Complete Blood Count (CBC)

The complete blood count is the most frequently requested test in laboratory hematology. It quantifies the three principal populations of formed elements in blood, the red cells, white cells, and platelets, and reports a set of derived parameters such as haemoglobin concentration and the red cell indices. Performed on automated analysers, it provides a broad, rapid screen of the cellular compartment of blood.

Definition

The complete blood count is a panel of automated measurements that enumerates red blood cells, white blood cells, and platelets and reports associated quantities including haemoglobin, haematocrit, and the red cell indices, often together with a leukocyte differential count.

Scope

The entry covers what the CBC measures, how automated analysers generate its parameters, the place of the leukocyte differential, and the way abnormal or flagged results lead to further review. It treats the CBC as a foundational laboratory test within hematology and hemostasis; reference intervals and diagnostic thresholds are deliberately not given.

Core questions

• Which cellular populations and derived parameters does the CBC report?
• How do automated analysers count and size blood cells?
• What does the leukocyte differential add to the basic count?
• When does a CBC result prompt a manual blood film review?

Key concepts

• Red blood cell count and haemoglobin
• White blood cell count and leukocyte differential
• Platelet count
• Haematocrit and red cell indices (MCV, MCH, MCHC)
• Automated impedance and optical/flow cytometric counting
• Analyser flagging and reflex blood film review
• Preanalytical factors and sample integrity

Mechanisms

Modern analysers count and size cells by electrical impedance and by optical or flow-cytometric methods, deriving the cell counts, haemoglobin, haematocrit, and red cell indices, and many also generate a partial or full leukocyte differential (Buttarello, 2008). Results that fall outside expected patterns, or that trigger instrument flags, are referred for microscopic review of the stained blood film, which characterises morphology using standardised nomenclature (Palmer, 2015). Because the measured quantities are sensitive to specimen handling, preanalytical factors and sample integrity are integral to interpreting the count (Buttarello, 2008; Wintrobe, 2018).

Clinical relevance

Because it surveys all three cellular lineages at once, the CBC is a broad screening test that contributes to the laboratory recognition and monitoring of anaemias, infections and inflammatory states, and disorders affecting platelets and white cells. The entry describes what the test measures and how it is produced; it is educational and is not a basis for individual diagnostic or treatment decisions.

Evidence & guidelines

Analyser performance and quality specifications for the automated count have been reviewed in the laboratory literature (Buttarello, 2008), and the microscopic film review that follows abnormal counts is standardised through ICSH nomenclature and grading recommendations (Palmer, 2015).

Seminal works

• buttarello-2008
• palmer-2015

Frequently asked questions

What does a complete blood count actually measure?

It enumerates red blood cells, white blood cells, and platelets and reports derived quantities such as haemoglobin, haematocrit, and the red cell indices; many CBCs also include a leukocyte differential that breaks the white cell count into its subtypes.

Why might a CBC be followed by a manual blood film?

Automated analysers flag results that fall outside expected patterns or that they cannot fully characterise; these are referred for microscopic review of the stained blood film, which can describe cell morphology that the counter cannot.