Investigation of suspected glucose intolerance
Diagnosis should be based on two independent glucose measurements, unless the patient has symptoms of diabetes. All samples should be collected into fluoride oxalate blood tubes, as glucose deteriorates rapidly in inappropriate samples leading to potential errors in diagnosis.
The recommended initial test is either a two hour post-prandial or a random blood sample:
A non-fasting, venous plasma glucose concentration less than 6.1 mmol/L is normal; 11.1 mmol/L or greater is diagnostic for diabetes mellitus.
A non-fasting, venous plasma glucose concentration between 6.1 mmol/L and 11.1 mmol/L should be followed up by a fasting level.
A fasting venous plasma glucose concentration less than 6.1 mmol/L is normal and one of 7.0 mmol/L or greater, is diagnostic of diabetes mellitus. Between these levels an OGTT can confirm the degree of glucose intolerance.
By measuring venous plasma glucose concentration, four possible states of glucose metabolism may be defined:
2. Impaired fasting glycaemia – IFG
3. Impaired glucose tolerance – IGT
4. Diabetes mellitus – DM
IFG and IGT are intermediate states of carbohydrate intolerance and are risk factors not only for subsequent development of diabetes mellitus but also cardiovascular disease and should form part of a cardiovascular risk assessment.
These conditions are defined as follows in terms of plasma glucose concentration:
1. Normal: fasting venous plasma glucose of less than 6.1 mmol/L
2. IFG: fasting venous plasma glucose of 6.1 to less than 7.0 mmol/L and (if measured) 2 hr post 75g glucose load less than 7.8 mmol/L
3. IGT: fasting venous plasma glucose less than 7.0 mmol/L and 2 hr post glucose load of 7.8 to less than 11.1 mmol/L.
- Venous plasma glucose of 11.1 mmol/L or greater, at any time
- Fasting venous plasma glucose of 7.0 mmol/L or greater
- Post 75g OGTT – 2hr venous plasma glucose of 11.1 mmol/L or greater
Diagnostic threshold for diabetes is ≥ 48 mmol/L
(Last reviewed November 2019)