Plasma (yellow).

Paired venous plasma samples are required.

Sample collection in adults requires:

1) The first sample is taken in the fasting state (8 hours)

2) The second sample is taken 2 hours after ingestion of 75g oral glucose load.

Both samples MUST be labelled with the time taken.

Although it is considered to be more sensitive than a fasting plasma glucose, the OGTT is affected by a number of factors that can cause poor reproducibility.

Patient preparation: duration of fast; prior carbohydrate intake; medications (e.g. thiazides, oral contraceptives and corticosteroids); trauma; intercurrent illness; age; physical activity and weight.

Administration of oral glucose load: quantity of glucose ingested; volume in which administered; and rate of ingestion. 

Test conditions: posture; anxiety; caffeine; smoking; physical activity; and time of day.

Due to the number limitations the OGTT should be undertaken on two separate occasions before the results are considered abnormal (unless the initial results are grossly abnormal).

For a fasting state the patient should refrain from any foods or liquids other than water for 8hrs.

Diagnostic Criteria - Non-Pregnant Individuals:

Normoglycaemia:

Fasting Plasma Glucose ≤6.0 mmol/L AND 2-h Plasma Glucose <7.8 mmol/L

Diabetes:

Fasting Plasma Glucose ≥7.0 mmol/L OR 2-h Plasma Glucose ≥11.0 mmol/L

Impaired Glucose Tolerance:

Fasting Plasma Glucose <7.0 mmol/L AND 2-h Plasma Glucose ≥7.8 and ≤11.0 mmol/L

Impaired Fasting Glucose:

Fasting Plasma Glucose 6.1 to 6.9 mmol/L AND 2-h Plasma Glucose <7.8 mmol/L 

Source: Definition and Diagnosis of Diabetes Mellitus & Intermediate Hyperglycaemia (WHO, 2006)

Diagnostic Criteria - Pregnant Individuals:

Gestational Diabetes:

Fasting Plasma Glucose ≥5.6 mmol/L OR 2-h Plasma Glucose ≥7.8 mmol/L

(Gestational diabetes can be excluded if fasting plasma glucose ≤5.5 mmol/L AND 2-h plasma glucose ≤7.7 mmol/L)

Source: NICE Guideline [NG3] - Diabetes in Pregnancy (NICE, 2015)