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The ADA does not recommend it for common practice, unlike the WHO,
which proposes its use to diagnose asymptomatic diabetes. The test is not quite
reproducible (due to the difficulty to comply during its preparatory stages), more
expensive and uncomfortable (see table 2). Nevertheless, it should be taken into
consideration that it can be considered valid in some cases. Only with the fasting
plasma glucose (FPG), 30% of the diabetic population is not diagnosed (unknown
diabetes) (52). This figure is higher if the population group is elderly and even
more so if they are women. According to several studies, diagnose through plasma
glucose two hours after the OGTT is related to higher cardiovascular morbimortality
and diabetes microvascular complications than fasting plasma glucose(54).
The impaired glucose tolerance (IGT) stage can only be diagnosed by glycaemia
two hours after the OGTT.
Therefore, the OGTT is recommended in the following cases:
• When there is a strong suspicion of diabetes (microvascular complications,
symptoms, contradictory or doubtful results, etc.) and there are normal fasting
plasma glucose levels.
• In patients with repeatedly impaired fasting glucose (IFG) (110-125 mg/
dl), to check the diagnosis of diabetes, or with impaired glucose tolerance (IGT),
especially among the elderly and female population.
Table 2. Conditions to develop the Oral Glucose Tolerance Test (OGTT)
Do not carry out the
test in case of
Preparation
Method
Fasting plasma glucose >126 mg/dl.
Acute disease or post-surgical stress (delay for three months).
Pharmacological treatment which cannot be interrupted.
At least three days before follow a free and rich in carbohydrates diet (at least
150 g /day) and do as much physical exercise as desired.
Absolute fasting for 8-12 hours (except water).
Carry out a test in the morning (between 8-10 am).
Oral administration of 75 g of glucose in 250 ml of water (100 g in the case of
pregnant women and 1, 75 g/kg for children).
The patient shall remain seated and will not smoke during the test.
For the general population, a determination after two hours is enough.
Pregnant women will undergo three extractions (1, 2 and 3 hours after taking
100 g of anhydrous glucose).
Glycosylated haemoglobin (HbA 1
c)
This shows the average of the glycaemia determinations in the last two-three
months in a single measure and it can be carried out at any time of the day, without
any prior preparation nor fasting. It is the test recommended to control diabetes.
42 CLINICAL PRACTICE GUIDELINES IN THE NHS