Suxamethonium Apnoea Dibucaine Number — FRCA Final MCQ

Instant feedback + full explanation. One question, done properly.

HardGeneral AnaesthesiaSuxamethonium Apnoea Dibucaine NumberFRCA Final

A patient experienced approximately 4 hours of neuromuscular paralysis after receiving suxamethonium. Subsequent investigation demonstrates measurable plasma butyrylcholinesterase activity and a dibucaine number of 20. Using conventional inhibition phenotyping, which plasma cholinesterase classification is most consistent with these findings?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: E — Atypical–atypical homozygous phenotype

The correct answer is E. The dibucaine number is the percentage inhibition of plasma butyrylcholinesterase by dibucaine. Usual enzyme is strongly inhibited, giving a value around 80; a usual–atypical heterozygous phenotype typically gives an intermediate value around 60. A value of approximately 20–30 is classically associated with the atypical–atypical homozygous phenotype and markedly prolonged suxamethonium or mivacurium block. Fluoride-resistant variants are identified principally using the fluoride number rather than the dibucaine number. Silent variants produce absent or extremely low enzyme activity, so a dibucaine number may be unobtainable or uninformative. Modern molecular studies show that biochemical phenotype does not invariably prove genotype; therefore, this result is described as most consistent with the conventional homozygous atypical phenotype rather than as definitive genetic confirmation.

Reference: Zabihi E et al. Pseudo-cholinesterase polymorphism in Mazandaran province (North of Iran), 2012. https://pubmed.ncbi.nlm.nih.gov/42120224/