Effect of sample storage on human blood cholinesterase activity after inhibition by carbamates.

During an operational field trial with propoxur it was observed that the inhibition of whole blood cholinesterase was greater when samples were stored before the assay. Since measurement of cholinesterase activity is not always possible immediately after sampling, the effects of different storage conditions were evaluated. Human blood cholinesterases were inhibited in vitro by methylcarbamates and stored at different pH values, temperatures, and sample dilutions. The results showed that the degree of cholinesterase inhibition does not change if samples are diluted 300-fold with buffer at pH 5.0 at 4 degrees C and the enzyme activity measured within 4 hours after dilution. These conditions of storage were equally satisfactory for each of the three methylcarbamates studied and are therefore likely to apply to other carbamates as well.