CBC Blood Test Results Explainer

What is a CBC blood test?

A complete blood count (CBC) – called a full blood count (FBC) in the UK and many other countries – is the most frequently ordered blood test in the world. From a single small tube of blood, an automated analyser counts and measures the three families of cells that circulate in your blood:

• Red blood cells, which carry oxygen using the protein haemoglobin.
• White blood cells, which fight infection and are part of the immune system.
• Platelets, small cell fragments that help blood clot.

A CBC is used in routine check-ups, before surgery, to investigate tiredness, bruising, fever or infection, and to monitor conditions and treatments that affect the blood or bone marrow. A "CBC with differential" adds a breakdown of the white cells into their five main types.

The explainer above lets you enter whichever CBC values you have – haemoglobin, haematocrit, white cell count, platelets, MCV and RDW – and shows where each one sits against typical adult reference ranges, with anaemia flagged using the World Health Organization thresholds for your sex. It also recognises common combinations, such as the iron-deficiency pattern. Every field is optional.

CBC normal ranges (adults)

Ranges are from the NBME laboratory reference values used in US medical licensing exams, with RDW from common laboratory practice. Your own lab's range, printed on your report, takes priority: ranges differ between analysers, populations and altitudes. "×10⁹/L" and "thousands per µL" (K/µL) are the same number – a WBC of 7.2 × 10⁹/L is 7,200 per µL.

White cell differential

Many labs now report absolute counts (for example, neutrophils 4.1 × 10⁹/L) rather than percentages, which is more useful clinically: a "normal" percentage can hide an abnormal absolute count when the total WBC is very high or low.

How to read the red cell section

Haemoglobin and anaemia

Anaemia means a haemoglobin below the normal threshold for your age, sex and pregnancy status. The WHO's 2024 guideline sets these thresholds for adults at sea level:

WHO grades anaemia as mild, moderate or severe according to how far haemoglobin falls below these thresholds. Haemoglobin is naturally higher in people who live at altitude and in smokers, so WHO also provides adjustments for both.

Red cell indices: MCV, MCH, MCHC and RDW

The indices describe what the red cells look like, and they are the key to working out why someone is anaemic:

RDW shows how varied the cells are in size. A high RDW with a low MCV favours iron deficiency, while a normal RDW with a low MCV is more typical of thalassaemia trait – a classic distinction described by Bessman and colleagues in the 1980s. Our MCV blood test page explains the indices in detail with its own calculator.

Haematocrit

Haematocrit usually tracks haemoglobin – roughly three times the haemoglobin value in g/dL. A high haematocrit can come from dehydration (less plasma), smoking, living at altitude, testosterone therapy, lung disease, or – rarely – a bone-marrow condition called polycythaemia vera.

How to read the white cell section

Benign ethnic neutropenia (also called Duffy-null associated neutrophil count) is a normal finding in a proportion of people of African and Middle Eastern ancestry, whose neutrophil counts are naturally lower than standard reference ranges without any increased infection risk.

A neutrophil count below 0.5 × 10⁹/L is severe neutropenia and, combined with fever, is a medical emergency – particularly in people having chemotherapy.

How to read the platelet section

The most common reason for a high platelet count is reactive thrombocytosis: a response to iron deficiency, infection, inflammation, surgery or bleeding. A low platelet count can come from viral infections, medicines, alcohol, liver disease with an enlarged spleen, immune thrombocytopenia (ITP), pregnancy (gestational thrombocytopenia) or, less often, bone-marrow disorders. Occasionally platelets clump in the sample tube and the machine undercounts them – "pseudothrombocytopenia" – which a repeat sample in a different tube resolves.

Common CBC patterns

If the iron-deficiency pattern applies to you, an iron panel with TIBC and ferritin are the usual next tests.

How a CBC is measured

The blood is drawn into a tube containing an anticoagulant (EDTA, usually a lavender or purple top) and run on an automated haematology analyser. The machine counts cells as they pass one by one through a narrow aperture, using changes in electrical impedance and the way each cell scatters laser light to sort them by size and internal structure. Haemoglobin is measured chemically after the red cells are broken open.

Some values on your report are measured directly – red-cell count, white-cell count, platelets, haemoglobin and MCV on most analysers – while others are calculated from them:

• Haematocrit = red-cell count × MCV.
• MCH = haemoglobin ÷ red-cell count.
• MCHC = haemoglobin ÷ haematocrit.

When the analyser detects something unusual – immature cells, clumped platelets, abnormal shapes – it flags the sample, and a laboratory scientist looks at a blood film under the microscope. Comments such as "target cells", "pencil cells", "hypersegmented neutrophils" or "blasts" come from that film and can be very informative.

A worked example: reading a CBC report

Imagine a 34-year-old woman with tiredness whose report shows: haemoglobin 10.2 g/dL, haematocrit 32%, MCV 74 fL, RDW 17.5%, WBC 6.1 × 10⁹/L, platelets 420 × 10⁹/L.

1. Haemoglobin 10.2 g/dL is below the WHO threshold of 12.0 g/dL for non-pregnant women – anaemia.
2. MCV 74 fL is under 80 – the cells are small (microcytic).
3. RDW 17.5% is high – the cells vary in size, which favours iron deficiency over thalassaemia trait.
4. Platelets 420 are mildly raised – a common reactive response to iron deficiency.
5. White cells are normal.

Together, these point strongly towards iron-deficiency anaemia, and the logical next tests are ferritin and an iron panel – plus, crucially, finding the cause, such as heavy periods. Try the "Iron-deficiency pattern" example in the explainer above to see how the tool describes it.

CBC values in different groups

Pregnancy

Plasma volume rises faster than red-cell mass in pregnancy, so haemoglobin naturally falls – the "physiological anaemia of pregnancy". That is why WHO uses lower thresholds in pregnancy. White cells rise, often to 10–15 × 10⁹/L in late pregnancy and higher during labour, and platelets may fall slightly.

Older adults

Reference ranges for most CBC values are similar across adult ages, but anaemia becomes more common with age and is more likely to have a medical cause, such as kidney disease, chronic inflammation or nutritional deficiency. Anaemia in an older adult is not simply "due to age" and is worth investigating.

Children

Children's ranges differ considerably by age: newborns have very high haemoglobin, which falls to a low point at around two months, while lymphocytes make up a larger share of white cells in young children. Always use paediatric ranges for anyone under about 16.

Athletes

Endurance training expands plasma volume, so well-trained athletes can have slightly low haemoglobin ("sports anaemia") despite having more red cells in total. Genuine iron deficiency is also common in athletes, particularly female runners, so a low haemoglobin should still be checked.

Preparing for a CBC

No fasting is needed for a CBC alone. Results can be affected by:

• Recent illness, which can change white cells and platelets for weeks.
• Strenuous exercise, which can temporarily raise white cells.
• Pregnancy, which lowers haemoglobin through dilution and raises white cells.
• Altitude and smoking, which raise haemoglobin.
• Medicines such as steroids (raise neutrophils), chemotherapy and some antibiotics (can lower counts).
• Sample problems – clotting or delay can give misleading results.

When to see a doctor

Contact your clinician about any CBC result flagged as abnormal, and seek urgent care if you have:

• Haemoglobin under 7 g/dL, or a large fall from your usual value, especially with breathlessness, chest pain or fainting.
• Fever with a low neutrophil count, particularly during chemotherapy.
• Platelets under 20 × 10⁹/L, or any bleeding that will not stop, blood blisters in the mouth or widespread pinpoint bruising.
• Very high white cell counts with unexplained weight loss, night sweats or bruising.