Bone Turnover Markers in Practice: A Guide for the Rheumatologist Save

Bone turnover markers (BTMs) have been eyed for decades as potential monitoring tools in osteoporosis.

However, there have been difficulties with poor between-lab reproductivity, insufficiently controlled heterogeneity, and pre-analytical variability of the assays; making BTMs vastly unpopular amongst practicing rheumatologists.

Two BTMS have garnered the most interest and study. A marker of bone formation, N-terminal collagen type I extension propeptide (PINP) and a marker of bone resorption, C-terminal cross-linking telopeptide of type I collagen (CTX-I) have been identified as the international reference standards for prediction of fracture risk and monitoring of osteoporosis treatment. Current guidelines for sample handling for CTX-I and PINP lack consistency and clarity.

The National Bone Health Alliance (NBHA) set out to review published information and provide recommendations for standardized use and testing of BTMs.

Based on the author’s extrapolation of published data further summary and recommendations were formed:

• Either serum or plasma samples must be used for CTX-I and PINP measurements
• For PINP levels, random non-fasting sample is acceptable
• EDTA plasma is more stable than serum and is preferred if delay is expected in sample processing. It is recommended to centrifuge the sample within 2 hour of collection for better stability
• Multiple freeze-thaw cycles considered to be acceptable for both CTX-I and PINP, although “freeze only once” is preferred for CTX-I and 5 or less for PINP

Patient related controllable factors were identified as follows:

• Collect blood after overnight fasting between 7:30 and 10 am. This recommendation is based on observation that CTX-I level peaks during early morning hours. Fasting markedly reduced the circadian variation from 36 to 8.7% in postmenopausal women
• Vigorous exercise should be avoided the day prior to sampling. Light exercise has no significant effect to serum CTX-I and PINP levels
• In premenopausal women, testing of BTMs is more accurate if done in the early-mid-follicular phase
• Seasonal variation was not observed in PINP and remained minimal for CTX-I
• BTMs levels are lower in 4th decade in women and in 5th decade in men. Postmenopausal levels rise in women and remain stable in men up until after 70 years of age.

Preexisting medical conditions and medications administered may significantly influence BTM levels.

BTM Levels Elevated In

• Disorders with high bone turnover (hyperparathyroidism, acromegaly, Paget’s disease) lead to increase of BMT.
• Patient with bone metastases lead to higher baseline levels of all BTMs. However, CTX-I levels do no increase as much due to release of immature forms of CTX-I that are not detected by assays.
• Vitamin D deficiency lead to higher BTM levels as it leads to secondary hyperparathyroidism and as a result elevated levels of PTH.
• BTM increase steeply following bone fracture with CTX reaching its peak concentration at 4 weeks and PINP after 12 weeks.
• Severely impaired kidney function is associated with increased CTX-I levels due accumulation of their degradation products.

In certain cases, it is expected to see dissociation of bone turnover markers. Disorders implicated in such dissociation are rheumatoid arthritis, multiple myeloma (MM) and Cushing’s disease. Acute phase of RA is associated with increased CTX-I without parallel elevation of PINP signifying decreased bone formation. Both markers tend to normalize in remission. Similar discrepancies observed in MM were PINP remains slow normal in stable disease and decrease in the progressive disease.

Anti-resorptive medications lead to significant decrease pf BTMs after 2-3 months of treatment. It is recommended to use CTX-1 after 3 months of therapy and PINP may be used after 6 months in case of oral bisphosphonates and in 3  months after denosumab or IV bisphosphonates. With therapeutic doses, at least a 30-40% decrease of CTX-I is expected and is medication dependent.

Anabolic regimen (teriparatide) leads to increase of BTM levels with PINP levels peaking after 1-3 months of treatment/ CTX-I levels increase with a delay and attain highest levels at 6-12 months. IT is recommended to use PINP after 3 months of teriparatide therapy.

In conclusion, when proper sample collection technique is followed and patient and disease-specific variables are taken into consideration, BTMs such as CTX-I and PINP may help significantly in monitoring of osteoporosis treatment response in-between periodic bone density measurements.