How to read a full blood count

A full blood count, also called a complete blood count, is the most frequently ordered laboratory test in most health systems. It is also the one that produces the most numbers, many of which are calculated from others.

Read as twenty independent results it is bewildering. Read as three groups with a handful of derived measurements, it becomes considerably more manageable.

The three groups

Red cells and haemoglobin describe the blood's capacity to carry oxygen. White cells describe the immune system's current state. Platelets describe the ability to form clots.

The three are measured from the same sample and reported together, but they answer largely separate questions. A change in one does not imply anything about the others, though certain patterns across all three are meaningful to a clinician.

Red cells: the count and the indices

Haemoglobin is the headline number, describing how much oxygen-carrying protein is present. Haematocrit describes the proportion of blood volume occupied by red cells and moves closely with haemoglobin. The red cell count is the number of cells.

The indices are calculated from those three. Mean cell volume is the average size of a red cell. Mean cell haemoglobin describes how much haemoglobin an average cell carries, and mean cell haemoglobin concentration describes how densely packed it is.

The indices matter because different causes of a low haemoglobin tend to produce differently sized cells. This is why a clinician looking at a low haemoglobin looks immediately at the mean cell volume: it narrows the list of things worth investigating before any further test is ordered.

Red cell distribution width describes how much variation there is in cell size within the sample. A single average can hide two populations of cells, and this number reveals when that is happening.

White cells and the differential

The total white cell count is rarely interpreted alone. The differential breaks it into types, each with a different role, and the pattern is far more informative than the total.

Neutrophils are the most numerous and respond strongly to bacterial infection and to physical stress. Lymphocytes are central to viral responses and to longer-term immunity. Monocytes, eosinophils and basophils make up the remainder, each associated with different processes.

Counts are reported both as percentages and as absolute numbers. The absolute numbers are what clinicians generally use, because a percentage can change simply because another cell type changed.

Reference ranges for neutrophils differ between populations. Some people have persistently lower neutrophil counts as a normal variant, and this is well described in several ethnic groups.

Platelets

The platelet count is reported alongside a measure of average platelet size in many laboratories. Counts respond to inflammation, recent bleeding and a long list of medicines.

A low platelet count is one of the results most likely to be an artefact. Platelets can clump in the collection tube, and the analyser counts a clump as a single cell. Where this is suspected, the laboratory examines a blood film or requests a sample in a different tube type.

What a blood film adds

When the automated count finds something unexpected, a laboratory scientist examines a stained film of the blood under a microscope. This is the oldest part of the test and still the most informative.

A film shows the shape of cells, the presence of immature forms, and features an analyser cannot classify. A comment on a report describing cell appearance comes from this examination rather than from the machine.

Reading the whole thing together

The most useful question when looking at a full blood count is which of the three groups, if any, is affected, and whether the pattern within that group is consistent.

A single value at the edge of its range, with everything else unremarkable and no change from previous counts, carries much less weight than a coherent pattern across several related measurements.

Previous results matter more here than almost anywhere else. A haemoglobin that has been stable for five years at slightly below the printed range is a different situation from the same number arriving after a run of higher ones, and only a clinician with access to the history can tell the two apart.