The effect on cholinesterase activity of storage of undiluted whole blood sampled from men exposed to o-isopropoxyphenyl methylcarbamate (OMS-33).

An observation made during an operational field trial with o-isopropoxyphenyl methylcarbamate (OMS-33) suggested that the cholinesterase in undiluted blood samples from workers exposed to that insecticide was inhibited during the storage of those samples before assay.A laboratory investigation has now shown that rapid falls of whole-blood cholinesterase activity do indeed occur during the storage of undiluted samples collected from workers at different times after exposure to OMS-33, but that no such change occurs during the same period of time in circulating blood.Several possible explanations are put forward to account for the fact that, although the carbamate persists in the circulating blood for several hours after cessation of exposure, it does not appreciably inhibit cholinesterase in vivo but only under storage conditions in vitro. The implications of this phenomenon, in so far as methods for determining cholinesterase activity in persons exposed to carbamate insecticides are concerned, are discussed; and the authors point out that, whenever storage of undiluted samples precedes the assay, the spectrophotometric method should be regarded as giving results reflecting daily fluctuations of inhibitor concentration rather than as measuring the actual whole-blood cholinesterase in the body.