Insulin stress test

Investigation Protocol

Indications

To test anterior pituitary growth hormone and ACTH reserve in patients suspected or confirmed as having anterior pituitary failure.

Preparations and precautions

This test is absolutely contra-indicated in patients with epilepsy, cardiac rhythm disturbances, ischaemic heart disease, previous cerebrovascular accidents, or any unexplained fits or collapses.

Estrogen containing medications such as HRT or the pill must be stopped six weeks prior to this test. Patients should be fasting from midnight before the test, and are asked to bring their medications with them for documenting at the start of the test, and to be taken at the end of it.

ECG, 9am cortisol, free thyroxine must all be performed and checked by a doctor before proceeding with this test.

Protocol

Document that the ECG, thyroid function and cortisol have all been checked by a doctor, and that the patient has no history of fits, faints or funny turns, before proceeding with the test. Patients should be fasted from midnight, with only water to drink. The patient should then be weighed without shoes and asked to lie down throughout the test.

Insert an intravenous cannula, preferably into a large antecubital vein, and flush with 0.9% Normal Saline. Start a slow running 0.9% saline infusion to ensure patency throughout the test. This should be prescribed by medical staff prior to starting the test.

Take blood into one fluoride oxalate, and one plain clotted bottle, for glucose, cortisol and GH, and label this as time -10 minutes. Repeat this blood test after 10 minutes recumbence, labelled 0 minutes.

Administer 0.15units/kg body weight human actrapid insulin intravenously, as prescribed. Where requested, and in patients known to have acromegaly, administer 0.3units/kg. Take a fluoride oxalate blood sample for glucose alone at 20 minutes, and perform near patient testing of venous blood for glucose at this point.

Repeat near patient glucose testing with every sample throughout the test, or whenever the patient becomes symptomatic. Repeat sampling in both fluoride oxalate, and plain bottles at +30, 45, 60, 90, 120 minutes, for glucose, GH and or cortisol, and label the samples with the patient details and time.

If the patient does not achieve clinical hypoglycaemia, i.e. is not sweaty, light headed, hungry or tachycardic, or the blood glucose level does not fall below 2.2 mmol/l, administer a repeat full dose of intravenous insulin at 45 minutes. When this is necessary, continue sampling at 60, 75, 90, 120 and 150 minutes.

As soon as the patient becomes symptomatically hypoglycaemic, and this is confirmed biochemically, treat immediately with oral glucose, fruit juice, lucozade or carbohydrate meal. Severe symptoms may require intravenous glucose: 25ml of 50% dextrose followed by a 5% dextrose infusion. Treating the hypoglycaemia has no effect on the results of the test, which should be continued in all circumstances. Very severe reactions may require emergency treatment with hydrocortisone and intravenous fluids, which should be prescribed and supervised by a doctor.

At the end of the test the cannula may be removed and the patient is given a high carbohydrate meal and a sweet drink. The patient should then be observed for a minimum of two hours following the hypoglycaemic stimulus. If the patient normally takes steroid replacement they should be given their usual dose of steroids at the end of the test.

Patients with diabetes

Patients with diet or tablet controlled diabetes can undergo this test with no special precautions. However, patients with insulin treated diabetes require a special procedure. These patients should fast from midnight and omit their morning dose of insulin. On arrival at 8am, they should start an insulin infusion at 0.5units/hour. At 9am the blood glucose (BG) should be rechecked and the infusion continued if the BG is <9. If the BG >9 the infusion should increase to 1unit /hour. At 10am they should then start the IST protocol as above. 

In the unlikely event that they become hypoglycaemic (<2.2) during the infusion, there is no need to give an actrapid bolus, the infusion can be stopped and sampling started as per protocol. At the end of the test, give lunch time insulin with lunch if on basal bolus, or give half usual morning dose if on a twice daily regime. 

Pre-pubertal adolescents and adults with untreated hypogonadism

These patients will need priming in advance with stilboestrol 1mg twice daily for 48 hours prior to the test in both sexes, which should be arranged by the referring endocrinologist before arranging the test.

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Patient information sheet

Insulin stress test

Your doctor has arranged for you to undergo a special series of blood tests, known as the insulin stress test. The test will be explained to you again on the day, but it is important that you have read this information beforehand, so that you can ask any questions before proceeding. You will also be asked to give consent for the procedure on the day of the test.

The insulin stress test is designed to measure the function of the pituitary gland under stress. This is achieved by obtaining blood samples for hormone measurement, through a small tube (cannula) inserted into a vein in your arm. Following baseline blood samples, an injection of insulin is given through the cannula by a specialist nurse or doctor. This will lower your blood sugar level, inducing what is medically termed hypoglycaemia. This counts as the stress, as people usually feel very hungry, faint and sweaty. These symptoms are generally not severe and last for about 10-15 minutes.

Once hypoglycaemia has been achieved, the test will be reversed by giving you food and drink usually in the form of a jam sandwich and tea/coffee, or by giving a glucose injection through the cannula, depending on how you feel at the time. Blood sampling continues for two hours after the initial injection.

After lowering your blood sugar levels, we measure your stress hormones cortisol and growth hormone. This will tell us if your pituitary gland is working normally or if you require tablet treatment.

This test should not be performed on anyone who has epilepsy or heart problems such as angina, previous heart attack or coronary artery bypass surgery. If you have or have had any of these it is important that you notify us before the test date. To be sure that this test is safe for you, if you have not had one done recently, we will arrange a routine ECG tracing of your heart.

Please come fasting, i.e. nothing to eat or drink from midnight except water. Do not take any medication (with the exception of any type of desmopressin) on the day of this test, but please bring it all with you, so that we can check and document it, and so that you can take it after the test. If you usually take hydrocortisone tablets, please omit the lunchtime and teatime doses on the day before, and on the morning of the test. If you take any diabetes medication, please contact us, to discuss special arrangements for the test. Please refrain from smoking on the day of the test.

We will feed you after the test. You should expect your stay at the hospital to last about three hours. You may wish to bring a book or magazine to read. We do have a radio, or you can access Patientline, alternatively you may like to bring your own personal radio/cd player (please bring your own headphones). You may bring someone to stay with you during the test but there is not enough space for more than one person.

Children are discouraged, as this can be disruptive to other patients. However if this poses a particular problem for you, please discuss this with the senior nurse in the unit. The test can be quite tiring so it is best if you get someone else to bring you in and take you home.

You should not drive for two hours after the test.

If you have any queries about the test, please contact: 

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