Full blood count: what it includes, what it tells you and how to read the values

The full blood count is the most common blood test. It counts the cells in your blood: red cells, which carry oxygen, white cells, which defend you, and platelets, which stop bleeding. It does not give one number but about fifteen values, read together.

Why we do it

The full blood count, or complete blood count (CBC), looks at the three families of cells made in your bone marrow. It counts them, and for some it also measures how big they are.

It is a quick look at three things: whether your blood carries enough oxygen, how your defences are doing and whether your blood stops well when you get hurt.

That is why it is one of the tests doctors ask for most often. Mild anaemia, for example, often causes no symptoms and is found this way.

What it tells you, and what it does not

A value outside the limits means there is something to understand, not a diagnosis. Often the cause is temporary: a virus, too little water, a hard workout the day before.

The values are read together. Haemoglobin shows whether there is anaemia, MCV helps show where it comes from. For white cells, what matters is which type went up or down.

And the blood count does not measure everything. It does not show how much iron you have stored, or how much vitamin B12: those need other tests. So a perfect blood count can sit next to a low ferritin.

That is why it is read together with other tests:

• Ferritin: How much iron you have stored: it falls before haemoglobin
• Vitamin B12: If it runs short, your red cells get bigger
• CRP: Whether there is inflammation

What the full blood count includes and how to read the values

The full blood count includes three groups of values: red cells with their measurements, platelets and white cells. Complete usually means with the white cell differential (in Italian, formula leucocitaria): the white cells split into their five types.

Next to each value, your report prints the lab’s limits. For haemoglobin, haematocrit and red cells they differ for women and men. Compare each value with the limits on your own report, not with another lab’s.

The five types of white cell appear twice: as a percentage and as an absolute count, how many there really are. The absolute count matters more.

The limits on your report leave out some healthy people too. With so many values, finding one just outside is common even when you are well.

The lines you find on your report, each with its own page:

• Red blood cells (RBC): How many cells carry oxygen
• Haemoglobin (Hb): The protein that carries oxygen: it shows whether there is anaemia
• Haematocrit (HCT): How much of your blood is red cells
• MCV: How big your red cells are: it helps explain the cause of anaemia
• MCH: How much haemoglobin an average red cell holds
• MCHC: How concentrated the haemoglobin in your red cells is
• RDW: How much your red cells differ in size
• Platelets (PLT): The cell fragments that stop bleeding
• MPV: How big your platelets are
• White blood cells (WBC): The total of your immune cells
• Neutrophils: The first defence against bacteria, and the most common type
• Lymphocytes: The cells that remember infections you have met before
• Monocytes: They clean up your tissues and alert the rest of your defences
• Eosinophils: They react to allergies and parasites
• Basophils: The rarest: they often read zero, which is normal

In the Rosa report you see every blood count value explained in plain words, read together with the others and the rest of the panel.

If a value is out of range

• Low haemoglobin: This is anaemia, often from too little iron. If your red cells are small but numerous and ferritin is normal, it can be an inherited trait that is common in Italy.
• High haemoglobin and haematocrit: Often down to too little water before the draw. Sometimes smoking or testosterone. The test is usually repeated well hydrated.
• High or low white cells: High, often from an infection, smoking or exercise. Low, often after a virus or from a medicine. What matters is which type changed.
• High or low platelets: If they are low, sometimes it is the tube or a virus. If they are high, it is usually the body responding to inflammation or too little iron.

Low blood count: what it can mean

A low blood count usually means one or more values are below the limit on your report. What matters is which one, and by how much. The cause is often temporary.

• Low haemoglobin: This is anaemia. The most common cause is a lack of iron, from heavy periods, a pregnancy or small blood losses. Vitamin B12 or folate can run short too.
• Small red cells, low MCV: Usually too little iron, or being a healthy carrier of thalassaemia trait (Mediterranean anaemia). Ferritin helps tell the two apart.
• Low white cells: Often after a virus or from a medicine. Many people of African descent, and some of Middle Eastern descent, naturally have lower counts, with no problems.
• Low platelets: Sometimes they stick together in the tube and the machine counts fewer. Other times a virus, a medicine or alcohol is behind it.
• Several low values together: For example haemoglobin, white cells and platelets. Vitamin B12 or folate can be short. More rarely the problem is in the bone marrow, where blood is made. It always needs to be seen by a doctor.
• A value just below the limit: With the rest normal and no symptoms, it usually does not matter. Your doctor decides whether to repeat it.

Low blood count: symptoms

Often none: many low values are found by chance, with a blood test. When there are symptoms, they depend on which value fell.

• Low haemoglobin: tiredness that does not go away, breathlessness when you exert yourself, pale skin, dizziness, a fast heartbeat.
• Low white cells: infections that come more often or last longer, fever, mouth ulcers.
• Low platelets: bruises from small knocks, tiny red spots on the skin, nosebleeds or bleeding gums.

These symptoms can have other causes too. And if a value falls slowly the body adapts, so you can feel well even with a low value.

Blood count and infections

An infection changes the blood count, and sometimes the mark stays for a few weeks after you recover. In general:

• A bacterial infection usually raises neutrophils, and with them the white cell total.
• A virus can lower white cells, neutrophils or platelets for a while. Other times it raises lymphocytes, as in glandular fever.
• After an infection, monocytes can stay high for a few weeks while the body cleans up.
• Inflammation, or the healing phase, can raise platelets.

