A young adult develops prolonged apnea after succinylcholine anesthesia. Her brother had a similar episode years ago. Serum cholinesterase activity is low. Which additional test BEST distinguishes homozygous atypical cholinesterase deficiency from a heterozygous carrier state?
- A Serum fluoride level
- B Dibucaine inhibition number ✓
- C Genotyping for CYP2D6 polymorphisms
- D Urinary pseudocholinesterase excretion
Correct answer: B. Dibucaine inhibition number
Explanation
Dibucaine inhibits the normal but not the atypical variant of pseudocholinesterase. The percentage inhibition, called the dibucaine number, is about 80 in homozygous normal individuals, 40 to 60 in heterozygotes, and below 30 in homozygotes for the atypical enzyme who suffer severe apnea. This functional assay separates genotypes better than activity alone, since heterozygotes may have borderline activity. Fluoride number exists as a supplementary test, but dibucaine number is the classic discriminator.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.