Bone turnover markers

The two reference markers

	Marker	Reflects
Resorption	serum CTX (beta-CrossLaps, C-telopeptide of type I collagen)	Osteoclast activity
Formation	serum P1NP (procollagen type I N-propeptide)	Osteoblast collagen synthesis

Others: NTX (urine), TRAP-5b, bone-specific ALP, osteocalcin. CTX and P1NP are the IOF/IFCC reference markers - use them rather than a miscellany

They measure the rate of turnover across the whole skeleton right now - DXA measures accumulated mass months to years later
*They do not diagnose osteoporosis and they do not by themselves indicate treatment*
They answer three questions well:

1. Is she taking and absorbing the drug?

2. Has turnover been suppressed enough?

3. Has turnover come back after stopping?

Wide biological and analytical variability - least significant change ~25-40% for CTX, ~25% for P1NP
- A 15% change is noise. Do not act on it
Higher in adolescence, after fracture, in pregnancy/lactation, in CKD (renal clearance)

Osteoblast - bone-forming; secretes type I collagen (-> P1NP), alkaline phosphatase, osteocalcin
- *Secretes RANK ligand - the essential mediator of osteoclast formation, function and survival*
- Also secretes osteoprotegerin (OPG), a decoy receptor that binds RANKL and prevents it reaching RANK
- The RANKL:OPG ratio sets the resorption rate
Osteoclast - multinucleate, haematopoietic (monocyte/macrophage) lineage; resorbs bone releasing collagen fragments (-> CTX/NTX) and TRAP-5b
- Has no PTH receptor - all PTH effects on resorption are relayed through the osteoblast lineage
Osteocyte - terminally differentiated osteoblast entombed in matrix; ~90-95% of bone cells
- The skeleton's mechanosensor
- Secretes sclerostin (inhibits Wnt -> inhibits bone formation) and FGF-23 (phosphaturic, suppresses calcitriol)
- Mechanical loading suppresses sclerostin -> more formation. Immobilisation does the opposite

Oestrogen deficiency -> inc RANKL, dec OPG -> osteoclast activation -> accelerated resorption
Turnover markers rise 50-100% at menopause and remain high

32-aa peptide from thyroid parafollicular C cells; inhibits osteoclasts via the calcitonin receptor
*Physiological role in human calcium homeostasis is negligible* - thyroidectomy causes no calcium disturbance, and medullary thyroid carcinoma with massive calcitonin excess causes no hypocalcaemia
Used therapeutically in Paget disease and formerly for hypercalcaemia (rapid onset but tachyphylaxis within 48-72 h)
Its real clinical value is as a tumour marker for medullary thyroid carcinoma

Pre-analytical rules - most "abnormal" results are collection errors

CTX has marked circadian variation (peak ~0500, trough ~1300) and falls sharply after food
- *Collect fasting, in the morning* - non-fasting afternoon samples can be 30-50% lower
P1NP is far more stable - not food- or time-dependent
Both accumulate in renal impairment (CTX more than P1NP - use P1NP or bone ALP in CKD)
Avoid within ~6-12 months of a fracture

1. Adherence and response to an oral bisphosphonate

Recheck CTX at 3-6 months; expect a fall of >=25-50% into the lower half of the premenopausal reference range
No fall = not taking it, or not absorbing it (taken with food, coffee, calcium, or PPI/coeliac malabsorption) - check that before switching drug

2. Bisphosphonate drug holiday

A rise in CTX back toward pre-treatment values supports resumption of therapy

3. After stopping denosumab - markers rebound above baseline, matching the rebound fracture risk

4. Unexplained very high turnover - suggests Paget disease, malignancy, hyperparathyroidism, osteomalacia, or hyperthyroidism

5. Very low turnover - over-suppression, adynamic bone disease in dialysis, hypoparathyroidism

Pattern	Suggests
inc CTX + inc P1NP (coupled)	Postmenopausal osteoporosis, hyperthyroidism, hyperparathyroidism, Paget (isolated inc ALP with normal LFTs)
dec CTX + dec P1NP	Antiresorptive effect; adynamic bone disease
dec CTX + inc P1NP	*Romosozumab* - the only agent that uncouples them
inc P1NP >> CTX	Teriparatide/abaloparatide anabolic window
inc ALP + inc P1NP + low calcium/phosphate, high PTH	Osteomalacia
inc ALP with normal P1NP and normal calcium	Check the ALP is bone and not liver - GGT/isoenzymes

Drug	Effect on markers	Practical use
Bisphosphonate	CTX and P1NP both fall; effect persists after stopping	3-6 month CTX for adherence; rising CTX ends a drug holiday
Denosumab	Profound, rapid suppression; complete reversal within 6-12 months of a missed dose	*Never allow a gap - rebound resorption causes multiple vertebral fractures.* Always follow cessation with a bisphosphonate
Teriparatide / abaloparatide	P1NP rises early (within 1-3 months)	Early P1NP rise confirms an anabolic response
Romosozumab	inc formation AND dec resorption (dual, uncoupled)	12 months only; *must be followed by an antiresorptive*
Calcitonin	dec CTX	Paget disease; largely superseded by bisphosphonates

*Sequence matters: build bone then lock it in. Anabolic before antiresorptive; never anabolic after denosumab without cover*

Do not use them to decide who to treat - that is DXA plus fracture risk
Do not use them to diagnose osteoporosis
Do not act on a single value or a change under ~25%
Do not use CTX to predict or exclude medication-related osteonecrosis of the jaw - the evidence does not support a "safe" CTX threshold before dental extraction

Postmenopausal osteoporosis; glucocorticoid-induced osteoporosis (P1NP falls - formation is suppressed)
Paget disease - very high ALP and P1NP; ALP is the standard monitoring marker
Primary hyperparathyroidism, hyperthyroidism, acromegaly - high turnover
CKD-MBD - markers renally retained; PTH and bone ALP are preferred in dialysis
Bone metastases and myeloma - raised resorption markers
Osteomalacia, vitamin D deficiency - raised ALP and PTH with low/normal calcium
Immobilisation, weightlessness - raised resorption

Turnover markers change within weeks of starting or stopping therapy; DXA takes 1-2 years to move
- This is the whole reason markers are useful - they close the feedback loop early
The fall in CTX explains only part of the fracture-risk reduction; do not treat marker normalisation as the therapeutic goal in itself
Turnover rises with age, and remains permanently elevated after menopause without treatment
Denosumab cessation rebound: markers overshoot baseline within 6-9 months, with multiple vertebral fractures reported - the single most important marker-related clinical fact

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