Role of lab tests in diabetes

Laboratory tests are used both to screen for diabetes, and to monitor ongoing management in those with diabetes. Screening blood tests include random blood glucose, fasting blood glucose, oral glucose tolerance test (OGTT), and glycosylated haemoglobin (HbA1c). Tests for those with diabetes include the HbA1c, which measures the average blood glucose over the previous weeks, giving an indicator of longer-term blood glucose control. Other blood testing may look at levels of blood lipids (fats and cholesterol). Urine testing (eg, microalbuminuria) is used to look for signs of diabetes complications.

Understanding your lab tests

If you already know you have diabetes, go to the section: 'Tests for diabetes management'.

Testing for diabetes

There are a number of tests that may be done to find out if you have diabetes. These include:

• fasting blood glucose
• oral glucose tolerance test (OGTT)
• random blood glucose
• islet cell antibody test (for type 1 diabetes).

Fasting blood glucose

A fasting blood glucose level is a measure of how much glucose is in your blood when you have not eaten anything for at least 10 hours (check with your doctor or the local laboratory for the number of hours they would like you to fast before the test). The fasting blood test is normally taken first thing in the morning. When you wake up in the morning you visit the laboratory for the test before you have had anything to eat or drink (but you can have a glass or two of plain water while fasting).

A fasting blood glucose level of 7mmol/L or greater is indicative of diabetes. If you have no symptoms of diabetes (thirst, tiredness, repeated infections and needing to pass urine often) the test should be repeated on another day to confirm the diagnosis. Fasting blood glucose level is now the recommended test for finding out if you have diabetes.

Pre-diabetes or impaired fasting glycaemia (IFG)
A fasting blood glucose level can tell you other things as well. If your fasting blood glucose level is from 6.1mmol/L to 6.9mmol/L you may have a condition called 'impaired fasting glycaemia' (IFG) or pre-diabetes. This means that the level of glucose in your blood after eating nothing overnight is higher than it is in people without pre-diabetes or IFG, but not as high as in diabetes.

Most doctors will ask you to have a test called an 'oral glucose tolerance test' (OGTT) if your fasting blood glucose shows that you have IFG. The oral glucose tolerance test may show that you actually have diabetes or 'impaired glucose tolerance' (IGT) instead of IFG.

If your tests show that you have either IFG or IGT you need to take action to manage these conditions. Having either of these conditions means you are more likely to go on to develop diabetes. You are also at higher risk of developing cardiovascular disease, that is, disease of your heart and blood vessels.

If you have IFG or IGT you should:

• Organise to be checked once a year for diabetes (and at any time that you have the symptoms of diabetes). Ask your doctor to put you on an annual recall for this test.
• Make sure you eat in a healthy way
• Keep your weight in a healthy range
• Have your cholesterol checked every year
• Have your blood pressure checked often
• Have 20 - 30 minutes of moderate exercise on most days of the week.

Oral glucose tolerance test (OGTT)

An oral glucose tolerance test is a test where you go to the laboratory before eating anything in the morning. A blood glucose level is taken. Then you will be asked to drink a glass of fluid containing 75 grams of glucose (this is a lot!). You stay in the laboratory and another blood glucose level is taken at one hour and two hours after the drink. These blood glucose levels tell you how well your body uses and stores the glucose you have taken in the drink.

You will normally be asked to do an OGTT if your doctor is unsure whether you have diabetes or not. This is normally if your fasting blood glucose is less than 7mmol/L but more than 6.1mmol/L.

Pregnancy
You will also sometimes be asked to do an OGTT when you are pregnant. All pregnant women in New Zealand are asked to have a 'glucose challenge' test when they are 28 weeks' pregnant. A glucose challenge test is similar to an OGTT but the amount of glucose you are asked to drink is 50g, not 75g. If the results of this test show your body is not using glucose in the normal way you are usually asked to do a full OGTT.

An OGTT is how diabetes of pregnancy (gestational diabetes) is usually diagnosed in New Zealand.

Preparing for an OGTT
An OGTT may not be accurate if you have either eaten a very low carbohydrate diet or been having very strenuous exercise in the three days before taking the test. If you are doing either of these things talk to your doctor before having an OGTT.

The most accurate results of an OGTT will be achieved if it is done after you have had a diet with normal amounts of carbohydrate and moderate exercise only over the previous three days.

What do the results of an OGTT mean?
If you are not pregnant and your blood glucose two hours after an OGTT is 11.1mmol/L or more you have diabetes. However, a second test on another day may be done to confirm the diagnosis.

If you are having a glucose tolerance test to see whether you have diabetes of pregnancy, and your blood glucose two hours after the glucose drink is 9mmol/L or higher, you have diabetes of pregnancy (gestational diabetes).

Random blood glucose level

If you have the symptoms of diabetes you may be asked to go to the laboratory for a random blood glucose test. This is a blood glucose level taken at any time of the day after you have eaten at any time (ie. not in a fasting state). If this level is 11.1mmol/L or more, you have diabetes. However, a fasting blood test may also be carried out on another day to confirm the diagnosis.

Islet cell antibody tests

Many people with type 1 diabetes have antibodies in their blood that show the autoimmune process that results in type 1 diabetes. If you have a direct relative (parent, sibling or child) with type 1 diabetes you can have this test done. If you have these antibodies it's extremely likely that you will develop type 1 diabetes.

Tests for diabetes management

Managing your diabetes means managing a number of factors in your life. You can get a good idea of how effective this management is by the results of some of the laboratory tests you have done. A number of laboratory tests are currently available in New Zealand to measure your blood glucose levels.