So if you had an infection recently, tell your doctor: the check is often done once you have recovered.

What to do if a value is out of range

Usually nothing urgent. Where to start:

• Look at the value together with the others: a single one just outside, with the rest normal, usually matters little.
• For white cells, look at the absolute count, not only the percentage.
• Think back to the days before the draw: a virus, too little water, a hard workout or a new medicine often explain the value.
• Show your report to your doctor, who decides whether and when to repeat the test.
• Do not take iron without checking your ferritin: if iron is not low, it does not help.
• If you take medicines or supplements, bring the list to your doctor, and do not stop anything on your own.
• Come to your next draw rested and well hydrated: no hard workouts the day before, and a glass of water in the morning.

What can skew the result

A few things change the values for a while. It helps to know them before you read them.

• Too little water: Your blood gets more concentrated, and haemoglobin, haematocrit and red cells go up. You can drink water even while fasting.
• Exercise or stress: A hard workout or strong stress raises white cells, especially neutrophils, for a few hours. Best to arrive at the draw rested.
• An infection or a vaccine: Even flu or a vaccine in the days before can shift white cells and platelets for a while.
• The tube: Sometimes platelets stick together in the tube and the machine counts fewer. And if the sample waits a long time before it is analysed, MCV comes out higher.
• Smoking and cortisone: People who smoke often have slightly higher haemoglobin and white cells. Cortisone raises neutrophils and lowers lymphocytes and eosinophils: tell your doctor, and do not stop it on your own.
• Pregnancy: The blood gets diluted: haemoglobin and haematocrit fall a little, white cells rise a little. This is usually normal.

When to worry

Talk to your doctor if:

• A value is outside the limits on your report and you do not know why, or it still is at the next check.
• Several values are out of range together, for example haemoglobin, white cells and platelets.
• You have anaemia or low iron, and you are a man or a woman after menopause.
• You have night sweats, lose weight without trying, bruise for no reason or have swollen glands in your neck, armpits or groin.
• A value is very far from the limits, such as very low haemoglobin, platelets or neutrophils, very high or very low white cells, or a very high haematocrit: see a doctor the same day, even if you feel well. The thresholds are on each value’s page.
• You have a fever with low white cells or neutrophils, or tiny red spots appear on your skin for no reason: see a doctor the same day.
• You have a fever with very low neutrophils, or a fever and you feel confused, short of breath or faint: call 112 or go to the emergency department.
• You see black stools or blood in your stools, or bleeding will not stop: call 112 or go to the emergency department.
• You are short of breath at rest or suddenly, have chest pain or a swollen and painful leg, or feel faint: call 112 or go to the emergency department.

With Rosa, a licensed physician approves every report before you see it, your blood count included.

Frequently asked questions

What does the full blood count include? Red cells with haemoglobin, haematocrit and their measurements (MCV, MCH, MCHC, RDW), platelets with MPV, and white cells with the differential: neutrophils, lymphocytes, monocytes, eosinophils and basophils. Some labs add a few more lines.

What is the difference between a blood count and a complete blood count? Often none. Complete, or with differential, usually means the white cells are split into their five types. If your report shows neutrophils, lymphocytes and the others, yours is complete. The Rosa panel always includes the differential.

Does the blood count include iron, cholesterol or ESR? No. The blood count counts your blood cells. Iron, cholesterol, glucose and ESR are separate tests, even if they are often done from the same draw. The Rosa panel includes iron, ferritin, cholesterol and glucose, not ESR.

Are there blood count values that point to cancer? Not on their own. A blood count cannot find a cancer, and a normal blood count does not rule one out. Some changes, though, make a doctor look further: iron deficiency anaemia in a man or a woman after menopause, platelets that stay high at the next check, or several values out of range together for no reason. Even then, the cause is usually something else. That is why a blood count that is out of range with no explanation should be shown to a doctor.

Do I need to fast for a blood count? For the blood count itself, no. But it is usually done with other tests that need fasting, so it is best to arrive fasting: with Rosa, 8 to 12 hours. You can drink water, and it helps keep the result accurate. Avoid hard workouts the day before.

Do normal blood count values change with age? In adults, usually not: some limits differ between women and men, such as those for haemoglobin and haematocrit, not with age. In children they are different, and in pregnancy some values shift a little. With age, though, anaemia becomes more common, and the reason always needs to be found.

When is a blood count worrying? When a value is very far from the limits, when several values are out of range together, or when a value stays outside for no reason at the next check too. A single value just outside, with the rest normal and no symptoms, usually is not. With a fever and low white cells, see a doctor the same day.

How often should it be done? For people who feel well there is no fixed rule: Rosa does it twice a year, in the full draw and in the check six months later. If a value is out of range, your doctor decides when to check it again.

Sources

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• British Society of Gastroenterology, guidelines for iron deficiency anaemia in adults. Gut, 2021. PMID 34497146
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• Fioredda and colleagues, European guidelines on the diagnosis and management of neutropenia in adults and children. HemaSphere, 2023. PMID 37008163
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