Blood Cyanide Unit Converter
Blood Cyanide Unit Converter
Convert whole blood cyanide between mg/L, µg/mL, µg/L and µmol/L. The factor depends on whether the measurand is the cyanide ion or hydrogen cyanide, and the two are 3.9% apart; the published laboratory pairs settle it for the ion.
Blood Cyanide converter
Mass ⇄ molarWhole blood cyanide 2.6 mg/L — the threshold Borron 2007 calls typically lethal
The conversion, and which cyanide is being weighed
mg/L = µmol/L × 0.026018
µg/mL = mg/L · µg/L = mg/L × 1,000
- 38.4349
- the factor on the cyanide ion CN−, molar mass 26.018 (carbon 12.011 plus nitrogen 14.007). This is what the published pairs reproduce
- 37.0014
- the factor on hydrogen cyanide HCN, 27.026 — 3.9% lower, and what the circulating species actually is at blood pH. No laboratory’s published pair of numbers matches it
- mg/L = µg/mL
- the same quantity. Mayo reports mcg/mL and most other sources mg/L, which has caused more confusion on this analyte than the molar mass
Worked example
Whole blood cyanide 2.6 mg/L — the threshold Borron 2007 calls typically lethal
2.6 × 38.4349 = 99.9 µmol/L
Borron and colleagues print exactly this pair as "100 µmol/L (2.6 mg/L)", and the cyanide-ion mass reproduces it; the HCN mass would give 96.2 µmol/L
= 2.600 µg/mL = 2,600.0 µg/L
Which molar mass? The published pairs decide it
| Published pair | Source | On CN− (26.018) | On HCN (27.026) |
|---|---|---|---|
| 0.05 mg/L = 1.92 µmol/L | EBM Consult | 1.92 ✓ | 1.85 |
| 0.5 mg/L = 19.2 µmol/L | EBM Consult | 19.22 ✓ | 18.50 |
| 0.041 mg/L = 1.57 µmol/L | EBM Consult | 1.58 ✓ | 1.52 |
| 1 µg/mL = 38.5 µmol/L | EBM Consult | 38.43 ✓ | 37.00 |
| 100 µmol/L = 2.6 mg/L | Borron 2007 | 2.60 ✓ | 2.70 |
Blood cyanide concentrations and what has been described at them
| mg/L (µg/mL) | µmol/L | Described |
|---|---|---|
| 0.025 | 1.0 | Upper limit for non-smokers (Akron) |
| 0.05 | 1.9 | Mayo: upper limit of normal |
| 0.41 | 15.8 | Akron: smoker average |
| 0.5 – 1.0 | 19 – 38 | Tachycardia, flushing; Mayo calls 0.50 potentially toxic |
| 1.0 – 2.5 | 38 – 96 | Obtundation, coma, respiratory depression |
| 2.6 | 100 | Borron 2007: the typically lethal threshold |
| > 3 | > 115 | Death described |
The one analyte here where the molar mass had to be decided
Seven of the eight analytes in this batch are elements, with exact published atomic weights and nothing to argue about. Cyanide is the exception. A result reported as "cyanide" could be converted on the cyanide ion CN−, molar mass 26.018, or on hydrogen cyanide HCN, 27.026. The two are 3.9% apart, so the choice is visible in the output: a milligram per litre is 38.4349 µmol/L on one and 37.0014 on the other.
The chemistry argues for HCN. Hydrocyanic acid has a pKa of about 9.2, so at blood pH the overwhelming majority of circulating cyanide is the undissociated acid. The published arithmetic argues the other way, and more decisively. EBM Consult’s reference table prints the nitroprusside whole blood window as 0.05 to 0.5 mg/L and 1.92 to 19.2 µmol/L; both ends reproduce on CN− and neither on HCN, which would give 1.85 and 18.50. Independently, Borron and colleagues state the typically lethal threshold as 100 µmol/L (2.6 mg/L) — 2.6018 mg/L on the ion, 2.7026 on HCN. Five pairs from two unrelated sources all reproduce on the ion. This page uses 26.018 and prints the alternative beside it, because a laboratory reporting on the HCN convention would read 3.9% lower in molar units and that is outside what a conversion check would tolerate.
