Blood sugar tests

Blood sugar tests measure glucose, the sugar in your blood. A fasting sugar test measures it after at least 8 hours without food. A post-meal sugar test measures it about 2 hours after eating. A random sugar test can be done at any time. HbA1c shows your average blood sugar over the past two to three months. In a glucose tolerance test, you give a blood sample, drink a very sweet glucose drink, and give more samples over the next hours.

These tests are used to find prediabetes (sugar that is higher than normal but not yet diabetes) and diabetes, and to check sugar control in people who already have diabetes. A doctor may test you because of symptoms such as extreme thirst, passing urine often or losing weight without trying, or because you are at higher risk, for example if you are overweight or have a family history of type 2 diabetes, high blood pressure or PCOS. In pregnancy, doctors screen for gestational diabetes, usually between weeks 24 and 28.

For a fasting sugar test or a glucose tolerance test, you go without food and drink, except water, for at least 8 hours, and the sample is usually taken in the morning. HbA1c and random sugar tests need no fasting. For a post-meal test, the lab will tell you when to eat and when to come.

A fasting sugar, random sugar or HbA1c test is a simple blood draw:

1. A health worker puts a small needle into a vein in your arm. You may feel a brief sting.
2. Blood runs into a small tube, usually in under five minutes, and goes to the lab.

A glucose tolerance test takes longer. After the fast, a first sample is taken, you drink a very sweet glucose drink, and more samples are taken over the next 2 to 3 hours.

You may have a small bruise where the needle went in. After a glucose tolerance test you may feel light-headed, so have someone take you home.

These tests carry very little risk. You may have slight pain or bruising where the needle went in, and this usually fades quickly. A glucose tolerance test can make you feel light-headed.

The report prints the lab's own normal range, which depends on the test and the method the lab uses. Doctors use set cut-off points to tell normal sugar from prediabetes and diabetes, and your doctor will tell you where your result falls. In India, diabetes can be diagnosed from a fasting sugar, a sugar level 2 hours into a glucose tolerance test, an HbA1c, or a random sugar together with symptoms. A high result is usually confirmed with a second test.

HbA1c can be unreliable with anaemia or another blood disorder, kidney or liver disease, an inherited change in haemoglobin, or the later months of pregnancy, so the doctor may use other tests. People with diabetes often check their fasting and post-meal sugar, and have HbA1c checked at times their doctor sets.

People with diabetes also have regular checks of the kidneys, cholesterol, heart, eyes and feet, such as kidney function tests, a lipid profile and an ECG.