What should you know about complete blood test?

A complete blood test measures different parts of your blood to help investigate symptoms and monitor health. Depending on the request, it may include a full blood count, blood sugar, kidney and liver markers, cholesterol, thyroid tests, iron or inflammation markers. The clinician chooses tests based on your symptoms, medicines, examination and medical history.

Results can show patterns that need follow-up, such as changes in red cells that may occur with Anaemia, altered kidney markers in Chronic Kidney Disease, or signs that fit an infection or inflammation. One result rarely gives a diagnosis on its own. Hydration, recent illness, medicines, food intake and the time of collection can affect some measurements. Your clinician compares the findings with the laboratory's reference range and your overall situation.

Ask whether fasting is needed; many blood tests do not require it, while some measurements may be more useful after avoiding food for a specified period. Drink water unless you have been given different instructions, and bring a list of medicines and supplements. The sample is usually taken from a vein in your arm. Keep pressure on the site afterwards and mention if you have previously felt faint during blood tests.

Arrange medical review for persistent unexplained tiredness, fever, weight change, unusual bruising, ongoing diarrhoea or symptoms that are getting worse. Seek urgent care for severe breathing difficulty, chest pain, confusion, fainting, heavy bleeding or a rapidly worsening illness. A test result marked outside a reference range is not automatically an emergency, but it should be interpreted with the clinician who ordered it.

Ask which measurements were requested, whether you need to repeat the sample, what an unexpected result could mean in your context, and when follow-up is planned. If digestive symptoms prompted testing, ask whether Coeliac Disease or Crohn's Disease needs separate assessment.