Is succinyldicholine the substrate of choice for the measurement of cholinesterase activity?

We have developed a succinyldicholine-based assay for serum cholinesterase (EC 3.1.1.8) to help establish whether patients with suspected sensitivity to drugs of this type have enzyme abnormalities that cannot be detected by conventional laboratory techniques. Although the method discriminates between cholinesterase activities of drug-sensitive and nonsensitive people as well as an assay involving propionylthiocholine does, it has not revealed cholinesterase abnormalities in patients who experienced prolonged apnea after succinyldicholine, in whom no enzyme defect could be demonstrated by other procedures. Of 50 individuals who were apneic for between 20 and 180 min, only one had a cholinesterase activity less than the mean for E1u homozygotes by more than 2.5 SD. We conclude that the number of patients who experience clinical problems due to enzyme abnormalities that at present go unrecognized is small. Consequently, although succinyldicholine might eventually become the substrate of choice for cholinesterase, its advantages over propionylthiocholine are not yet sufficient to encourage its use.