Before it gets too sweet: Testing for diabetes

Diabetes is a condition that leads to high levels of glucose in the blood.

Diabetics either produce too little insulin or their bodies do not respond well to insulin, with some people having a combination of both. Because the insulin is not functioning as it should, glucose cannot get into the body’s cells to be used for energy.

Many people have blood glucose levels above the normal range, but not high enough to be diagnosed as diabetes, a condition called pre-diabetes. If the blood glucose level gets above the normal range, the risk of developing full-blown diabetes is increased.

It is important for diabetes to be diagnosed and treated as early as possible because uncontrolled diabetes worsens with time.

If untreated or treated inadequately, it will cause multiple health problems.

Test results constitute part of the information in the diagnosis of diabetes. In making a decision on testing, the doctor takes into account the person’s symptoms; medical, surgical and family history; and physical examination findings.

Urine tests

Urine tests are common, and high urine glucose levels indicate there may be a problem.

However, the blood glucose level at which glucose appears in the urine varies with different persons. For some, blood glucose may be high, but glucose may not appear in the urine; for others, blood glucose may be normal, but glucose may appear in the urine.

As such, urine glucose tests are not as accurate as blood tests.

However, urine tests for ketones are very useful in diabetic ketoacidosis, which is an acute, life-threatening complication that occurs mainly in type 1 diabetes, but is not uncommon in type 2 diabetes.

The condition is a complex disordered metabolic state characterised by very high blood glucose (hyperglycaemia); ketoacidosis, which are toxic compounds; and ketones in the urine (ketonuria).

The only way to confirm a diagnosis of diabetes is to have a blood test done.

There are three tests used to diagnose diabetes – fasting blood glucose and glucose tolerance test.

Fasting blood glucose – The fasting blood glucose (FBS) is a reliable diagnostic test. A single blood sample is taken after an overnight fast. Normal levels are below 6.0 mmol/L. Levels above 7.0 mmol/L on two or more occasions is indicative of diabetes, and no other tests are needed.

Random blood glucose – Random blood glucose (RBS) measures the blood glucose irrespective of when the last meal was. If the RBS is more than 11.1 mmol/L and there are symptoms of diabetes, there is no doubt about the diagnosis. An RBS between 7.8 and 11.1 mmol/L is not necessarily abnormal.

Glucose tolerance test – The interpretation of blood glucose tests may be problematic when the FBS or RBS are not obviously normal or abnormal. In such situations, a glucose tolerance test (GTT) is done.

The GTT is done after an overnight fast of at least 10 but not more than 16 hours. Blood is taken for FBS, followed by consumption of 75 grams of glucose. Blood samples are taken on two or four subsequent occasions, i.e. half hourly or hourly, for up to two hours, to measure the blood glucose.

The blood glucose levels rise and then fall rapidly in a non-diabetic. In a diabetic, it increases to a higher level than normal and does not fall as rapidly.

The following diagnoses would be made:

• Normal response – the FBS is less than 7.0 mmol/L and the two-hour glucose level is less than 7.8 mmol/L

• Diabetes – the FBS is 7.0 mmol/L or more and the two-hour glucose level is more than 11.1 mmol/L

• Impaired glucose tolerance (IGT) – the FBS is less than 7.0 mmol/L and the two-hour glucose level is between 7.8 and 11.1 mmol/L

About one in two of those with IGT have a normal GTT upon repeat testing. Weight loss and exercise may help in the return to normal blood glucose levels in those with IGT. However, about 1-5% of those with IGT develop diabetes annually.

To ensure a reliable result when testing, a person must not have other illnesses, e.g. a cold, is normally active and is not taking medicines that affect blood glucose.

There should be no smoking or drinking on the morning of the test. During the test, one should sit or lie down quietly.

The body’s response to a GTT may vary and requires interpretation by a doctor. An obviously abnormal GTT is diagnostic. If the blood glucose levels are mildly raised, the GTT will be repeated to ensure that a correct diagnosis is made.

What is glycohaemoglobin?

Haemoglobin in the red blood cells (RBC) carries oxygen to the body’s cells. Glycohaemoglobin (also known as glycated haemoglobin), which comprises 6-8% of haemoglobin, is formed when blood glucose attaches itself to haemoglobin.

The glycation of haemoglobin occurs throughout the lifespan of red blood cells, which is normally 120 days. This glycation occurs mostly in days 90-120.

The relative proportion of glycated haemoglobin at any one time depends on the average glucose level over the previous 120 days.

There are three different glycations, with HbA1c being the most useful. Hence, HbA1c is a reliable indicator of the average blood glucose level over the previous four months.

The HbA1c test requires only one sample of blood, which can be taken at any time, even after a meal.

A high HbA1c is found in diabetes and IGT. However, a normal HbA1c can also be found in diabetes or IGT.

As such, HbA1c is not used to diagnose diabetes but to monitor blood glucose control in diabetics.

HbA1c is not useful in conditions in which the average RBC lifespan is much less than 120 days, e.g. blood loss, RBC disorders and when the levels fluctuate between very high and very low.

Other tests

Many diabetics have related conditions, e.g. hypercholesterolaemia or complications such as kidney and eye problems, all of which would require other tests.

It is important to remember that tests are just part of the information required. The doctor will also consider the patient’s history and physical examination findings in planning the diabetic’s management.

Dr Milton Lum is a member of the board of Medical Defence Malaysia. The views expressed do not represent that of any organisation the writer is associated with. The information provided is for educational and communication purposes only and it should not be construed as personal medical advice. Information published in this article is not intended to replace, supplant or augment a consultation with a health professional regarding the reader’s own medical care.