What should you know about an FBS blood test?

An FBS blood test measures the amount of glucose in your blood after a period of fasting, helping a clinician assess how your body regulates blood sugar. FBS stands for fasting blood sugar; you may also hear fasting blood glucose. A sample is usually taken from a vein in your arm, although the exact collection method depends on the clinic. The result is interpreted with your symptoms, medicines, examination and other tests rather than in isolation.

A fasting glucose result can fall within the expected range, be higher than expected, or be lower than expected. A raised result may prompt repeat testing or another type of glucose test because illness, stress, medicines and the length of the fast can affect the reading. A low result may be relevant if you use glucose-lowering medicine or have symptoms such as sweating, shaking, weakness or confusion. One result does not by itself establish the cause of an abnormal reading.

The test may be ordered during a general health assessment or when a clinician is checking a possible blood sugar problem. Other health concerns can also affect energy, appetite or weight, so your clinician may consider conditions such as [Anaemia](/conditions/anaemia/) or [Chronic Kidney Disease](/conditions/chronic-kidney-disease/) when deciding what to investigate.

Follow the clinic’s fasting instructions exactly. Ask whether you should avoid food and which drinks are allowed during the fasting period; do not assume that your usual coffee, juice or supplements are suitable. Tell the clinician about every medicine and supplement you take, especially medicines that affect blood sugar. Do not stop a prescribed medicine unless the clinician who ordered the test tells you to. If you eat, drink something outside the instructions or cannot complete the fast, mention it before the sample is taken so the result can be interpreted correctly.

After the blood draw, you can usually return to your normal eating plan unless your healthcare professional gives different advice. Bringing a snack may be sensible if you tend to feel faint or unwell after fasting.

Contact a healthcare professional promptly if you have repeated intense thirst, frequent urination, vomiting, marked weakness, new confusion or difficulty staying awake. Seek urgent help for severe confusion, collapse, a seizure, or breathing difficulty. These symptoms need clinical assessment regardless of whether an FBS test has already been booked. If you feel dizzy during fasting or after the blood draw, tell the staff immediately and sit or lie down safely.

Ask what fasting instructions apply to you, whether your regular medicines need special timing, and what the result is being used to assess. You can also ask whether the test should be repeated, whether another glucose test is needed, and which symptoms should prompt you to contact the clinic.