Suxamethonium Apnoea — FRCA Primary MCQ
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Correct answer: D — Homozygous atypical plasma cholinesterase
Explanation lettering: E = shown as B · B = shown as E
D is correct. The dibucaine number is the percentage inhibition of plasma butyrylcholinesterase by dibucaine and therefore characterises enzyme phenotype rather than measuring its absolute concentration. Usual enzyme is strongly inhibited, giving a number around 80; heterozygous atypical enzyme gives an intermediate value around 60; and homozygous atypical, dibucaine-resistant enzyme gives a value around 20. The latter has reduced affinity for suxamethonium and may cause paralysis lasting several hours. Acquired cholinesterase deficiency reduces enzyme activity or concentration but does not itself produce the characteristic low dibucaine number. A fluoride-resistant variant is distinguished principally by its fluoride number, not by a dibucaine number of 20.
Reference: Arnold WP. A rapid, semi-automated method for determining dibucaine numbers. Anesthesiology. 1981;55(6):676-679. https://pubmed.ncbi.nlm.nih.gov/7908176/