Preoperative biochemical screening.

Biochemical investigations are carried out before a surgical operation either because a patient's condition indicates the need or to detect clinically inapparent abnormalities that may affect surgical and anaesthetic management. Nobody would argue about the first indication, but there is little agreement on the second. The inexperience of junior staff together with anxiety about litigation may lead to excessive testing. Clinical laboratories are under increasing pressure to reduce expenditure , and eliminating unnecessary tests is an essential step. The experience of other disciplines is encouraging: when firm guidelines were issued for routine preoperative chest x ray investigations, for example, their frequency was reduced by half with no apparent increase in morbidity or mortality. I 2 The more common conditions that may be detected biochemically and are ofparticular concern in anaesthesia and surgery are renal and hepatic disease, diabetes mellitus, and abnormal serum electrolyte concentrations, which may occur in various circumstances. Operating on a patient with appreciable renal or hepatic impairment or on one who is incubating viral hepatitis may precipitate further deterioration in function and even overt failure.3-5 If muscle relaxants are given in normal doses to patients with severe renal impairment their effects may recur when the action of the reversing agent, usually neostigmine, wears off.6 The main concern in diabetes is unrecognised hypoglycaemia under general anaesthesia, but this is more likely to happen in patients with diabetes who are being treated. Diabetic ketoacidosis, on the other hand, may arise in undiagnosed diabetes and cause major problems, and indeed death, if unrecognised during a prolonged major operation. Serum sodium and potassium concentrations are the most frequently requested preoperative biochemical investigations,7 of which potassium is the more important. Hyperkalaemia predisposes to cardiac arrest, particularly when suxamethonium chloride (a depolarising muscle relaxant) is given, and hypokalaemia to cardiac arrhythmias, which may be exacerbated by other anaesthetic agents such as halothane. Routine preoperative biochemical screening has been examined in only four studies.8"I In three ofthem unsuspected abnormalities were found in 0.2%,8 0-2%,9 and 1%10 of cases. The fourth" found that 10% of patients over 40 had biochemical abnormalities but that under this age there were none; no indication was given about these abnormalities, but they are said to have made no difference to management. Screening of other patients-those being admitted to hospital,'2-'6 those in general practice,'7 blood donors,'8 and those undergoing health screening'9-has shown unsuspected abnormalities in serum electrolyte concentrations in around …