What should you know about a blood test?

Different tests examine different parts of blood. A full blood count can assess red cells, white cells and platelets, while other tests look at organs, inflammation, infection, hormones, glucose or cholesterol. A result outside the laboratory range does not identify a condition by itself. Your symptoms, medicines, medical history and the reason for testing help a clinician interpret it. Blood tests may support assessment of anaemia, kidney disease or inflammatory and infectious illnesses, but further tests may be needed.

Ask whether you need to fast, change your usual medicines or avoid strenuous activity before the appointment. Drink water unless you were given different instructions; being well hydrated can make sampling easier. Wear clothing that allows access to your arm. Tell the clinician about pregnancy, allergies, medicines and previous fainting during blood collection. Afterward, keep pressure on the puncture site as advised and report continuing bleeding or increasing swelling.

Contact the clinician who ordered the test if you do not understand the result or if symptoms continue despite a reassuring result. Seek urgent help for severe breathlessness, chest pain, confusion, fainting, heavy bleeding or sudden weakness. A small bruise at the needle site can occur, but worsening pain, redness, warmth or swelling should be assessed.

Ask which tests were ordered, what question each test addresses, whether preparation is needed, when results are expected and what follow-up would be appropriate if a value is outside the laboratory range.