Blood Cobalt Unit Converter
Blood Cobalt Unit Converter
Convert whole blood cobalt between µg/L, ng/mL, nmol/L and µmol/L. The reference intervals from two major laboratories differ nearly fourfold, and the MHRA arthroplasty action level of 7 ppb is above both of them.
Blood Cobalt converter
Mass ⇄ molarWhole blood cobalt 7 µg/L, the MHRA action level
The conversion, and the regulator’s own arithmetic
µg/L = nmol/L × 0.0589332
µmol/L = µg/L × 0.0169684 · ppb = µg/L
- 16.9684
- from cobalt’s atomic weight, 58.933194 (CIAAW 2024). MDA/2017/018 prints 7 ppb as 119 nmol/L; this factor gives 118.78, so the alert’s figure is the rounding and not a different mass
- total cobalt
- not cobalamin. Vitamin B12 contains cobalt, but at a mass three orders of magnitude below what a cobalt assay is asked about, so the two tests are unrelated in practice
- whole blood vs serum
- ARUP states that blood is the preferred specimen for evaluating metal ion release from a metal-on-metal arthroplasty; serum cobalt is used where the question is not the implant
Worked example
Whole blood cobalt 7 µg/L, the MHRA action level
7 × 16.9684 = 118.8 nmol/L
MDA/2017/018 prints this level as "7ppb (119 nmol/L cobalt)", and the 2010 version of the guidance was quoted as 120 nmol/L for the same 7 ppb
Seven times Mayo's reference limit, 1.8 times ARUP's, and inside Labcorp's moderate-risk band for an implant
Cobalt figures, and what kind of figure each one is
| Source | µg/L (ppb) | nmol/L | Kind of figure |
|---|---|---|---|
| Mayo COWB, adults | < 1.0 | < 17.0 | Reference interval, no implant |
| Exeter, whole blood | 0 – 1.18 | 0 – 20 | Reference interval |
| ARUP 0099231 | ≤ 3.9 | ≤ 66.2 | Reference interval |
| Labcorp 738770 | < 3.0 / 3.0 – 10.0 / > 10.0 | < 50.9 / 50.9 – 169.7 / > 169.7 | Low, moderate and high risk bands |
| MHRA MDA/2017/018 | ≥ 7 | ≥ 119 | Device-surveillance action level |
| Exeter, quoted referral threshold | 20.0 | 340 | Referral, with or without symptoms |
| ACGIH BEI, cobalt in urine | 15.0, end of work week | — | Occupational exposure index |
Cobalt in arthroplasty: what the number is and is not
| Question | Answer |
|---|---|
| Specimen | EDTA whole blood, royal blue metal-free tube |
| Who should run it | MHRA: a laboratory in the TEQAS scheme |
| What 7 ppb triggers | Closer follow-up and cross-sectional imaging |
| What it does not decide | Revision surgery, on Labcorp’s and Mayo’s own statements |
| Systemic cobalt toxicity | Cardiomyopathy, hypothyroidism, neuropathy — at far higher levels |
Four reference intervals, and none of them is the action level
Whole blood cobalt in µg/L becomes nmol/L on multiplication by 16.9684, from cobalt’s atomic weight of 58.933194. In whole blood, parts per billion is µg/L. The factor checks against the regulator’s own figures: MDA/2017/018 prints 7 ppb as 119 nmol/L and this factor gives 118.78, a 0.18% rounding.
The reference intervals are the problem on this page. Mayo publishes under 1.0 ng/mL for an adult, ARUP 3.9 µg/L or less, and Exeter 0 to 20 nmol/L, which is 1.18 µg/L. Mayo’s and ARUP’s upper limits are almost fourfold apart for the same analyte in the same specimen, so a cobalt of 2 µg/L is twice the upper limit on one report and well inside it on the other. There is no way to reconcile them and no reason to prefer one; the interval that was printed beside your result is the one that applies to the method that produced it.
The reason most readers are here is an implant. Cobalt-chromium bearings release metal ions, and the MHRA’s alert on metal-on-metal hips sets an action level of 7 ppb for either metal — 119 nmol/L cobalt, 134.5 nmol/L chromium — above which it advises closer follow-up and cross-sectional imaging. That is seven times Mayo’s reference limit and it is a surveillance trigger rather than a concentration at which cobalt is toxic. Both Labcorp and Mayo are explicit that the level alone does not decide revision surgery. Exeter separately quotes a referral threshold of 340 nmol/L, with or without symptoms, which is nearly three times the MHRA figure.
