Glucose Tests

The fasting blood glucose test (collected after an 8 to 10 hour fast) is used to screen for and diagnose diabetes. A 2-hour 75 g oral glucose tolerance test (OGTT / GTT) may also be used to diagnose diabetes (and acromegaly). To be certain of a diagnosis of diabetes if the person has no symptoms the test should be repeated (fasting glucose, random glucose or HbA1c) at different times (e.g. within two weeks). In the OGTT, the person has a fasting glucose measurement, then drinks a glucose drink to 'challenge' their system, and has another glucose test two hours later.

Gestational diabetes is a temporary type of hyperglycaemia seen in some pregnant women, usually late in their pregnancy. The risk of gestational diabetes is assessed at the booking appointment, and at subsequent antenatal appointments.

The risk factors are:

• Body mass index above 30kg/m2
• Previous large baby weighing 4.5 kg or above
• Previous gestational diabetes
• Family history of diabetes
• Minority ethnic family origin with a high prevalence of diabetes

If the woman has any of these risk factors, she can be offered a 75 g 2-hour OGTT at 24-28 weeks.

However, if the woman has had gestational diabetes in a previous pregnancy, she may be offered early self-monitoring of blood glucose or a 75 g 2-hour OGTT as soon as possible after booking (whether in the first or second trimester), followed by a further 75 g 2-hour OGTT at 24-28 weeks if the results of the first OGTT are normal.

It is also noted that if glucose is detected in urine during routine antenatal care, this may indicate undiagnosed gestational diabetes and further testing as mentioned above may be recommended.

When a woman has gestational diabetes, her doctor will usually request glucose testing throughout the rest of her pregnancy and after delivery to monitor her condition.

People with diabetes who are taking insulin must monitor their blood glucose levels, several times a day, to see whether they need to alter their medications. This is usually done by placing a drop of blood from a finger prick onto a plastic indicator strip and then inserting the strip into a glucose meter, a small machine that provides a digital readout of the blood glucose. Many people with diabetes who are not taking insulin also monitor their blood glucose concentrations.

Urine glucose testing is seldom requested. At one time, it was used to monitor diabetes, but it has been largely replaced by the more sensitive and “real time” blood glucose. Urine glucose is, however, one of the substances measured when a urinalysis is performed. A urinalysis may be done as part of an examination or antenatal appointment, when a doctor suspects that a patient may have a urinary tract infection, or for a variety of other reasons. Urine glucose and other reducing sugars may be tested when doctors suspect inherited metabolic diseases. (eg: Fanconi syndrome, galactosaemia).

In those with suspected hypoglycaemia, glucose levels are used to help confirm a diagnosis (low glucose). Further investigations are needed to find the cause of hypoglycaemia.

Fasting blood glucose testing may be requested as part of a routine examination especially in those people at high risk of developing diabetes.
The risk factors are:

• Those with a strong family history of diabetes (first degree relative)
• Those who are overweight and obese
• High risk ethnicity/race (more prevalent among South Asian, Chinese, African-Caribbean and black African than white population)
• People with other health conditions (those who have had myocardial infarction or a stroke, poly cystic ovary syndrome (PCOS) or have a history of gestational diabetes)
• People with mental health conditions or learning disabilities
• People taking certain drugs such as steroids, anti-retroviral and some antipsychotic drugs
• Previous impaired glucose tolerance, impaired fasting glucose or elevated HbA1c

There are different risk scores available to identify those at risk of developing diabetes. The fasting blood glucose test may also be used to help diagnose diabetes when someone has symptoms of hyperglycaemia such as:

• Increased thirst
• Increased urination
• Urinary tract infections
• Tiredness
• Blurred vision
• Slow-healing infections

Blood glucose may also be tested when a person has symptoms of hypoglycaemia, such as:

• Sweating
• Hunger
• Trembling
• Anxiety
• Confusion
• Blurred Vision

Glucose testing is also done in emergency settings to determine if low or high glucose is contributing to symptoms such as fainting and unconsciousness.

High levels of glucose most frequently indicate diabetes but many other diseases and conditions can also cause raised glucose concentrations in the bloodstream. The following information summarises the meaning of the test results.

Fasting Blood Glucose

Oral Glucose Tolerance Test (OGTT)

Diagnosis of Gestational Diabetes (OGTT)

Either of below is used for diagnosis, however other thresholds are also in use.

• A fasting plasma glucose level of 5.6 mmol/L or above
• A 2-hour plasma glucose level of 7.8 mmol/L or above

Some of the other diseases and conditions that can result in elevated glucose levels include:

• Acromegaly
• Acute stress (response to trauma, heart attack, and stroke for instance)
• Long-term kidney disease
• Cushing syndrome
• Drugs, including: corticosteroids, tricyclic antidepressants, diuretics, adrenaline, oestrogens (birth control pills and hormone replacement therapy [HRT]), lithium, phenytoin (Dilantin), aspirin
• Excessive food intake
• Hyperthyroidism
• Pancreatic cancer
• Pancreatitis

Moderately increased levels may be seen with impaired glucose tolerance. This condition, if left un-addressed, can lead to type 2 diabetes mellitus.

Low glucose levels (hypoglycaemia) are seen with:

• Adrenal disease (Addison disease)
• Drinking alcohol
• Drugs, such as: paracetamol and anabolic steroids
• Extensive liver disease
• Hypopituitarism
• Hypothyroidism
• Insulin overdose
• Overdose of glucose lowering medications
• Insulinomas (insulin-producing pancreatic tumours)
• Starvation

Hypoglycaemia is characterised by a drop in blood glucose to a level where first it causes nervous system symptoms (sweating, palpitations, hunger, trembling, and anxiety), then begins to affect the brain (causing confusion, hallucinations, blurred vision, and sometimes even coma and death). An actual diagnosis of hypoglycaemia requires satisfying the following three criteria:

• Documented low glucose levels (the cut offs vary depending on the age of the person and where the sample was taken from)
• Symptoms of hypoglycaemia
• Reversal of the symptoms when blood glucose levels are returned to normal.

In some people with symptoms of low blood glucose, dietary changes such as eating frequent small meals and several snacks a day and choosing complex carbohydrates over simple sugars may be enough to help. Those with fasting hypoglycaemia may require IV (intravenous) glucose, if dietary measures are insufficient.

It should be remembered that the glucose level in a finger drops as the hand gets colder (as blood supply slows down) therefore in people who have fainted (and therefore have poor blood supply to their limbs also) low glucose can be detected on finger pricks. This does not mean however that the reason for the collapse was low blood glucose levels.

If you do not have diabetes there is usually no reason to test glucose levels at home. If you have been diagnosed with diabetes, however, your doctor may recommend a home glucose monitor (glucose meter). You will be given guidelines for how high or low your blood sugar should be at different times of the day. By checking your glucose regularly, you can see if the diet and medication schedule you are following is working properly.