Blood (or plasma) glucose level

This measures how much glucose you have in your blood at the time the test is taken. A person without diabetes would nearly always have a blood glucose level somewhere between 4 and 7.5 mmol/L (a 'normal' blood glucose level).
Also see: Home blood glucose testing

Blood glucose series

This is a series of three tests for blood glucose levels. There are different ways of doing this kind of test but a common way is as follows:

• Day one: Have your normal breakfast and medication or insulin. Have your normal mid-morning snack (if you have one) and go to the laboratory for the first blood glucose test before your lunch. Have your usual lunch (and medication or insulin if you take it), then go back to the laboratory 2 hours after lunch and have your second blood glucose test taken. Have your normal food and medication or insulin for the rest of the day.
• Day two: Have no insulin, medication or food when you wake up in the morning. Go to the laboratory as soon as you can and have your third blood glucose test taken. Once this is done have your medication or insulin and your breakfast. Your blood glucose series is now complete.

The aim of such a series is to get a 'picture' of your usual blood glucose levels on a close to normal day.

What are healthy levels for a blood glucose series?
Everybody will have different target levels depending on lots of individual factors. You and your doctor need to work out what are realistic and safe target levels for you.

A general guide to target levels for a blood glucose series is:

• Pre-lunch between 4.5 and 7.8mmol/L
• 2 hours after lunch between 4.5 and 8.5mmol/L
• First thing in the morning (fasting) between 4 and 6mmol/L.

HBA1c level (glycosylated haemoglobin level)

The HbA1c measures your average blood glucose over the past 4 to 6 weeks (possibly longer). It measures how much glucose is stuck onto your red blood cells. Red blood cells have a lifespan of about 6 weeks and so the test gives a good indication of what your overall blood glucose levels have been through that time.

Most diabetes specialists and GPs have a lot of confidence in this test and will use it to help show you how you are going with your blood glucose management.The HBA1c level is not directly equivalent to blood glucose levels. For example, an HbA1c level of 13% means that your average blood glucose for the past 6 weeks has been around 18 - 19mmol/L.

Change to reporting values from October 2011
HbA1c levels have previously been measured as a percentage. However, from October 2011, New Zealand laboratories will be reporting HbA1c values in IFCC (International Federation of Clinical Chemistry and Laboratory Medicine) format, which is in mmol/mol.

What are healthy HBA1c levels?
Target HbA1c levels will vary from person to person. Work out a safe target HbA1c for you with your doctor.

A general range for HbA1c levels [IFCC values in square brackets] is:

• Less than or equal to 7% [up to 53mmol/mol] is a very healthy HbA1c level
• Between 7% and 8% [54 - 64mmol/mol] is a fair HbA1c level and needs work to improve
• Between 8% and 10% [65 - 86mmol/mol] indicates your blood glucose levels are much too high
• Above 10% [87mmol/mol or higher] indicates your blood glucose levels are extremely high.

Note: If you are taking insulin and your HbA1c level is less than 6.2% [44mmol/mol] this almost certainly indicates that you are having lots of low blood glucose levels. Having HbA1c levels this low is not safe when you are taking insulin.
Also see: Low blood glucose type 1 diabetes and Low blood glucose type 2 diabetes

Fructosamine level

This measures your average blood glucose levels over the previous 2 weeks. This measure is not as commonly used in New Zealand as HbA1c levels. This is because many people feel HbA1c levels tend to be more accurate and reliable.

Some diabetes services use fructosamine as a useful measure for women who are pregnant. Because insulin needs (and consequently blood glucose levels) change so rapidly during pregnancy, some services prefer to use fructosamine rather than HbA1c as it looks back over a shorter time and therefore may reflect rapid changes more effectively.

What is a healthy fructosamine level?
If you are not pregnant, the target healthy level is less than 300. When you are pregnant, the target healthy level is less than 200.

Measuring blood fats (cholesterol or lipid levels)

The level of cholesterol or fats in your blood is measured by a test called a blood lipid profile. This measures the level of a number of different fats in your blood.

The fats that are measured are:

• Total cholesterol
• HDL cholesterol (high density lipoproteins)
• LDL cholesterol (low density lipoproteins
• Triglycerides
• Total cholesterol/HDL ratio.

Healthy levels of these fats are*:

• Total cholesterol less than 4.0mmol/L
• HDL cholesterol greater than 1.0mmol /L
• LDL cholesterol less than 2.0mmol/L
• Triglycerides less than 1.7mmol/L
• Total cholesterol:HDL ratio less than 4.0.

[*Heart Foundation of New Zealand figures]

Also see: Cardiovascular risk and diabetes

What other lab tests am I likely to have done?

A test you will have done regularly (it should be done at least once a year) is your 'microalbuminuria' level. This is a laboratory test that is done on a sample of urine. It involves checking for microscopic amounts of protein in your urine.

Protein in urine
Small amounts of protein leaking out into your urine is an early sign that your kidneys are developing damage from your diabetes (diabetic nephropathy). The amount of protein leaking into your urine can tell if you have kidney damage and also if it is getting worse or not.

Your microalbuminuria levels should be plotted on a graph over time. Hopefully, these levels will stay low, but if they are rising then this shows there is a problem happening with your kidneys. It is best if your microalbuminuria levels are less than 2.

If your test comes back with raised microscopic protein levels your doctor may ask for you to have the test again. This is because, sometimes, your microalbuminuria levels can be up if you have an infection or even if you have been exercising hard. If they are up for this reason this is just temporary and not a problem. Repeating the test helps to eliminate a 'false positive' for either of these reasons.

Thyroid function
You may also have your thyroid hormone levels tested (especially if you have type 1 diabetes). Checking your thyroid function involves having a sample of blood taken at the laboratory. Some diabetes clinics will check your thyroid function on a fairly regular basis. Your doctor will discuss the results with you.

Original material provided by Diabetes New Zealand, 2008. Edited by everybody, August 2011.