An audit of the insulin tolerance test in adult subjects in an acute investigation unit over one year

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Abstract
OBJECTIVE We audited our practice of insulin tolerance testing (ITT) in terms of safety and technical success. We reviewed the results of those tests performed over a 12-month period. By relating peak Cortisol response to 0900 h screening Cortisol level, we determined whether we could reduce the number of tests performed. DESIGN The results of all ITTs performed on our unit between 1 January and 31 December 1991 were reviewed. PATIENTS AND MEASUREMENTS Patients were pre-screened by measurement of serum Cortisol and thyroxine, and recording of an electrocardiogram. A subnormal serum thyroxine (prior to the ITT from 97 to 431 nmol/l. The lowest level of screening Cortisol above which all patients would be expected to achieve the normal peak Cortisol of 580 nmol/l or over is therefore 494 nmol/l. If this cut-off level had been adopted, 10 (8%) ITTs need not have been performed if their only purpose had been to assess Cortisol reserve. Altering the criterion for the necessary peak Cortisol to 500 nmol/l did not affect the number of ITTs required. Our lower limit for testing could not be revised upwards from 100 nmol/l. Adequacy of Cortisol reserve did not predict a normal GH response to insulin-induced hypoglycaemia. CONCLUSIONS When performed in an experienced endocrine unit with adequate supervision, the insulin tolerance test is a safe procedure. According to the current sample, fewer tests would be performed without detriment to patient care if those with a screening Cortisol of greater than 500 nmol/l did not proceed to testing, unless the purpose of the test was also to exclude GH deficiency. A lower limit of 100 nmol/l appears reasonable and need not be revised upwards.