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Glucose Tolerance Tests in Primary Care

Glucose tolerance tests are rarely needed to diagnose diabetes mellitus. If the patient has suggestive symptoms, measure the fasting blood glucose. A fasting venous plasma glucose of 7.0 mmol/L or greater on two different days is consistent with diabetes mellitus even in the absence of symptoms under the 1998 W.H.O. criteria. These criteria also defined a new category of intolerance called Impaired Fasting Glycaemia, where fasting venous plasma is 6.1 – 6.9 mmol/L. Currently, the advice of the British Diabetic Association is that people with IFG should have a full GTT, but normally the full GTT is of very limited value and is only indicated to exclude a diagnosis of diabetes mellitus or to establish a diagnosis of gestational diabetes or impaired glucose tolerance.

Procedure

The patient should continue with their normal diet (i.e. a normal intake of carbohydrate) for at least three days prior to the test. The test should be conducted in the morning.

Ideally, any medication known to affect glucose metabolism should be discontinued for a period equal to five times the effective half life. If this is not possible, please indicate the drug(s) on the laboratory request form.

The patient must not have anything to eat or drink (other than plain water) after 2200h on the evening prior to the test or on the day of the test, until it is completed. During the test, the patient should be at rest and should NOT smoke.

Glucose load (Adults)

The equivalent of 75g of anhydrous glucose in solution may be provided as either:

Measure 140ml only of Maxijul and make up to 200ml with water. Ask the patient to drink all the Maxijul followed by a further 100ml of water.

Please note that other flavours of Lucozade, Lucozade Light and Lucozade Sport Isotonic drinks contain different amounts of glucose. The glucose (energy) content of the product must be checked before use.

The volume of Lucozade required for adults is 394ml. (To avoid excessive fizziness, open the bottle 30 mins before the test begins).

Whichever glucose solution is chosen, the patient must consume the whole drink as quickly as possible and in no more than 5 minutes. Time ‘0’ minutes is when the patient begins the drink.

The following venous blood samplesare required:-

Time (min) Blood for Glucose
  (fluoride oxalate – YELLOW)
Basal 0.5ml
0 give glucose drink
60 0.5ml (this may be omitted)
120 0.5ml

Label all specimens clearly with date, time and stage of test and send together to the laboratory with a request form for a glucose tolerance test.

NB – Glucose values required to diagnose DM vary with the type of sample. The values for whole blood, capillary blood and venous plasma differ. Normally, the laboratory only accepts fluoride-oxalate samples (yellow top) for glucose analysis and reports values for their venous plasma glucose concentration. The WHO and British Diabetic Association recommend that point of care testing equipment should NOT be used for the diagnosis of diabetes but that blood glucose specimens should be analysed in an accredited laboratory.

Interpretation (WHO criteria)

Venous Plasma Glucose mmol/L

  Normal Diabetes Mellitus Diabetes Mellitus
(“Provisional”)
Impaired
Glucose
Tolerance
Impaired
fasting
glycaemia
Fasting
6.0 and below 7.0 and above 7.0 and above Below 7.0 6.1 – 6.9
  and and or and and
2 hour 7.7 and below 11.1 and above 11.1 and above 7.8 – 11.0 7.7 and below
      Note: If at least
one other
abnormal level on
another occasion,
then diagnosis of
DM can be made
   

Notes