Complete blood count (CBC) explained: haemoglobin, white cells, platelets and indices

What a CBC is for

A complete blood count (CBC), also called a full blood count or haemogram, is one of the most commonly ordered blood tests. A small sample of blood is run through an automated analyser that counts the cells floating in it and measures some of their features.

Doctors order it for many reasons: to look for anaemia, to check for signs of infection or inflammation, to monitor the effects of some medicines, to investigate bruising or bleeding, or simply as part of a routine health check. A CBC does not diagnose a condition by itself. It points the doctor towards what to look at next.

The report is usually divided into three families of cells: red blood cells, white blood cells and platelets. Each family has its own measurements, and they are easiest to understand one group at a time.

Red cells: haemoglobin, haematocrit and the indices

Red blood cells carry oxygen using a protein called haemoglobin. The headline numbers are:

• Haemoglobin (Hb or Hgb): the amount of haemoglobin in the blood. In India it is usually printed in grams per decilitre (g/dL); many other countries use grams per litre (g/L), which is ten times larger, so 12.0 g/dL equals 120 g/L.
• Red cell count (RBC): the number of red cells, often printed as millions per cubic millimetre or ×10^12 per litre.
• Haematocrit or packed cell volume (Hct or PCV): the percentage of blood volume taken up by red cells.

Beneath these you will find the red cell indices, which describe the cells themselves:

• MCV (mean corpuscular volume): the average size of a red cell, in femtolitres (fL). Small cells (low MCV) and large cells (high MCV) point doctors towards different groups of causes.
• MCH and MCHC: the average amount and concentration of haemoglobin per cell.
• RDW: how much the cells vary in size. A higher value means the cells are less uniform.

The World Health Organization defines anaemia in adults as a haemoglobin below roughly 13.0 g/dL in men and 12.0 g/dL in women who are not pregnant, with lower cut-offs in pregnancy and for children. Your laboratory’s printed range may differ slightly, and what matters most is the reason behind a low value. Iron deficiency is a common cause, but not the only one, which is why a doctor looks at the indices and often orders further tests before deciding anything.

White cells and the differential

White blood cells (WBC or leucocytes) are part of the immune system. The total count is commonly printed as cells per cubic millimetre, for example 7,800 /cumm, or in the international form ×10^9 per litre. The two forms are the same number scaled: 7,800 /cumm equals 7.8 ×10^9/L.

A typical adult reference range is about 4,000 to 11,000 per cubic millimetre, but it varies by laboratory and age.

The differential splits the total into types of white cell:

• Neutrophils: the most numerous type, often involved in bacterial infections and some inflammation.
• Lymphocytes: important in the response to viral infections and in long-term immunity.
• Monocytes, eosinophils and basophils: smaller groups, with eosinophils sometimes linked to allergies or parasites.

Differentials are usually printed as percentages. Percentages can mislead, because if one type rises the others fall as a share even when their own numbers have not changed. Where the report gives absolute counts (for example, absolute neutrophil count), those are more reliable than the percentages.

Counts rise and fall for many harmless reasons: a recent infection, stress, strenuous exercise, smoking, or certain medicines. A single slightly high or low count is common and often means little. A persistent or marked change is something for the doctor to investigate.

Platelets

Platelets are small cell fragments that help blood to clot. Indian reports commonly print platelets in lakhs per cubic millimetre. One lakh is 100,000, so 2.4 lakh/cumm equals 240,000 per microlitre, which is 240 ×10^9/L.

The usual adult range is about 1.5 to 4.0 lakh/cumm (150 to 400 ×10^9/L). Platelet counts can be temporarily low during some infections and are affected by several medicines and conditions. They can also be falsely low because of the sample itself: in some people platelets clump together in the collection tube, and the analyser counts the clumps as one. Laboratories usually flag this and may check a blood film or repeat the sample.

Why reference ranges differ between laboratories

You may have noticed that two laboratories print different “normal” ranges for the same test. This is expected, and it is not a sign that one is wrong. There are several reasons:

• Different analysers and methods give slightly different results.
• Ranges are built from different groups of healthy people, and local populations differ.
• Ranges depend on age and sex. Haemoglobin is normally lower in women than men and differs in children and in pregnancy.
• Altitude and smoking raise haemoglobin.
• Some laboratories use age-specific ranges for children and older adults, while others print one adult range.

