The Complete Blood Count (CBC), Line by Line
Haemoglobin, haematocrit, red and white cell counts, and platelets — what each line of the most commonly ordered blood test actually measures.
The complete blood count is the most frequently ordered blood test in the world. It is really three tests printed together — one for each cell line the bone marrow produces — plus a set of indices calculated from them.
Haemoglobin and haematocrit
Haemoglobin is the iron-containing protein inside red blood cells that carries oxygen, reported as a concentration in blood. Haematocrit is the proportion of blood volume occupied by red cells, expressed as a percentage. They measure the same thing from two angles and normally move together.
Because both are concentrations, hydration shifts them. Dehydration concentrates blood and raises both; fluid overload dilutes it and lowers both, without a single red cell having been gained or lost.
The red cell indices
The indices describe the cells themselves rather than how many there are, and they are what makes a CBC informative when haemoglobin is low:
| Index | What it describes |
|---|---|
| MCV — mean corpuscular volume | Average size of a red blood cell |
| MCH — mean corpuscular haemoglobin | Average amount of haemoglobin per cell |
| MCHC | Haemoglobin concentration within the cell |
| RDW — red cell distribution width | How much cell size varies across the population |
MCV in particular is how anaemias are grouped: cells that are smaller than expected, normal-sized, or larger than expected point investigation in different directions. RDW adds whether the cells are uniform or mixed, which can indicate two processes overlapping.
White cells and the differential
The total white blood cell count is rarely the interesting number. The differential — the breakdown by type — carries most of the information:
- Neutrophils — the largest fraction in adults, the first responders to bacterial infection
- Lymphocytes — central to viral response and to longer-term immune memory
- Monocytes — clear debris and mature into tissue macrophages
- Eosinophils — associated with parasitic infection and allergic responses
- Basophils — the smallest fraction, involved in inflammatory and allergic signalling
Normal proportions also change with age. Young children run lymphocyte-predominant counts where adults run neutrophil-predominant ones, which is why paediatric reference ranges differ rather than being scaled-down adult ones.
Platelets
Platelets are cell fragments that initiate clotting. The count is worth reading with one laboratory artefact in mind: platelets sometimes clump in the collection tube, and an analyser counts a clump as a single platelet. This produces a falsely low result, which is why a surprising platelet count is often confirmed on a fresh sample or by manual review of a blood film.
Reading a CBC as a set
The three cell lines share a factory. When all of them move in the same direction at once, that pattern points somewhere different from a change confined to one line — which is why the CBC is reported and read as a whole rather than line by line.
Reference ranges for these markers
Open the library entry for the published range, sample type, and citations.
Keep your own results in one place
File the reports behind these numbers under each family member and watch repeat values build into a trend. Free to start.
Educational content only. This article describes what a test measures and how published reference ranges are defined. It is not a diagnosis, does not interpret your personal results, and is not a substitute for a qualified doctor. Reference thresholds cited are those published by bodies such as the WHO, NIH, and CDC, and can differ between laboratories.
