Bone Turnover Markers Explained: CTX, P1NP and What They Tell You

What are bone turnover markers?

Bone turnover markers are substances measured in a blood or urine sample that reflect how quickly your skeleton is being remodelled — broken down and rebuilt. Your bones are living tissue in constant renewal: two types of cell are always at work inside them, and they leave behind traces, or markers, in the blood. Royal Osteoporosis Society, 2024

That renewal is a two-part cycle. Cells called osteoclasts break down old bone (resorption), and cells called osteoblasts lay down new bone (formation). BTMs are the by-products of that cycle, and they respond quickly and noticeably to changes in the rate of bone turnover. StatPearls, 2024 A high level suggests bone is being remodelled rapidly; a low level suggests slower turnover. Crucially, that's a measure of activity, not of bone mass.

What are CTX and P1NP?

CTX and P1NP are the two reference bone turnover markers — one for each half of the remodelling cycle. The International Osteoporosis Foundation and the International Federation of Clinical Chemistry designated procollagen type I N-propeptide (P1NP) in blood as the reference marker of bone formation, and the C-terminal telopeptide of type I collagen (β-CTX) as the reference marker of bone resorption. IOF/IFCC review, 2021

In plain terms:

• CTX (a "CTX blood test") is released when old bone is broken down. A higher CTX points to faster bone resorption.
• P1NP is released when new bone is being built. A higher P1NP points to more bone formation.

You may also see NTX, another marker of bone resorption, on a report StatPearls, 2024 — the Royal Osteoporosis Society lists it among the result names you might come across Royal Osteoporosis Society, 2024. But CTX and P1NP are the pair a specialist is most likely to use, because they've been chosen internationally as the standard against which the others are compared. IOF joint consensus, 2025

What do bone turnover markers tell you — and what don't they?

They tell you the speed of bone remodelling. They do not tell you your bone density, your bone strength, or whether you have osteoporosis. This is the distinction that trips people up, so it's worth being blunt: BTMs measure the rate of an ongoing process, not the amount of bone you're standing on.

That has a direct consequence. The Royal Osteoporosis Society states plainly that blood and bone marker tests "are not used to diagnose osteoporosis or to find out your chance of breaking a bone." Royal Osteoporosis Society, 2024 Osteoporosis is defined by low bone mineral density, and density is measured by a scan — most commonly a DEXA scan. If you want to know what a scan actually measures, see our guide to what a DEXA scan shows, and how the T-score is read.

What are bone turnover markers actually used for?

Their main role is monitoring — checking whether an osteoporosis treatment is working, and whether someone is taking it. Because BTMs shift within weeks to months — far faster than bone density, which changes only slowly StatPearls, 2024 — a marker change can signal a response early, and it is a practical way to confirm a person is actually taking their medicine. IOF joint consensus, 2025

The direction of change depends on the type of drug:

• On antiresorptive medicines — which slow bone breakdown, such as bisphosphonates and denosumab (Prolia) — CTX and P1NP are expected to fall. The markers are usually re-checked over the first few months of treatment to confirm bone turnover has been adequately suppressed — for intravenous bisphosphonates and denosumab, CTX-1 at around 1 month and P1NP at 3 months, with oral bisphosphonates checked a little later. StatPearls, 2024
• On anabolic (bone-building) medicines, such as teriparatide, the formation marker is expected to rise. P1NP is the preferred marker here, beginning to rise soon after treatment starts and typically peaking within 1 to 3 months. StatPearls, 2024

It's important to be realistic about how routine this is in the UK. The Royal Osteoporosis Society notes that many people do not need a bone marker test at all — because there are other ways to know if treatment is working, such as your DEXA results — and that bone markers "are only used in specialist centres and normally for research." Royal Osteoporosis Society, 2024 This is not a test to request from your GP; it's one a specialist orders when it will genuinely add something.

How is a CTX or P1NP blood test done?

Usually as a morning blood sample — and for CTX, after an overnight fast — because CTX in particular varies with food and the time of day. CTX shows circadian fluctuation and is influenced by eating, so if CTX and P1NP are both being measured, the sample should be taken in the morning after fasting overnight. P1NP is more forgiving, being minimally affected by food and circadian rhythm. StatPearls, 2024

Two practical caveats matter. First, results vary between laboratories and assays: the reference markers are still being harmonised precisely because different labs don't yet report identically, which is why you can't casually compare a result from one lab with another. IOF/IFCC review, 2021 Second, the results can be difficult to interpret — different things, including the medicines you take, can affect them, and you often need several tests over a period of time to get a meaningful picture. That interpretation is a job for the specialist who ordered the test. Royal Osteoporosis Society, 2024

Do bone turnover markers replace a DEXA or REMS scan?

No — they answer a different question, and the fullest picture comes from reading turnover and density together. A CTX or P1NP result tells you how fast bone is changing; a density scan tells you how much bone there is. Neither substitutes for the other, which is why a bone turnover marker is used alongside density measurement rather than instead of it.

For the density half of that picture, the amount of bone you have is what a scan measures — and if you're building a long-term view of your bone health, you'll be doing it more than once. A proactive bone health assessment is designed around that baseline-then-track approach. Because REMS uses no ionising radiation, there's no cumulative-dose consideration to weigh up when a density measurement is repeated over the years. If you'd like to establish where your bones stand today, you can book a Screen My Bones assessment — and, where relevant, discuss with your specialist whether bone turnover markers have a role in your care.

---

This article is part of our screening technology series.