CBC test: full form, normal ranges and what your blood count means

CBC is short for complete blood count. You'll also see it called CBP (complete blood picture), a full blood count (FBC) or a haemogram. It's a single blood sample that counts and measures the three kinds of cells in your blood:

• Red blood cells, which carry oxygen using haemoglobin. The report includes haemoglobin, the red cell count, haematocrit (PCV) and the red cell indices (MCV, MCH, MCHC, RDW).
• White blood cells, which fight infection. The total count (WBC or TLC) and the differential (DLC), which splits it into five types.
• Platelets, which help blood clot.

Doctors order it at routine check-ups, when you have tiredness, fever, infection, bruising or bleeding, before surgery, and to follow conditions or medicines that affect the blood. Some reports add the ESR, which isn't part of the CBC but is often done with it.

Low haemoglobin means anaemia. The WHO uses below 13 g/dL in men and below 12 in women; in pregnancy it's below 11 in the first and third trimesters and below 10.5 in the second, when the blood naturally becomes more dilute. Anaemia can cause tiredness, breathlessness, a fast heartbeat, pale skin, dizziness and headaches.

The MCV (red cell size) points to the cause:

• Small red cells (low MCV): most often iron deficiency, from heavy periods, pregnancy, a low-iron diet or slow bleeding from the gut. Thalassaemia trait, common in South Asia, the Mediterranean and the Middle East, also gives small cells, often with a normal or high red cell count.
• Large red cells (high MCV): low vitamin B12 or folate, regular alcohol, an underactive thyroid, liver disease or some medicines.
• Normal-sized cells: long-term illness, kidney disease, recent blood loss, or a mix of causes.

High haemoglobin or haematocrit is usually from dehydration, smoking or living at high altitude, and less often from lung or heart disease, or a bone-marrow condition (polycythaemia).

A high white cell count (above about 11,000 per µL) usually means the body is fighting something: a bacterial infection (often high neutrophils), inflammation, an injury or surgery, stress or steroid medicines. It also rises a little in pregnancy and in smokers. Very high counts, or immature cells (blasts) on the report, need prompt review, as they can rarely mean a blood cancer such as leukaemia.

A low white cell count (below about 4,000 per µL) is common for a short time with viral infections. It can also come from some medicines (including chemotherapy and some antibiotics, anti-thyroid and psychiatric medicines), low B12 or folate, autoimmune conditions and, rarely, bone-marrow problems. A low neutrophil count raises the risk of infection; with fever it can be urgent.

High eosinophils most often mean allergy, asthma, eczema or worm infections, which are common in many countries.

Low platelets (thrombocytopenia), below about 150,000 per µL, are common with viral fevers, especially dengue, and can also come from some medicines, heavy alcohol, liver disease, an enlarged spleen, pregnancy and immune conditions (ITP). Lab clumping can make a count look falsely low, so an unexpected result is often repeated.

In dengue, platelets often fall sharply around days 3 to 7 of the fever, as the fever settles. This is the critical phase, when doctors watch closely for warning signs: severe belly pain, repeated vomiting, bleeding gums or nose, blood in vomit or stools, restlessness or unusual drowsiness. A number alone doesn't decide treatment; how you are matters more. Don't take aspirin or ibuprofen for a fever that might be dengue; ask a doctor or pharmacist what's safe.

High platelets (thrombocytosis), above about 450,000 per µL, usually follow iron deficiency, infection or inflammation, surgery or removal of the spleen. Rarely they come from a bone-marrow condition, which a doctor looks for if the count stays high.

• It's a small blood sample from a vein (or a heel or finger prick in babies). No fasting is needed for a CBC, unless other tests at the same visit need it.
• Results usually come back the same day, often within a few hours.
• Drink as usual: dehydration can make haemoglobin and haematocrit look higher.
• Tell the lab about recent illness, pregnancy, and medicines you take, as they can shift the counts.
• If something is off, doctors often look at a peripheral smear (the cells under a microscope) or repeat the test.

• Look at the whole picture. One value just outside the range, with everything else normal and no symptoms, is common and often means nothing.
• Recheck after an illness. Colds and other infections can shift white cells and platelets for a couple of weeks.
• Anaemia needs its cause found, not just treatment: iron studies (ferritin), B12 and folate are the usual next tests.
• Iron, B12 and folate supplements are your doctor's decision. Taking iron without a test can hide a cause that needs finding, and too much iron is harmful.
• Eat for healthy blood: iron (lentils, beans, green leafy vegetables, meat, fish, eggs, fortified cereals) with vitamin C, and B12 from dairy, eggs, fish or fortified foods if you're vegetarian or vegan.