Beta-CrossLaps (CTX) Unit Converter
Beta-CrossLaps (CTX) Unit Converter
Convert serum beta-CrossLaps between ng/L, pg/mL, µg/L and ng/mL — and read the result knowing that CTX must be sampled fasting and before 10 a.m., because a fed afternoon sample can be half the fasting morning value.
Beta-CrossLaps (CTX) converter
Mass ladder onlyBeta-CrossLaps 350 ng/L, premenopausal woman, fasting 08:30 sample
The conversion — a mass ladder, with no molar unit
1,000 ng/L = 1 µg/L = 1 ng/mL
µg/L = ng/L ÷ 1,000
- ng/L = pg/mL
- a nanogram per litre and a picogram per millilitre are the same concentration. Most European reports use ng/L or pg/mL; the Roche assay literature and many North American reports use ng/mL, which is a thousand times larger — a 350 ng/L result is 0.350 ng/mL
- µg/L = ng/mL
- also identical to each other. These are the units in which the commonly quoted treatment thresholds appear, which is why a result of "0.573" and one of "573" can both be correct and describe the same sample
- no molar unit, deliberately
- CTX is not one molecule. It is a family of isomerised eight-amino-acid fragments released as type I collagen is degraded, detected by antibodies against the beta-isomerised octapeptide. There is no single molecular weight to convert with, so this converter offers a mass ladder only and no nmol/L or pmol/L
- the decimal-point trap
- the thousandfold step between ng/L and ng/mL is where CTX results go wrong. Before comparing a result with a threshold or a previous value, check that both are in the same unit
Worked example
Beta-CrossLaps 350 ng/L, premenopausal woman, fasting 08:30 sample
350 ng/L = 350 pg/mL — the same concentration written the other way
350 ÷ 1,000 = 0.350 µg/L = 0.350 ng/mL, which is how the Roche assay literature usually expresses it
350 ng/L sits inside Mayo's premenopausal interval of 136–689 ng/L
Now change only the sampling: the same woman, sampled at 4 p.m. after lunch, could return something closer to 180 ng/L. Nothing about her skeleton would have changed
Which is why the interval above applies only to a fasting specimen taken before 10 a.m., and why a CTX drawn at an afternoon clinic appointment should not be compared with it
Why the sampling conditions matter more than almost anything else on this page
| Condition | Effect on CTX | What to do |
|---|---|---|
| Eating | Falls sharply within an hour or two of a meal — the fall is large, and is mediated by gut hormones including GLP-2 rather than by calcium | Sample after a 12-hour fast. A fed CTX can be close to half the fasting value in the same person |
| Time of day | Peaks in the early hours of the morning and falls through the day; the circadian amplitude is substantial | Sample before 10 a.m., and keep the time consistent between visits |
| Renal impairment | Rises as eGFR falls, independently of bone | Check renal function. CTX is unreliable as a turnover marker in advanced chronic kidney disease |
| Recent fracture | Rises, and stays raised for months | Avoid measuring within several months of a fracture if the aim is a treatment baseline |
| Antiresorptive treatment | Falls, quickly and substantially | This is the intended effect and the main use of the test |
The recommended pair: one resorption marker, one formation marker
| Marker | Side of the remodelling cycle | Recommended status |
|---|---|---|
| CTX — beta C-terminal telopeptide of type I collagen | Resorption. Released when osteoclasts degrade the collagen matrix | The IOF/IFCC reference marker for bone resorption |
| P1NP — procollagen type I N-terminal propeptide | Formation. Cleaved from procollagen as osteoblasts lay down new matrix | The IOF/IFCC reference marker for bone formation |
| Osteocalcin | Formation | Superseded by P1NP; assays are not interchangeable |
| Urinary NTX | Resorption | Largely superseded by serum CTX, which avoids the urine collection and the creatinine correction |
What the test is actually for
| Question | Does CTX answer it? |
|---|---|
| Does this patient have osteoporosis? | No. Osteoporosis is diagnosed on bone mineral density and fracture history. A turnover marker neither makes nor excludes the diagnosis |
| Is this patient taking and absorbing their bisphosphonate? | Yes — this is its best use. A substantial fall from baseline, conventionally beyond the least significant change for the assay, a few months after starting treatment is good evidence of adherence and absorption |
| Has turnover returned after stopping treatment or during a drug holiday? | Reasonably. A rise back towards the pretreatment value suggests the antiresorptive effect has worn off |
| Should this patient have a dental extraction — what is their osteonecrosis risk? | No. The proposal to use a CTX threshold to stratify the risk of medication-related osteonecrosis of the jaw has not been validated and is not endorsed by the major oral surgery position papers |
| How high is this patient’s fracture risk? | Weakly, at best. High turnover carries some independent fracture risk in cohort studies, but the effect is not strong enough to guide an individual decision |
A resorption marker that has to be sampled properly
Beta-CrossLaps, usually shortened to CTX, is the beta-isomerised C-terminal telopeptide of type I collagen: an eight-amino-acid fragment released into the blood when osteoclasts degrade the bone matrix. It is the reference marker of bone resorption recommended by the International Osteoporosis Foundation and the IFCC, paired with P1NP as the reference marker of formation. Because it is a heterogeneous degradation fragment rather than a defined molecule, it is reported as a mass concentration and there is no molar unit: this converter offers ng/L, the identical pg/mL, and the thousandfold-larger µg/L and ng/mL, and nothing else. Most of the errors in reading a CTX come from that thousandfold step — a result of 0.350 ng/mL and one of 350 ng/L are the same measurement.
