What does a Full Blood Count test for?

A Full Blood Count (FBC) measures red blood cells (haemoglobin, haematocrit, MCV), white blood cells (total count and differential — neutrophils, lymphocytes, and others), and platelets. It is used to investigate anaemia, infection, immune disorders, bleeding problems, and medication effects on the blood.

Do I need to fast before a Full Blood Count?

No. Fasting is not required for a Full Blood Count. It can be done at any time of day.

Can a Full Blood Count diagnose iron deficiency?

A Full Blood Count can suggest iron deficiency (low haemoglobin and small red cells) but cannot confirm it. Iron studies — including ferritin — are needed to diagnose iron deficiency. Your GP may order both tests together.

Haemoglobin (Hb) is the protein inside red blood cells that carries oxygen. It is the most clinically important number in the Full Blood Count. Low haemoglobin indicates anaemia. Australian laboratories report haemoglobin in g/L.

What Is a Full Blood Count (FBC)?

📋 Source: Royal College of Pathologists of Australasia (RCPA) Pathology Manual — FBC reference ranges. rcpa.edu.au. Reference ranges may vary slightly between laboratories.
Your doctor or healthcare provider knows your history and circumstances — always follow their personalised advice over general information.

What a Full Blood Count measures

A Full Blood Count (FBC) — also called a Complete Blood Count (CBC) — measures the three main types of cells in your blood:

🔴 Red blood cells (RBCs)

Carry oxygen from your lungs to the rest of your body. The FBC measures the number of red cells, haemoglobin (the oxygen-carrying protein), haematocrit (the percentage of blood made up of red cells), and the size and shape of red cells (MCV, MCH, MCHC, RDW).

⬜ White blood cells (WBCs)

Fight infection and are part of the immune system. The total white cell count and a differential (breakdown by type — neutrophils, lymphocytes, monocytes, eosinophils, basophils) are measured.

🩷 Platelets

Small fragments that help blood clot when you bleed. The FBC reports the platelet count and mean platelet volume (MPV).

Why your GP ordered it

The FBC is one of the most commonly ordered tests in general practice. It may be requested to:

Investigate tiredness, weakness, or pallor — anaemia is a common finding
Look for signs of infection or inflammation (elevated white cells)
Screen for blood or bone marrow disorders
Monitor the effects of chemotherapy, immunosuppressants, or other medications on blood cells
Check for unexplained bruising or bleeding (platelet problems)
Routine pre-operative assessment
Annual chronic disease reviews (diabetes, kidney disease, heart disease)

What the test involves

The FBC requires a blood sample, usually from a vein in the arm. No fasting is required. Results are typically available the same day or within 24 hours.

If your GP is checking for iron deficiency anaemia, they may order iron studies at the same time — the FBC alone cannot confirm iron deficiency, only suggest it (through low haemoglobin and small red cells).

Australian laboratory context

In Australia, FBC results are reported in standardised units: haemoglobin in g/L, white cell and red cell counts in ×10⁹/L or ×10¹²/L, and platelets in ×10⁹/L.

Reference ranges differ between adult males and females (particularly haemoglobin), and between adults and children. Your laboratory report will show the reference range appropriate to your age and sex.

A mildly abnormal FBC often requires context to interpret — a slightly low neutrophil count in an otherwise well patient may be a normal variant, while the same result in someone with fever and fatigue may be significant.

Your GP will interpret your results in the context of your full medical history, your symptoms, your medications, and any previous results. A result outside the reference range does not automatically mean something is wrong, and a result within range does not always mean there is no problem. Always discuss your results with your GP or the clinician who ordered the test.