Glucose Tolerance Test (GTT)

The Glucose Tolerance Tests is now only recommended for the diagnosis of Gestational Diabetes.

It may occasionally be used where HbA1c is contraindicated for diagnosis eg in patients with abnormal red cell turnover or if there is a presence of a haemoglobin variant.

Results are interpreted as follows:
Normal Impaired fasting glycaemia (IFG) Impaired glucose tolerance (IGT) Diabetes mellitus Gestational Diabetes
Fasting glucose (mmol/L) < 6.1 6.1 – 6.9 < 7.0 ≥ 7.0 ≥ 5.6
2 hour glucose (mmol/L) < 7.8 (< 7.8 if measured) 7.8 – 11.0 ≥ 11.1 ≥ 7.8

Synacthen Test

A Synacthen Test is unnecessary if a 9:00 a.m. or random cortisol level is > 400 nmol/L.

Take a blood specimen for cortisol at around 9:00 a.m. and send to the lab with the form marked -"Emergency - pre-Synacthen sample", also telephone extension 4755 to inform the Duty Biochemist that you are sending a pre-Synacthen cortisol, giving the patient’s name and your Bleep number. The baseline cortisol result will be phoned within 2 hours.

Cortisol > 400 no need for Synacthen Test

Cortisol < 400 : Give 250µg Synacthen intramuscularly and take serum cortisol specimens at 30 and 60 minutes post Synacthen. Send both blood specimens together to the laboratory, clearly identified with times of specimens and patient’s details..


Cortisol 450 - 500 nmol/L at 30 minutes indicates adequate adrenal reserve. Cortisol > 500nmol/L at 30 minutes indicates adequate adrenal reserve. If this is not achieved until the 60 minute sample, then hypopituitarism should be considered - contact the laboratory for further advice.

Dexmethasone Suppression Test

Patient is given a 2 mg tablet of Dexamethasone between 22.00 and 24.00 hrs. A blood specimen is taken at 09.00hrs for cortisol.


Cortisol suppressed to <50nmol/l excludes Cushing's Syndrome.