How to Read a Blood Count Report Without Panicking
A complete blood count is the most ordered blood test in the world. Most people receive one with a few values flagged in red and no explanation. Here is what the lines actually mean.
The three cell families
Red blood cells carry oxygen. White blood cells fight infection. Platelets help blood clot. The CBC counts all three and describes their characteristics.
Haemoglobin and the red cell indices
Haemoglobin is the oxygen-carrying protein and the headline number. Approximate lower limits are around 13 g/dL in men and 12 g/dL in women, though laboratory ranges differ.
Haematocrit is the proportion of blood volume made up of red cells, and tracks haemoglobin closely.
MCV, mean corpuscular volume, is the average size of a red cell and is the single most useful clue when haemoglobin is low.
- Low MCV, microcytic: iron deficiency, thalassaemia trait, anaemia of chronic disease
- Normal MCV, normocytic: acute blood loss, chronic disease, kidney disease, early iron deficiency
- High MCV, macrocytic: vitamin B12 or folate deficiency, alcohol, hypothyroidism, liver disease, some medicines
RDW, red cell distribution width, measures how variable the cell sizes are. Iron deficiency typically raises it, while thalassaemia trait typically does not, and that distinction saves people from being treated with iron for a condition iron will not fix.
Anaemia: find the cause, not just the number
Treating anaemia without establishing why is a persistent error.
Iron deficiency is the commonest cause worldwide. Confirm with ferritin, which reflects stores. Important nuance: ferritin is an acute phase reactant, so infection or inflammation falsely raises it. In that setting, transferrin saturation is more reliable.
The crucial question is where the iron went. In menstruating women, heavy periods are the usual answer. In men and post-menopausal women, unexplained iron deficiency requires investigation of the gastrointestinal tract, because slow bleeding from an ulcer, polyp or cancer is a common cause and finding it early matters.
B12 and folate deficiency produce macrocytic anaemia. B12 deficiency also damages nerves, and the neurological damage can occur before anaemia appears.
Thalassaemia trait produces lifelong mild anaemia with small cells and a normal or high red cell count. It needs no iron and no treatment, but it matters for genetic counselling, because two carriers can have a child with the severe form. Haemoglobin electrophoresis confirms it.
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Taking iron properly
- Alternate-day dosing often absorbs better than daily. Daily doses raise hepcidin, which blocks absorption of the next dose
- Take with vitamin C or orange juice
- Avoid tea, coffee, milk, calcium and antacids within two hours
- Expect dark stools, which is normal
- Continue for three months after haemoglobin normalises, to rebuild stores
- Recheck ferritin, not just haemoglobin
- Intravenous iron is an option where oral iron fails or is not tolerated
White blood cells
The total count matters less than the differential.
Neutrophils rise with bacterial infection, inflammation, stress, steroids and smoking.
Lymphocytes rise with viral infections and some chronic leukaemias.
Monocytes rise with chronic infections including tuberculosis.
Eosinophils rise with allergy, asthma, drug reactions and parasitic infections. A persistently high eosinophil count in a returning traveller is worth investigating.
Low neutrophils, neutropenia, reduces the ability to fight bacterial infection. Common causes include viral infection, medicines, chemotherapy and autoimmune disease. A specific point worth knowing: benign ethnic neutropenia is a harmless inherited lower baseline neutrophil count, common in people of African and Middle Eastern ancestry, which is frequently mistaken for a problem.
Platelets
Normal is roughly 150,000 to 450,000 per microlitre.
Low platelets. Causes include viral infections including dengue, immune thrombocytopenia, medicines, liver disease, alcohol and pregnancy. Bleeding risk rises substantially below about 50,000 and seriously below 20,000.
A common false alarm: clumping in the test tube can produce a falsely low count. If a low platelet count appears unexpectedly in someone with no bleeding and no symptoms, repeating the sample in a citrate tube is the first step rather than panic.
High platelets are usually reactive, from infection, inflammation, iron deficiency or after surgery.
In dengue-endemic areas
Platelet counts fall in dengue and are tracked closely, but the number alone is not what determines danger. Warning signs are what matter: severe abdominal pain, persistent vomiting, bleeding from gums or nose, lethargy, and a rising haematocrit indicating plasma leakage.
Avoid NSAIDs and aspirin in suspected dengue; use paracetamol. Fluid management matters more than platelet transfusion, which is reserved for specific situations rather than given routinely at a threshold.
When a flagged value is not a problem
- Values slightly outside the range are common; reference ranges are defined so that a small percentage of healthy people fall outside them
- Dehydration concentrates everything and can raise haemoglobin and haematocrit
- Pregnancy lowers haemoglobin through dilution
- Living at altitude raises haemoglobin normally
- Recent exercise, stress or a heavy meal shift several values
- A trend across repeated tests is far more meaningful than one reading
When to seek prompt attention
- Haemoglobin that has fallen substantially and quickly
- Breathlessness, chest pain or fainting with anaemia
- Unexplained bruising, bleeding gums, nosebleeds, or petechiae
- Fever with a very low neutrophil count, which is a medical emergency in anyone on chemotherapy
- Unexplained weight loss, night sweats or lumps alongside abnormal counts
The short version
Look at MCV before anything else when haemoglobin is low, confirm iron deficiency with ferritin rather than assuming it, and never treat iron deficiency in a man or post-menopausal woman without asking where the blood went. A single flagged value in isolation usually means less than the trend across two or three tests.