Cholinesterase Unit Converter
Cholinesterase Unit Converter
Convert plasma (pseudo)cholinesterase between U/L, IU/L, µkat/L and nkat/L, and keep it separate from the red-cell acetylcholinesterase used in organophosphate poisoning.
Cholinesterase converter
Activity unitsPlasma cholinesterase 8,500 U/L
Formula and conversion factor
U/L = µkat/L × 60
nkat/L = U/L × 16.667
- U
- 1 unit = 1 µmol of substrate hydrolysed per minute
- kat
- 1 katal = 1 mol per second, so 1 U/L = 1/60 µkat/L
- two enzymes
- plasma butyrylcholinesterase is a different protein from red-cell acetylcholinesterase; the units are the same but the tests are not interchangeable
- dibucaine number
- the percentage inhibition of the enzyme by dibucaine — a phenotype, not an activity, and the measurement that identifies an atypical variant
Worked example
Plasma cholinesterase 8,500 U/L
8,500 ÷ 60 = 141.667 µkat/L
= 141,667 nkat/L = 8,500 IU/L
Within the adult reference interval of 5,300–12,900 U/L
Reference interval in both conventions
| U/L | µkat/L | |
|---|---|---|
| Adult reference interval | 5,300 – 12,900 | 88 – 215 |
| Below the interval — investigate | < 5,300 | < 88 |
| Marked depression, as in significant organophosphate exposure | < 2,650, roughly half the lower limit | < 44 |
Plasma cholinesterase against red-cell acetylcholinesterase
| Plasma (butyryl)cholinesterase | Red-cell acetylcholinesterase | |
|---|---|---|
| Also called | Pseudocholinesterase, BChE | True cholinesterase, AChE |
| Site | Synthesised in the liver, circulates in plasma | Red cell membrane; the same enzyme as at the synapse |
| Falls after organophosphate exposure | Early, within hours | More slowly |
| Recovery | Weeks, as the liver resynthesises it | Months, with red cell turnover |
| Best reflects | Recent exposure and liver synthetic function | The true burden of synaptic inhibition |
| Used for suxamethonium apnoea | Yes, with the dibucaine number | No |
Two different enzymes, two different questions
This page converts plasma cholinesterase, also called pseudocholinesterase or butyrylcholinesterase. It is reported as catalytic activity in U/L, the numerically identical IU/L, or in µkat/L and nkat/L, and since a unit is one micromole per minute while a katal is one mole per second, the conversion is a division by 60. The consequential point is not arithmetic: plasma cholinesterase is a different enzyme from the red-cell acetylcholinesterase used to assess organophosphate exposure, and the two are constantly confused because they share a name, a unit and an indication.
The first clinical use is prolonged apnoea after suxamethonium or mivacurium. Both drugs are cleared by plasma cholinesterase, so a deficient or structurally atypical enzyme leaves the patient paralysed long after the expected duration. Activity alone does not settle it. The dibucaine number — the percentage by which dibucaine inhibits the enzyme — characterises the variant, and a patient with an atypical enzyme can have activity within the reference interval. Testing the family matters, because the trait is inherited and the anaesthetic implications carry across relatives.
The second use is organophosphate and carbamate poisoning. Plasma cholinesterase falls early, within hours of exposure, which makes it a sensitive marker that exposure has occurred, and it recovers over weeks as the liver resynthesises the protein. Red-cell acetylcholinesterase is the same enzyme found at the synapse, falls more slowly, and recovers only over months with red cell turnover, so it better reflects the true burden of inhibition and the depth of poisoning. Neither should be used to withhold atropine and pralidoxime from a patient with a cholinergic syndrome.
A low result is not specific. Plasma cholinesterase is synthesised in the liver, so it falls in advanced liver disease and has been used as a marker of synthetic function; it also falls in malnutrition, in protein-losing states, in pregnancy and with the oral contraceptive pill. Because reference intervals differ so much between methods, a percentage fall from a patient’s own baseline is far more informative than any single absolute figure.
Frequently asked questions
How do I convert cholinesterase from U/L to µkat/L?
Divide by 60. A unit is one micromole of substrate hydrolysed per minute and a katal is one mole per second. A plasma cholinesterase of 8,500 U/L is 141.667 µkat/L.
Is plasma cholinesterase the same as red-cell acetylcholinesterase?
No. Plasma cholinesterase is butyrylcholinesterase, made in the liver; red-cell acetylcholinesterase is the same enzyme found at the synapse. They share units and some indications but are different proteins, and requesting the wrong one is a common error.
Which test is better after organophosphate poisoning?
Plasma cholinesterase falls early and is a sensitive marker that exposure has occurred, but it recovers within weeks. Red-cell acetylcholinesterase recovers only over months with red cell turnover and better reflects the true burden of synaptic inhibition.
What is the dibucaine number?
The percentage by which dibucaine inhibits the patient’s enzyme. It characterises the variant rather than measuring how much enzyme is present, which matters because a patient with an atypical enzyme can have activity within the reference interval.
What else lowers plasma cholinesterase?
It is made in the liver, so advanced liver disease lowers it, and it has been used as a marker of synthetic function. Malnutrition, protein-losing states, pregnancy and the oral contraceptive pill also lower it, so a low value on its own is not specific.
Related calculators
References
- Eddleston M, Buckley NA, Eyer P, Dawson AH. Management of acute organophosphorus pesticide poisoning. Lancet. 2008;371(9612):597–607.
- Soliday FK, Conley YP, Henker R. Pseudocholinesterase deficiency: a comprehensive review of genetic, acquired, and drug influences. AANA J. 2010;78(4):313–320.
- Rifai N, Horvath AR, Wittwer CT, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018.
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.