Osteocalcin Unit Converter
Osteocalcin Unit Converter
Convert serum osteocalcin between ng/mL, µg/L and nmol/L — with the caution that osteocalcin assays measure different mixtures of intact protein and fragments, so results are not comparable between methods.
Osteocalcin converter
Mass ⇄ molarSerum osteocalcin 22 ng/mL, adult
The conversion, and why the molar figure is approximate
ng/mL = nmol/L ÷ 0.172414
because 0.172414 = 1 ng/mL ÷ 5,800 g/mol, the mass of the intact 49-residue protein
- ng/mL = µg/L
- identical concentrations written two ways; no arithmetic is needed between them. Almost every osteocalcin report uses one of these two, and molar reporting is uncommon
- MW ≈ 5,800 — osteocalcin
- bone Gla protein: 49 amino acids, three gamma-carboxyglutamate residues, made by the osteoblast. A small protein rather than a small molecule, so the molecular weight is an average of a real species rather than a formula mass
- why the molar conversion is approximate
- the assay does not see one molecule. Osteocalcin is cleaved in the circulation, and the N-MID method deliberately measures the intact protein plus the large N-terminal-mid fragment, which is lighter. A single molecular weight applied to a mixture gives an approximate molar concentration — which is one reason laboratories report the mass concentration
- the specimen
- serum or EDTA plasma, separated promptly and kept cold. Osteocalcin degrades at room temperature, and a delayed sample reads low
Worked example
Serum osteocalcin 22 ng/mL, adult
22 ng/mL = 22.0 µg/L — the same concentration written the other way
22 × 0.172414 = 3.793 nmol/L, using 5,800 for the intact protein
22.0 ng/mL is inside Mayo's adult interval of 9–42 ng/mL for the Roche N-MID assay
Going back: 3.793 ÷ 0.172414 = 22.0 ng/mL
The number that matters clinically is usually not this one but the change from a baseline on the same assay — osteocalcin is used to follow bone turnover over time, not to classify a single result
The three bone turnover markers, and where osteocalcin sits among them
| Marker | What it reports | Standing |
|---|---|---|
| P1NP — procollagen type I N-terminal propeptide | Bone formation: collagen being laid down | The IOF/IFCC reference formation marker. What a formation marker should be measured as |
| CTX — beta C-terminal telopeptide | Bone resorption: type I collagen being broken down | The IOF/IFCC reference resorption marker |
| Osteocalcin | Bone formation, indirectly — osteoblast product, though a fraction comes from resorption of the bone matrix | Superseded by P1NP for most purposes. Still widely measured, and useful where it is followed serially on one assay |
| Bone-specific alkaline phosphatase | Bone formation | Useful where renal impairment complicates P1NP, and in Paget’s disease |
What moves an osteocalcin, other than bone
| Factor | Effect | Why |
|---|---|---|
| Renal impairment | Raises it, sometimes substantially | Osteocalcin and its fragments are cleared by the kidney. In chronic kidney disease a high result does not reliably mean high turnover |
| Time of day | Higher overnight and in the early morning | Bone turnover has a circadian rhythm. Standardise the sampling time if results are being compared |
| Sample handling | Delay and warmth lower it | The protein is proteolysed in the tube. Separate promptly and keep cold or frozen |
| Glucocorticoids | Lower it, quickly | Direct suppression of osteoblast function — one of the mechanisms of steroid-induced osteoporosis |
| Antiresorptive treatment | Lowers it | Turnover is coupled: suppressing resorption suppresses formation too. This is the intended effect |
| Warfarin | Lowers the carboxylated fraction | Osteocalcin is a vitamin K-dependent Gla protein. Assays differ in how much they see undercarboxylated osteocalcin |
Mayo's paediatric osteocalcin intervals, for orientation
| Age | Male (ng/mL) | Female (ng/mL) |
|---|---|---|
| Under 5 years | 19–75 | 14–126 |
| 5–9 years | 21–108 | 16–152 |
| 10–15 years | 19–159 | 15–151 |
| 16–17 years | 12–114 | 9–70 |
| 18 years and over | 9–42 | 9–42 |
A rate of bone formation, measured differently by every assay
Osteocalcin, also called bone Gla protein, is a 49-residue protein made by the osteoblast and incorporated into the bone matrix, with a fraction escaping into the circulation as it is made. That circulating fraction is what the assay measures, and it is read as an index of the rate at which bone is being formed. Reports are almost always in nanograms per millilitre or the identical micrograms per litre; the molar conversion, at 0.172414 nanomoles per litre per nanogram per millilitre, uses 5,800 as the mass of the intact protein and should be treated as approximate, because the assay also detects fragments that are lighter than that.