The most useful fact about CTX is pre-analytical. Bone resorption has a marked circadian rhythm, peaking in the early hours and falling through the day, and CTX falls sharply after eating — a gut-hormone-mediated suppression, largely attributed to GLP-2, that begins within an hour of a meal. The two effects together are large: a sample taken in the afternoon after lunch can read around half the value the same person would give fasting at eight in the morning. That variation is bigger than most of the treatment effects anybody is trying to detect. A CTX is therefore taken after a 12-hour fast, before 10 a.m., and at a consistent time between visits — and a result from an unfasted afternoon clinic appointment should not be read against a reference interval or compared with an earlier value at all.
Renal function is the other thing to check before interpreting the number. CTX is cleared by the kidney and rises as the glomerular filtration rate falls, independently of what the skeleton is doing; the effect is larger for CTX than for P1NP, and in advanced chronic kidney disease neither marker is a reliable index of turnover. Recent fracture raises CTX for months, and immobilisation raises it too.
What is the test actually for? Not for diagnosing osteoporosis, which is a bone mineral density and fracture-history diagnosis that no turnover marker makes or excludes. Its real value is in monitoring: starting an oral bisphosphonate should produce a large fall in CTX within about three months, and a fall of that size is good evidence that the patient is taking the drug and absorbing it — which matters, because oral bisphosphonates are absorbed poorly and only if taken fasting, upright and with water. A CTX that has not moved is a prompt to ask how the tablets are being taken before concluding the drug has failed. Using a CTX threshold to decide whether dental extraction is safe, by contrast, has never been validated and is not supported by the oral surgery position papers.
Frequently asked questions
How do you convert CTX from ng/L to ng/mL?
Divide by 1,000. A beta-CrossLaps of 350 ng/L is 0.350 ng/mL, which is the same as 0.350 µg/L. Going the other way, multiply ng/mL by 1,000. ng/L and pg/mL are numerically identical, so no conversion is needed between those two. There is no molar unit for CTX, because it is a heterogeneous collagen fragment rather than a single molecule with a defined molecular weight.
Does a CTX have to be taken fasting?
Yes, and in the morning. CTX falls sharply after eating and has a marked circadian rhythm that peaks in the early hours, so a fed afternoon sample can be around half the value the same person gives fasting at 8 a.m. Reference intervals are established on 12-hour fasting specimens collected before 10 a.m., and a sample taken outside those conditions cannot be compared with them or with an earlier result.
What is the difference between CTX and P1NP?
They report opposite halves of the remodelling cycle. CTX is a fragment of collagen being broken down, so it measures bone resorption; P1NP is a propeptide cleaved from procollagen as new matrix is laid down, so it measures bone formation. The International Osteoporosis Foundation and IFCC recommend them as the reference markers for their respective sides, and they are usually measured together.
Does kidney disease affect the CTX result?
Yes. CTX is cleared renally and rises as the glomerular filtration rate falls, independently of bone turnover — more so than P1NP. In moderate to advanced chronic kidney disease a raised CTX cannot be read as high bone turnover, and turnover markers in general become unreliable there.
Can a CTX be used to decide whether a dental extraction is safe on a bisphosphonate?
No. The suggestion that a serum CTX below a particular threshold predicts medication-related osteonecrosis of the jaw has not been validated, and the American Association of Oral and Maxillofacial Surgeons position paper does not support using it to decide whether to proceed with dental surgery. The test has no established role in that decision.
Does a normal CTX mean the bones are fine?
No. CTX measures how fast bone is being resorbed, not how much bone there is. Osteoporosis is diagnosed on DXA and fracture history, and someone with severe osteoporosis can have a perfectly normal CTX. The marker earns its place in monitoring change, particularly the fall that should follow effective antiresorptive treatment.
Related calculators
References
- Mayo Clinic Laboratories. Test ID: CTX — Beta-CrossLaps, Serum. Reference values: premenopausal females 136–689 pg/mL, postmenopausal females 177–1015 pg/mL, with separate male intervals by decade. Electrochemiluminescence immunoassay; 12-hour fast required; collect prior to 10 a.m.
- Vasikaran S, Eastell R, Bruyère O, et al. Markers of bone turnover for the prediction of fracture risk and monitoring of osteoporosis treatment: a need for international reference standards. Osteoporos Int. 2011;22(2):391–420. doi:10.1007/s00198-010-1501-1
- Bjarnason NH, Henriksen EEG, Alexandersen P, Christgau S, Henriksen DB, Christiansen C. Mechanism of circadian variation in bone resorption. Bone. 2002;30(1):307–313. doi:10.1016/S8756-3282(01)00662-7
- Eastell R, Szulc P. Use of bone turnover markers in postmenopausal osteoporosis. Lancet Diabetes Endocrinol. 2017;5(11):908–923. doi:10.1016/S2213-8587(17)30184-5
- Ruggiero SL, Dodson TB, Aghaloo T, Carlson ER, Ward BB, Kademani D. American Association of Oral and Maxillofacial Surgeons’ position paper on medication-related osteonecrosis of the jaws — 2022 update. J Oral Maxillofac Surg. 2022;80(5):920–943.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.