The unit confusion on this analyte is worse than the molar one. Mayo reports in mcg/mL and almost everyone else in mg/L, and the two are the same number — a "potentially lethal" 2.0 mcg/mL is 2.0 mg/L, not 0.002. µg/L, a thousand times smaller, also appears.
Clinically the test is nearly always retrospective. Cyanide assays are referred out and take days, so a patient from a house fire or an industrial exposure is treated on the clinical picture — a profound lactic acidosis with a high venous oxygen saturation and no other explanation. A lactate above 8 to 10 mmol/L in smoke inhalation is the practical surrogate; see raised lactate and the anion gap. Smoke inhalation also delivers carbon monoxide, so carboxyhaemoglobin is measured on the same sample, and hydroxocobalamin treatment interferes with co-oximetry afterwards.
Frequently asked questions
How do I convert blood cyanide from mg/L to µmol/L?
Multiply by 38.4349, from the cyanide ion’s molar mass of 26.018. A whole blood cyanide of 2.6 mg/L is 99.9 µmol/L, which is the figure the literature prints as 100.
Should cyanide be converted using CN− or HCN?
The published pairs say the cyanide ion, 26.018. EBM Consult prints 0.05 to 0.5 mg/L as 1.92 to 19.2 µmol/L and Borron 2007 prints 100 µmol/L as 2.6 mg/L; both reproduce on the ion and neither on HCN’s 27.026. The difference is 3.9%, so it is not cosmetic.
Is mcg/mL the same as mg/L for cyanide?
Yes, exactly the same number. Mayo’s "potentially lethal: 2.0 mcg/mL" is 2.0 mg/L. The unit that differs is µg/L, which is a thousand times smaller, so a cyanide of 2,000 µg/L and one of 2.0 mg/L are the same result.
Do smokers have higher blood cyanide?
Several-fold higher. One laboratory gives a smoker average of 0.41 mg/L against a non-smoker upper limit of 0.025 mg/L, so smoking status changes what counts as a background concentration.
Is a cyanide level used to decide treatment?
Not in practice. The assay is referred out and takes days, so cyanide poisoning is treated on the clinical picture — typically a severe lactic acidosis with preserved venous oxygen saturation after smoke inhalation or an industrial exposure. The level confirms afterwards.
Related calculators
References
- Mayo Clinic Laboratories. Cyanide, blood (FCYNB). Test catalogue, test ID 75370; 2026. "Normal: Up to 0.05 mcg/mL"; "Potentially toxic: 0.50 mcg/mL and greater"; "Potentially lethal: 2.0 mcg/mL and greater"; "Gray top (potassium oxalate/sodium fluoride)".
- EBM Consult. Cyanide level — reference ranges and toxic thresholds. ebmconsult.com; accessed October 2026. Nitroprusside therapy, whole blood "0.05-0.5 mg/L (1.92-19.2 micromol/L)"; serum "0.01-0.06 mg/L (0.38-2.3 micromol/L)"; smoker whole blood "0.041 mg/L (1.57 micromol/L)"; "1 mcg/mL (38.5 micromol/L) or greater are typically associated with significant symptoms". These paired figures fixed the molar mass used here.
- Borron SW, Baud FJ, Mégarbane B, Bismuth C. Hydroxocobalamin for severe acute cyanide poisoning by ingestion or inhalation. Am J Emerg Med. 2007;25(3):551–558. "the typically lethal threshold of 100 µmol/L (2.6 mg/L)" — a pair that reproduces on the cyanide-ion mass and not on HCN’s.
- Akron Children’s Hospital, Laboratory Services. Cyanide. Laboratory test directory; accessed October 2026. Non-smoker "less than 0.025 mg/L", smoker "average 0.41 mg/L"; "0.5 to 1.0 mg/L…tachycardia and flushing"; "1.0 to 2.5 mg/L…obtundation, coma and respiratory depression"; "death with values greater than 3 mg/L".
- PubChem. Cyanide, CID 5975 (CN−), and hydrogen cyanide, CID 768 (HCN). National Library of Medicine; accessed October 2026.
- Agency for Toxic Substances and Disease Registry. Toxicological Profile for Cyanide. Atlanta, GA: US Department of Health and Human Services.
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/