A complete blood count is ordered for almost everything: fever, tiredness, before surgery, as part of a routine package. It comes back as twenty or so numbers with a few marked high or low, and most people read exactly one of them. The rest of the report is where the useful information usually is.
Three cell lines, three different jobs
Blood carries three families of cells, and a CBC counts all three separately. Red cells carry oxygen. White cells fight infection. Platelets stop bleeding. They are made in the same bone marrow but they fail for different reasons, which is why a report that disturbs one line reads very differently from one that disturbs all three.
The red cell block
Haemoglobin (Hb) is the headline. The World Health Organization's cut-offs for anaemia in adults are below 13 g/dL in men, below 12 g/dL in non-pregnant women and below 11 g/dL in pregnancy. Its 2024 revision of those thresholds left most adult figures where they were, and added something worth knowing: haemoglobin should be adjusted for altitude and for smoking. Smokers carry a higher haemoglobin for the same amount of usable oxygen, so a smoker sitting just inside the normal range may still be anaemic.
Haematocrit (PCV) is the percentage of blood made up of red cells, and it tracks haemoglobin closely, roughly three times the Hb figure. Both rise when you are dehydrated, which is one reason a sample taken after a long gap without water can read slightly better than the truth.
Then come the red cell indices, which are calculated rather than measured, and which are the part of the report doctors actually use to work out why:
| Index | What it describes |
|---|---|
| MCV | Average size of a red cell. Small is microcytic, large is macrocytic. |
| MCH | Average amount of haemoglobin inside one cell. |
| MCHC | How concentrated the haemoglobin is within the cell volume. |
| RDW | How much the cells vary in size from one another. |
MCV and RDW read together narrow the field faster than anything else on the page:
- Low MCV with a high RDW is the classic iron deficiency pattern: the marrow is turning out small cells now while normal-sized older cells are still in circulation, so the population is mixed.
- Low MCV with a normal RDW is more suggestive of a thalassaemia trait, where every cell has been small for life and the population is uniform.
- High MCV points towards vitamin B12 or folate deficiency, and also towards alcohol, thyroid disease and certain medicines.
- Normal MCV with a low haemoglobin is the pattern of chronic disease, kidney disease, recent blood loss, or two deficiencies cancelling each other out.
None of these are diagnoses. They are the reason your doctor asks for the next test rather than a different one.
The white cell block, where percentages mislead
The total white cell count in an adult usually sits between about 4,000 and 11,000 per microlitre. Underneath it, the differential splits that total into neutrophils, lymphocytes, monocytes, eosinophils and basophils.
Read the absolute counts, not the percentages. Sixty per cent neutrophils of a total of 3,000 and sixty per cent of a total of 12,000 are two completely different clinical situations, and the percentage column looks identical in both.
As a rough guide to which way to look: neutrophils rise in bacterial infection, and also with physical stress, steroid medicines and even a hard run. Lymphocytes rise in many viral infections. Eosinophils rise with allergy, asthma and parasitic infection, which is common enough in India to be worth taking seriously rather than ignoring. A low count in any line, particularly a persistently low one, matters more than a modest high one.
Platelets, and the count that is wrong
Platelets normally run between about 1.5 and 4.0 lakh per microlitre. They fall in dengue and several other infections, in some autoimmune conditions, and with certain medicines. The number that gets attention in the monsoon is covered in more detail in our note on which fever tests actually matter.
There is one artefact worth knowing about, because it saves people a frightening afternoon. The EDTA anticoagulant in the lavender-capped tube occasionally makes platelets clump together, and an automated counter reads a clump as a single platelet. The result is a low count in a person who is not bleeding and has nothing wrong with them. A laboratory that suspects it checks the smear, sees the clumps, and asks for a repeat sample in a different tube. If you are told a platelet count needs repeating for this reason, it is the laboratory doing its job, not doubting yours.
What a peripheral smear adds
An analyser counts and measures. A trained eye on a stained slide looks at shape: target cells, sickle forms, fragmented cells, immature white cells, the clumping above. That is why a CBC is often reported with a smear examination when a result is unusual, and why an abnormal count sometimes comes back with a comment rather than only a figure.
Things that move a CBC without any disease at all
- Dehydration concentrates every cell line and can lift haemoglobin by a full point.
- Pregnancy lowers haemoglobin by dilution, which is normal and expected, and is read against pregnancy-specific ranges.
- Hard exercise in the hours before a draw raises white cells.
- Living at altitude and smoking both raise haemoglobin.
- A difficult draw can damage cells in the tube and skew potassium and some counts, which is another reason a repeat is occasionally requested.
Fasting is not needed for a CBC. Drinking water normally beforehand is genuinely helpful.
Read the range on your own report
Every analyser and method carries its own reference interval, and ranges differ by age, sex and pregnancy. Compare your value to the range printed beside it, and compare this year's report to your own previous ones rather than to someone else's. A trend within one laboratory says far more than a single value read anywhere.
What to do with a flagged value
A single mildly abnormal count in someone who feels well is usually repeated before it is investigated. What deserves prompt attention rather than a wait-and-see is a haemoglobin that has fallen sharply, a very low white cell or platelet count, a count that is abnormal alongside fever, bruising, bleeding or weight loss, or an abnormality in more than one cell line at once.
A CBC is on almost every health package we run, and the Anemia Profile is the panel that goes looking for the cause when the haemoglobin is low. If travelling is difficult, a sample can be collected at home. The report is the beginning of the conversation with your doctor, not a substitute for it.
- CBC
- Reading reports
- Anaemia
