Nitrous oxide and formiminoglutamic acid: excretion in surgical patients and anaesthetists.

We have investigated the possible toxicity of nitrous oxide on vitamin B12 and its sequelae upon folic acid metabolism using the urine formiminoglutamic acid excretion test, an index of the functional state of folate metabolism. Ten control subjects not exposed to nitrous oxide and five patients receiving limb surgery under local anaesthesia excreted normal amounts of formiminoglutamic acid in urine for 6 days. Fifty patients received nitrous oxide anaesthesia for similar surgery and, of these, 22 had a dose-dependent increase in excretion on the first 2 days after operation. There were large individual variations. Exposure to 70% nitrous oxide appeared to cause abnormal metabolism of folate when exposure was greater than 90 min. Ten anaesthetists demonstrated normal excretion of formiminoglutamic acid; their exposure to nitrous oxide was typical of that in other studies of theatre environmental pollution.