's disease Hyperthyroidism Chronic renal failure (chronic renal insufficiency) Osteomalacia Bone metastases (some types) Anticonvulsants Elevated levels of growth hormone (Acromegaly)	Age (Generally lower pre-puberty/young adult) Pregnancy Early morning Summertime (may relate to Vit D) Growth hormone deficiency Hypothyroidism Hypoparathyroidism Estrogen Therapy Glucocorticoid Therapy Calcitonin Therapy Immobilization

• PINP: Recommended reference marker for formation. Reflects collagen synthesis. Increased in high turnover states. Less influenced by renal function or time of day than OC.

Bone Resorption Markers

These reflect osteoclast activity and the breakdown of bone matrix.

Clinical Significance (Resorption Markers):

• PYD & DPD: Increased after menopause, in Paget's, hyperthyroidism, hyperparathyroidism, osteoporosis. DPD preferred for bone specificity. Urine measurement less common now.
• CTX & NTX: Recommended reference markers for resorption (serum CTX preferred). Increase after menopause, in high turnover states. Decrease rapidly with anti-resorptive therapy (within months). Affected by food intake (CTX especially) and circadian rhythm (highest early morning) - fasting morning sample needed for serum CTX. Affected by renal function. High levels associated with increased fracture risk.
• TRACP 5b: Reflects osteoclast activity. Less affected by renal function/circadian rhythm. Useful in Paget's, bone metastases, myeloma, high turnover osteoporosis. Activity increases in conditions listed previously.
• OHP: Largely obsolete due to lack of specificity and dietary influence (requires special diet). Reflects total collagen turnover.

Interpretation & Prognostic Significance

Key points for interpreting BTMs:

1. Reference Population: Results are often compared to reference ranges from healthy, premenopausal women (approx. age 30-45). Age- and sex-specific ranges should be used when available.
2. Fracture Risk: Elevated levels of bone resorption markers (e.g., CTX or NTX > 1-2 standard deviations above the mean for premenopausal women) indicate increased bone turnover and are associated with a higher risk of osteoporotic fractures, adding information beyond BMD alone.
3. Monitoring Therapy: The primary use is monitoring response. The expected change depends on the drug type (decrease for anti-resorptives, increase for anabolics). The magnitude of change needed to be considered significant (Least Significant Change, LSC) varies by marker and lab but is generally around 25-30%. Monitoring helps assess adherence and efficacy.
4. Differential Diagnosis: Very high BTM levels (>3 times upper normal limit) in suspected osteoporosis should prompt investigation for other underlying conditions like Paget's disease, hyperparathyroidism, or malignancy with bone involvement.

Assessing the relationship between formation and resorption markers helps understand the overall bone remodeling balance.

Testing Procedure

• Sample Type: Serum or plasma for most markers (BSAP, OC, PINP, CTX, TRACP 5b); Urine (second morning void preferred, or 24-hour collection less commonly) for crosslinks (PYD, DPD), NTX, OHP.
• Patient Preparation:
  • Fasting/Timing: Crucial for serum resorption markers CTX and NTX due to significant circadian rhythm and food effects - collect fasting (overnight fast) sample in the morning (e.g., before 10 AM). Fasting morning samples are also often preferred for PINP and OC to minimize variability. BSAP and TRACP 5b are less affected by fasting/time of day.
  • Diet: Special collagen-restricted diet needed only for urinary Hydroxyproline.
  • Other: Avoid vigorous exercise before testing. Ensure consistent timing for serial samples.
• Collection & Handling: Standard venipuncture for blood. Follow lab instructions for urine collection and preservation. Some markers require specific handling (e.g., immediate separation, freezing).
• Analysis: Primarily automated immunoassays (ELISA, CLIA, ECLIA).

Limitations

• Variability: Significant intra-individual (circadian rhythm, diet, exercise, season, menstrual cycle) and inter-individual variability. Analytical variability between assays/labs exists.
• Lack of Standardization: While improving, standardization across different assays remains a challenge for some markers.
• Influence of Comorbidities: Kidney function significantly affects levels of renally cleared markers (OC, CTX, NTX, crosslinks). Liver function affects BSAP and PINP. Thyroid and parathyroid status influence most markers.
• Recent Fractures: Increase BTM levels for several months.
• Overlap: Significant overlap in values between healthy individuals and those with osteoporosis.
• Not for Diagnosis Alone: BTMs supplement, but do not replace, BMD testing and clinical assessment for osteoporosis diagnosis.

References

1. International Osteoporosis Foundation (IOF) and International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) Working Group on Bone Marker Standards. (2017). Recommendations for the measurement of bone turnover markers. Retrieved from https://www.osteoporosis.foundation/educational-hub/topic/bone-turnover-markers
2. Eastell, R., & Szulc, P. (2017). Use of bone turnover markers in postmenopausal osteoporosis. *The Lancet Diabetes & Endocrinology*, 5(11), 908–923. https://doi.org/10.1016/S2213-8587(17)30184-5
3. Vasikaran, S., Eastell, R., Bruyère, O., Foldes, A. J., Garnero, P., Griesmacher, A., ... & IFCC-IOF Committee for Bone Metabolism (C-BM). (2011). Markers of bone turnover for the prediction of fracture risk and monitoring of osteoporosis treatment: a need for international reference standards. *Osteoporosis International*, 22(2), 391–420. https://doi.org/10.1007/s00198-010-1501-1
4. Lab Tests Online. (n.d.). Bone Markers. Retrieved from https://labtestsonline.org/tests/bone-markers
5. Rosen, H. N. (Ed.). (UpToDate - subscription required). Use of biochemical markers of bone turnover in osteoporosis. Retrieved from https://www.uptodate.com/contents/use-of-biochemical-markers-of-bone-turnover-in-osteoporosis