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Haematology
A complete blood count looks at three separate cell lines in your blood at once: red cells, white cells, and platelets. Red cells carry oxygen around your body, white cells are your immune system's front line, and platelets stop bleeding by clotting. Because the test covers all three, a single CBC can point toward anaemia, an ongoing infection, a clotting problem, or a blood cancer, depending on which numbers move and in which direction. It's usually the very first test ordered when something feels generally wrong but the cause isn't clear yet, which is why it shows up in almost every routine health checkup.
A CBC reports red blood cell count, haemoglobin, and haematocrit together, since all three describe your oxygen-carrying capacity from slightly different angles. Separately it counts white blood cells, both the total and often a breakdown of the five types, which shifts depending on whether your body is fighting a bacterial infection, a viral one, or an allergic reaction. Platelet count is reported on its own, since a level that's too low raises bleeding risk and one that's too high raises clotting risk.
A CBC is a screening test, not a diagnosis on its own — it tells your doctor which direction to look next, not what's wrong. A low haemoglobin points toward anaemia, but the CBC doesn't say whether that's from low iron, blood loss, or a chronic condition; that needs a follow-up test like ferritin or iron studies. A raised white cell count usually means your body is actively fighting something, and which of the five white cell types is raised often hints at whether the cause is bacterial, viral, or allergic.
Most CBC reports also include a handful of red cell indices alongside the headline numbers — figures like mean corpuscular volume (MCV), which describes the average size of your red cells rather than how many there are. This matters because different causes of anaemia shrink or enlarge red cells in characteristic ways: iron deficiency tends to produce smaller-than-normal cells, while a B12 or folate deficiency tends to produce larger ones. Haematocrit, the percentage of your blood that's actually made up of red cells by volume, is reported alongside haemoglobin because the two normally move together, and a mismatch between them can itself be a useful clue.
No special preparation is needed for a CBC by itself. If your doctor has combined it with other tests on the same visit, one of those — not the CBC — may be the one that needs fasting, so it's worth asking what else is on the form. The sample is a small blood draw from a vein, usually at the inside of the elbow.
No. A CBC by itself needs no special preparation. If your doctor has ordered other tests on the same form, one of those may require fasting — check the full list, not just the CBC.
Not by itself. It's a screening test that tells your doctor which direction to investigate — for example, low haemoglobin points toward anaemia, but a separate test is usually needed to find out why.
A raised white cell count usually means your immune system is actively responding to something, such as an infection. Which of the five white cell types is raised can hint at whether the cause is bacterial, viral, or allergic.
It means your blood may take longer to clot, which raises bruising and bleeding risk. A high platelet count has the opposite effect — a higher tendency to clot.
No. ESR (erythrocyte sedimentation rate) is a separate marker of inflammation, often ordered alongside a CBC but measuring something different — how quickly red cells settle in a test tube, not their number.
That depends entirely on why it was ordered. A one-off screening checkup doesn't need repeating on a schedule, but monitoring a known condition like anaemia is usually done at intervals your doctor sets.
Yes. Being significantly dehydrated concentrates your blood and can make red cell counts and haemoglobin look higher than your true baseline.
It can raise a flag — certain patterns in white cell counts are followed up for blood cancers — but a CBC alone cannot diagnose cancer. Abnormal results lead to further, more specific testing.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