A reference range usually covers the middle 95 % of results from healthy people, so a few healthy people will fall just outside it for any one test. A value slightly outside the range is a prompt to look at the whole picture, not a diagnosis. Our guide to reading a laboratory report covers this idea in more detail.

Things that change the result

Several ordinary factors can move CBC numbers:

• Dehydration makes the blood more concentrated, so haemoglobin and haematocrit read higher.
• Being very well hydrated, or having a drip running in the arm used for the sample, can dilute the blood and give lower readings.
• Pregnancy lowers haemoglobin because blood volume expands.
• Recent infection, vaccination, injury or strenuous exercise can shift white cell counts.
• Some medicines, including certain antibiotics, anti-seizure drugs and chemotherapy, can lower counts.
• Delays in testing a sample can change cell appearance and counts, which is why laboratories have rules on how quickly samples must be processed.

Tell the person collecting the sample about any medicines you take, recent illness, or recent blood transfusion, and mention the same to your doctor.

A worked example (example values)

Imagine a CBC with these example values: haemoglobin 10.8 g/dL, MCV 72 fL, RDW 17 %, WBC 7,600 /cumm and platelets 3.1 lakh/cumm. The report flags haemoglobin and MCV as low and RDW as high.

Reading this the way a doctor might: the white cells and platelets are within the usual ranges. The haemoglobin is below the cut-off for anaemia, and the red cells are small and variable in size. That pattern is commonly seen when the body is short of iron, but it can also occur in some inherited conditions, so a doctor would usually order follow-up tests, such as ferritin or other iron studies, and ask about diet, periods, bleeding and medicines before concluding anything.

Notice what the report cannot say: it cannot say why the haemoglobin is low or what to do about it. Starting iron tablets without a doctor’s advice, based on a pattern like this, can delay finding the real cause and can have side effects.

Other blood-count terms you may meet

A few more items appear on some reports, and are worth recognising.

• ESR (erythrocyte sedimentation rate) is often printed beside the CBC. It measures how fast red cells settle in a tube in one hour and rises with inflammation, but it is non-specific and also rises with age, anaemia and pregnancy.
• Reticulocyte count measures young red cells and shows how actively the bone marrow is making new ones. Doctors use it to tell whether anaemia comes from reduced production or from loss or breakdown.
• Peripheral smear (blood film) is a pathologist’s description of what the cells look like under the microscope. It can reveal cell shapes that an analyser cannot, and it is often requested when a count is unexpected.
• Ferritin and iron studies are not part of a standard CBC, but are commonly added when haemoglobin is low or MCV is small.
• Flags such as “blast cells” or “atypical lymphocytes” come from the analyser and must be checked by a person. They are a reason for review, not a diagnosis.

Who might need repeat tests

A mild abnormality in a person who feels well is often repeated after a few weeks rather than investigated at once. Marked abnormalities, results that are falling steadily, or abnormalities in more than one cell line are looked into sooner. If you are told to repeat a CBC, ask when, and whether to use the same laboratory, because comparing results from one instrument is cleaner.

Questions worth asking your doctor

• Which of my results are outside the range, and does that matter for me?
• Is there a reason to repeat the test, and when?
• Do I need any further tests to understand the cause?
• Could my medicines, diet or recent illness explain the result?
• Which laboratory should I use next time so results are easy to compare?

Sources

• MedlinePlus: Complete Blood Count (CBC) — overview of what the test measures
• MedlinePlus: Red Blood Cell (RBC) Indices — MCV, MCH, MCHC and RDW explained
• Testing.com: CBC with differential — what each part of the report means
• WHO: Anaemia fact sheet — World Health Organization definition and causes of anaemia
• NHS: Iron deficiency anaemia — symptoms, causes and diagnosis

A note on this guide

This guide is general information about how a complete blood count is laid out. It is not medical advice, and it cannot tell you what your own results mean. Please discuss your report with your doctor.