That fragmentation is the central caution about osteocalcin. Intact osteocalcin is cleaved in the circulation into an N-terminal-mid fragment and a C-terminal fragment, and different commercial assays detect different mixtures of the three. The widely used N-MID method measures intact protein plus the large N-terminal-mid fragment deliberately, because that mixture is more stable in the tube than the intact protein alone. The practical consequence is that an osteocalcin from one laboratory cannot be compared with one from another, and a reference interval belongs to the assay that generated it. This lack of a single measurand is the main reason the International Osteoporosis Foundation and IFCC recommended P1NP rather than osteocalcin as the reference marker of bone formation, and why osteocalcin is used less than it once was.
What a bone turnover marker is genuinely good for is monitoring, not diagnosis. Osteoporosis is diagnosed on bone mineral density and fracture history; no turnover marker makes or excludes that diagnosis. What a marker can do is show whether treatment is working: turnover is coupled, so an antiresorptive drug lowers formation markers as well as resorption markers, and a substantial fall from baseline a few months after starting treatment is good evidence that the patient is taking the drug and absorbing it. A marker that has not moved should prompt a conversation about adherence, about whether an oral bisphosphonate is being taken correctly on an empty stomach, and about coexisting causes of persistently high turnover.
Interpretation needs the renal function beside it. Osteocalcin and its fragments are cleared by the kidney, so chronic kidney disease raises the measured concentration independently of bone. The same problem affects CTX and, to a lesser extent, P1NP. Sample handling matters too: the protein is proteolysed at room temperature, so a specimen that sat on a bench reads falsely low, and because turnover follows a circadian rhythm, serial samples should be taken at a consistent time of day.
Frequently asked questions
How do you convert osteocalcin from ng/mL to nmol/L?
Multiply by 0.172414, which is one nanogram per millilitre divided by 5,800 g/mol, the mass of the intact 49-residue protein. An osteocalcin of 22 ng/mL is about 3.79 nmol/L. Treat the molar figure as approximate: the assay measures a mixture of intact protein and fragments of different masses, which is one reason laboratories report the mass concentration. ng/mL and µg/L are the same number.
Why can't osteocalcin results from different laboratories be compared?
Because osteocalcin is not a single measurand. The intact protein is cleaved in the circulation, and different assays detect different mixtures of intact protein, the N-terminal-mid fragment and the C-terminal fragment. A result is only meaningful against the reference interval of the assay that produced it, and serial monitoring must stay on one method.
Is osteocalcin still used, or has P1NP replaced it?
Largely replaced for new work. The International Osteoporosis Foundation and IFCC recommended P1NP as the reference marker of bone formation and CTX as the reference marker of resorption, chiefly because they are better standardised. Osteocalcin is still measured, and it remains useful where a patient is followed serially on a single assay, but it is no longer the first choice.
Does a normal osteocalcin mean the bones are healthy?
No. Osteocalcin reports the rate of bone turnover, not the amount of bone. Osteoporosis is diagnosed from bone mineral density and fracture history, and a patient with severe osteoporosis can have an entirely normal turnover marker. The marker’s value is in showing change — particularly the fall that follows effective antiresorptive treatment.
Does kidney disease affect osteocalcin?
Yes, and substantially. Osteocalcin and its fragments are cleared renally, so a reduced glomerular filtration rate raises the measured concentration independently of what the skeleton is doing. In chronic kidney disease a high osteocalcin cannot be read as high bone turnover, and bone turnover markers generally lose reliability as renal function falls.
Related calculators
References
- Mayo Clinic Laboratories. Test ID: OSCAL — Osteocalcin, Serum. Reference values: 18 years and over 9–42 ng/mL. Electrochemiluminescence immunoassay detecting intact osteocalcin and N-terminal-mid fragments.
- 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
- Lee AJ, Hodges S, Eastell R. Measurement of osteocalcin. Ann Clin Biochem. 2000;37(Pt 4):432–446. doi:10.1177/000456320003700402
- 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
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.