Two things to check before believing a raised result. First the specimen: ARUP states blood is the preferred specimen for evaluating metal ion release from a metal-on-metal arthroplasty, and whole blood, serum and plasma are not interchangeable. Second the tube — Mayo warns that without metal-free collection an elevated cobalt "may be an incidental and misleading finding", and forbids opening the tube for any other test. Cobalt is read with blood chromium, and a total cobalt tells you nothing about vitamin B12 despite cobalamin containing the element.
Frequently asked questions
How do I convert blood cobalt from µg/L to nmol/L?
Multiply by 16.9684, from cobalt’s atomic weight of 58.933194. A whole blood cobalt of 7 µg/L is 118.8 nmol/L — the MHRA’s 7 ppb action level, which the alert itself prints as 119 nmol/L.
Why do Mayo and ARUP give different cobalt reference intervals?
They do, by nearly fourfold: Mayo publishes under 1.0 ng/mL and ARUP 3.9 µg/L or less. A cobalt of 2 µg/L is raised at Mayo and normal at ARUP. Different methods and different reference populations produce different intervals, and the one on your own report takes precedence.
What cobalt level is the MHRA threshold for a metal-on-metal hip?
MDA/2017/018 gives whole blood metal levels at or above 7 ppb — 119 nmol/L cobalt — as indicating the need for closer follow-up and cross-sectional imaging. The 2010 guidance was quoted as 120 nmol/L for the same 7 ppb.
Does a cobalt result say anything about vitamin B12?
No. Cobalamin contains a cobalt atom, but the amount is three orders of magnitude below what a cobalt assay measures, so the two tests are unrelated in practice.
Should whole blood or serum cobalt be used?
Whole blood for an arthroplasty question — ARUP states blood is the preferred specimen for evaluating metal ion release from a metal-on-metal joint, and the MHRA specifies whole blood. Serum cobalt is used where the question is not the implant. They are not interchangeable.
Related calculators
References
- Mayo Clinic Laboratories. Cobalt, blood (COWB). Test catalogue, test ID 60355; 2026. "≥18 years: <1.0 ng/mL"; "Unless these precautions are taken, elevated blood cobalt results may be an incidental and misleading finding".
- ARUP Laboratories. Cobalt, whole blood. Laboratory Test Directory 0099231; 2026. "Less than or equal to 3.9 µg/L"; "Blood is the preferred specimen type for evaluating metal ion release from metal-on-metal joint arthroplasty".
- Medicines and Healthcare products Regulatory Agency. Medical Device Alert MDA/2017/018: all metal-on-metal (MoM) hip replacements — updated advice for follow-up of patients. Issued 29 June 2017. "Whole blood metal levels ≥7ppb (119 nmol/L cobalt or 134.5 nmol/L chromium)" indicates "the need for closer follow-up and cross-sectional imaging".
- Labcorp. Chromium and cobalt, whole blood — test 738770. Test menu; 2026. Low risk <3.0 ng/mL, moderate 3.0–10.0, high >10.0, each analyte separately; "metal ion levels alone should not be relied on as the sole parameter to determine clinical recommendations for revision surgery".
- Exeter Clinical Laboratory International. Chromium and cobalt — metal ion monitoring. Test handbook; accessed October 2026. Chromium "0–100 nmol/L", cobalt "0–20 nmol/L"; quotes MDA/2010/033 as "above 120 nmol/L (cobalt) or 135 nmol/L (chromium)", and a referral threshold of "a cobalt level > 340 nmol/L, with or without symptoms".
- Mayo Clinic Laboratories. Cobalt occupational exposure, random, urine (COUO). Test catalogue, test ID 607762; 2026. "The…Biological Exposure Index for cobalt is a concentration of 15.0 mcg/L or above at the end of the work week."
- Commission on Isotopic Abundances and Atomic Weights, IUPAC. Standard Atomic Weights 2024. ciaaw.org; accessed October 2026. As 74.921595(6), Mn 54.938043(2), Al 26.9815384(3), Cr 51.9961(6), Co 58.933194(3), Tl [204.382, 204.385]; for the interval elements the IUPAC abridged values Tl 204.38, C 12.011, N 14.007 are used.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/