Insulin Tolerance Test
(ITT)

Sample type

Serum (Red-top tube or gel-barrier tube)

Uses

1. Assessing syndromes of extreme insulin resistance

2. Crude classification of insulin sensitivity

3. Assessing GH (Growth Hormone) deficiency

Precautions

The ITT test generally involves IV administration of insulin (e.g., 0.1 U/kg) following an overnight fast, with collection of samples at baseline, 30, 60, 90, and 120 minutes (times may vary) for cortisol, glucose, and ACTH testing. Hypoglycemia is achieved when the nadir glucose concentration is reduced >50% from baseline or <45 mg/dL . Under normal physiological conditions, ITT should induce a cortisol response of ≥20 μg/dL. In primary glucocorticoid deficiency, the cortisol response is deficient, and an elevated ACTH value is observed. Conversely, in secondary or tertiary glucocorticoid deficiency, both cortisol and ACTH responses are deficient. The ITT must be performed with caution under experienced medical supervision because of the risks of hypoglycemia (e.g., obtundation, seizures, and death).

Interfering factors

• In premenopausal women, the test can be performed at any phase of the menstrual cycle, because there are no cycle effects on the hypothalamic–pituitary–adrenal axis response to insulin-induced hypoglycemia.

• Almost all patients have some degree of perspiration. If the patient does not perspire, the adequacy of the stress stimulus must remain suspect irrespective of the serum glucose concentration.

• Most patients also have a hyperactive precordium (but not tachycardia or hypotension, because they are supine) and feelings of hunger, drowsiness, detachment, or anxiety. The last is common and sometimes severe, and many patients find this an unpleasant experience.

• Patients with primary or secondary adrenal insufficiency or long-standing DM have an impaired compensatory response to hypoglycemia.

• The criteria for a normal serum cortisol response ranged from 18 to about 22 μg/dL in multiple studies. Ideally, reference ranges would be determined locally, but this is rarely done in practice. If serum cortisol reaches this level, it is unimportant whether hypoglycemia was adequate. On the other hand, failure to reach this level is indicative of an inadequate response only if the serum glucose fell to 35 mg/dL or less. If this was not achieved, the stimulus was inadequate and the test must be repeated. It is the serum cortisol concentration that is achieved rather than the increment that is important.

Pre-analytical errors

Unlabeled samples

The corrective action

The sample must be rejected and another sample be obtained.

Post-analytical errors

1- reports sent to the wrong patient

2- write the wrong name in the report or the wrong results.

The corrective action

1-communication with patient, apologising for the error, and providing him with the correct report

2-If the report is not delivered to the patient and this error is discovered, the correct result or the correct name must be written, but if the report is delivered to the patient, you must communicate with him, apologise to him, and tell him that an error has occurred and replace it with the correct report.