The full blood count: what your haemoglobin, MCV, white cells and platelets reveal

The full blood count, often shortened to FBC, counts and measures the three main types of cell in your blood: red cells, which carry oxygen, white cells, which fight infection, and platelets, which help blood clot. It is cheap, fast and sensitive, which is why almost every doctor orders one. It rarely gives a diagnosis on its own, but it is very good at pointing to where to look next.

Haemoglobin: the oxygen carrier

Haemoglobin is the protein inside red cells that carries oxygen from your lungs to the rest of your body. When it is low, you have anaemia. Common signs are tiredness, breathlessness on stairs, headaches, pale skin and feeling the cold, although mild anaemia often causes no symptoms at all.

LifePrint uses the World Health Organization 2024 cut-offs. Anaemia is a haemoglobin below 13.0 g/dL in men and below 12.0 g/dL in women who are not pregnant. The WHO then grades it: mild from 11.0 g/dL up to the cut-off, moderate from 8.0 to 10.9 g/dL, and severe below 8.0 g/dL. A severe result is marked as urgent on your report.

A high haemoglobin is less common. It can come from dehydration, smoking, living at altitude, sleep apnoea or testosterone treatment. Occasionally it points to a bone marrow condition, which is why a persistently high result deserves a doctor's review.

Haematocrit and red cell count

Haematocrit is the share of your blood made up of red cells, and the red cell count is simply how many there are. Both usually move with haemoglobin. If you were short of fluids on the day of your draw, all three can read a little high.

MCV: the size of your red cells

MCV stands for mean cell volume, the average size of your red cells. Most labs print a range of around 80 to 100 fL. The size is a useful clue to why anaemia is present, and it can change before haemoglobin falls.

• Small red cells (low MCV) most often mean iron deficiency. An inherited trait such as thalassaemia can also cause small cells with a normal or near normal haemoglobin.
• Large red cells (high MCV) can come from low vitamin B12 or folate, regular alcohol use, an underactive thyroid, liver disease, or some medicines, including certain HIV and seizure treatments.
• A normal MCV with low haemoglobin can be seen in chronic illness, kidney disease, recent blood loss, or a mix of causes that cancel each other out.

The Essentials panel includes ferritin, your iron stores, so a low MCV can be read alongside it. If B12 or folate is the likely cause, your doctor may add those tests.

White cells: your immune system in numbers

The white cell count is the total number of infection fighting cells. The differential breaks it down into types, each with its own job. A raised count most often means your body is dealing with an infection or inflammation. Smoking, stress, pregnancy and steroid medicines can also push it up. A low count can follow a viral illness, some medicines, or, less often, a bone marrow or immune problem.

• Neutrophils are the first responders to bacterial infection.
• Lymphocytes rise with many viral infections.
• Eosinophils can rise with allergies, asthma, eczema and some parasite infections.
• Monocytes and basophils are smaller groups that are usually read in context.

Very high or very low white cell counts are treated differently. LifePrint marks a total white cell count below 1.5 or above 30, or a neutrophil count below 0.5 (all in 10⁹/L), as urgent, because these need a doctor promptly.

Platelets: the clotting cells

Platelets plug small breaks in blood vessels. A mildly low count is often found by chance and can follow a viral infection, heavy alcohol use, some medicines or liver disease. Sometimes it is a lab effect, where platelets clump together in the tube and are undercounted, and a repeat sample settles it. A mildly high count commonly follows infection, inflammation, iron deficiency or recent surgery.

A platelet count below 50 or above 1000 (10⁹/L) is marked as urgent. Unexplained bruising, nosebleeds or bleeding gums with a low platelet count also need prompt attention.

Reading the FBC as a whole

The blood count is most useful when the parts are read together and compared with earlier results. A single value just outside the lab range is common in healthy people and often sits in LifePrint's grey zone, with a suggestion to repeat at your next routine test. A pattern, such as low haemoglobin with small cells and low ferritin, tells a much clearer story.