Sugar and metabolism
Glucose
The blood sugar at the moment of the blood draw; measured fasting, this is the test the GP uses to diagnose or rule out diabetes.
Everything about this testWhat it measures
Glucose is the sugar the body gets from food and uses as fuel; insulin from the pancreas makes sure the cells take up the glucose. If insulin does not work properly or there is too little of it, the blood sugar rises. The value varies throughout the day and rises after eating; that is why the fasting value matters most (at least 8 hours without eating or drinking anything except water). A non-fasting value only says something if it is clearly too high.
- Diabetes mellitus: fasting value of 7.0 mmol/l or higher (venous plasma), or non-fasting 11.1 mmol/l or higher
- Impaired fasting glucose (6.1 to 6.9 mmol/l): a precursor stage with a higher chance of diabetes and of cardiovascular disease
- Blood sugar that is too low (hypoglycemia), especially when using insulin or some diabetes medicines
High
A fasting glucose of 7.0 mmol/l or higher fits diabetes. According to the Dutch GP guideline (NHG), the diagnosis is only certain with two fasting values of 7.0 or higher on different days, or with one high value together with symptoms such as great thirst, frequent urination, weight loss or tiredness. A fasting value between 6.1 and 6.9 mmol/l is called impaired fasting glucose: not yet diabetes, but a higher chance of it. A non-fasting value in healthy people is usually below 7.8 mmol/l; only above 11.1 does it point to diabetes. A temporarily high value also occurs with stress, fever, an acute illness, after a meal or due to medicines such as prednisone. Type 2 diabetes causes no symptoms for a long time.
Low
A low glucose (below about 3.5 to 4 mmol/l) causes symptoms such as trembling, sweating, hunger, palpitations and difficulty concentrating. In people without diabetes, true hypoglycemia is rare: prolonged fasting, heavy exertion, a lot of alcohol, a liver disease, an adrenal or pituitary problem or, very rarely, an insulin-producing tumor (insulinoma) can be the cause. In diabetes it is due to insulin or some tablets. A slightly low fasting value without symptoms is usually normal. A tube that stands for a long time before the laboratory measures it can also give a falsely low result.
Preparation
- Do not eat anything for 8 hours before the sample is taken and drink only water. Ideally have your blood taken in the morning before breakfast.
If your result is abnormal
With a raised fasting value, the GP repeats the fasting measurement in the laboratory; with impaired fasting glucose usually after 3 months and then every year, together with the cardiovascular risk profile. With two values of 7.0 mmol/l or higher, diabetes is diagnosed and care starts in the GP practice.
Reliability
According to the Dutch GP guideline (NHG) on type 2 diabetes mellitus, fasting glucose in venous blood is the test the GP uses to detect and diagnose diabetes; the cutoff values (normal below 6.1; impaired 6.1 to 6.9; diabetes 7.0 mmol/l or higher) follow the WHO criteria. One raised value is not yet a diagnosis: it calls for a second fasting measurement on another day. A normal fasting value does not rule out impaired glucose tolerance (a value that is too high after eating); that requires a separate test that is rarely done in practice. A non-fasting value is only useful if it is clearly above 11.1 mmol/l.
Limitations
- Not being fasting is the main source of error: food, sweet drinks and coffee with sugar or milk in the 8 hours before the blood draw raise the value; only water is allowed.
- Stress, fever, infections, surgery, poor sleep and medicines such as corticosteroids (prednisone), some diuretics and antipsychotics temporarily raise the blood sugar.
- Glucose in the tube is broken down by blood cells if the blood is not processed quickly or does not contain an inhibitor; this can give a falsely low result. Finger-prick meters have a measurement error of 10 to 15 percent; a borderline value must be confirmed in the laboratory.
- The cutoff values apply to venous plasma; capillary blood (finger prick) has slightly different limits.
How common is this
In 2024, about 1.2 million people in the Netherlands were known to their GP to have diabetes, of whom just over 1.1 million had type 2 (61 per 1000 inhabitants; 67 per 1000 men and 56 per 1000 women). It mainly occurs at older age: above the age of 85, about a quarter of men and a fifth of women have diabetes. According to the NHG guideline, about 30 percent of people over 60 have impaired fasting glucose or impaired glucose tolerance; at least half of them never develop diabetes.
Who is this useful for
According to the NHG guideline, useful with symptoms that fit diabetes (thirst, frequent urination, weight loss, recurrent bladder infections or fungal infections, tingling) and as a check every three years from age 45 with overweight (BMI 27 or higher), type 2 diabetes in parents, brothers or sisters, high blood pressure, a disturbed fat metabolism, (an increased risk of) cardiovascular disease, or a Turkish, Moroccan, Surinamese or Hindustani background (Hindustani people from age 35). After gestational diabetes: every year for the first 5 years. In young, slim people without symptoms or family history, the chance of an abnormal result is small. Measure fasting; otherwise the result is hard to interpret.
Downsides and risks
- A non-fasting or a single slightly raised value can cause unnecessary worry; conversely, a normal fasting value can miss raised blood sugar after meals.
Sources
Always discuss an abnormal result with your GP. If a result is strongly abnormal, we try to reach you as soon as possible. Read your result yourself as well and contact your GP if in doubt. A test does not make a diagnosis. Read more about risks and results