Infectious DiseasesTier 2Topic

Laboratory tests - full blood count (FBC) in infection

Role

The full blood count is a first-line, widely available test providing an indirect window into the presence and type of infection through characteristic white cell response patterns, alongside baseline haemoglobin and platelet assessment relevant to severity and complications
Performed in essentially every unwell patient given its speed, low cost, and broad diagnostic value beyond infection alone

Patterns

Neutrophilia typically suggests bacterial infection (via marrow release and demargination in response to acute inflammation); lymphocytosis is more typical of viral infection; eosinophilia suggests parasitic infection, allergic disease, or specific drug reactions
Left shift (immature neutrophil forms/band cells) suggests a more significant acute bacterial response than mature neutrophilia alone

Interpretation

Pattern recognition narrows the differential but is not organism-specific - severe bacterial infection can present with neutropenia (marrow suppression/consumption) rather than neutrophilia, particularly in overwhelming sepsis or in immunocompromised patients, which is a recognised pitfall of over-relying on a "typical" pattern

Acting on the result

Marked leucocytosis or leucopenia with clinical concern for sepsis should prompt urgent source identification and empirical antimicrobial treatment regardless of the specific pattern, given the poor prognosis of unrecognised severe sepsis
Trend over serial FBCs (rather than a single value) helps track treatment response

Associations

Bacterial vs viral differentiation (imperfect), sepsis-associated neutropenia (a pitfall), eosinophilia (parasitic/allergic/drug causes), monitoring response to treatment

Prognostic signs

Neutropenia in the context of severe sepsis is a poor prognostic sign (marrow exhaustion/consumption in overwhelming infection), not reassurance against bacterial infection - relying on the "expected" neutrophilia pattern can cause a dangerous bacterial infection to be under-recognised in this specific scenario

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