🔬 Full Blood Count

The Full Blood Count (FBC) is a fundamental screening test that measures the levels of red blood cells, white blood cells, and platelets. It provides crucial information on the oxygen-carrying capacity of the blood, the presence of an immune response, and the adequacy of the clotting system. It is used in nearly every specialty for screening, diagnosis, and monitoring of disease and treatment. Results include haemoglobin concentration, haematocrit, mean cell volume (MCV), and various cell counts.

Indicated for evaluating symptoms of anaemia (fatigue, pallor, dyspnoea), suspected infection or sepsis, and unexplained bruising or bleeding. It is a standard pre-operative screen and is used to monitor chronic conditions such as inflammatory bowel disease or chronic kidney disease. It is also required for routine monitoring of drugs that cause bone marrow suppression. Assessment of possible haematological malignancies also begins with an FBC.

A venous blood sample is collected into an EDTA (lavender top) tube. This anticoagulant prevents clotting and preserves the morphology of the blood cells for several hours. The sample is processed using an automated haematology analyser which uses laser flow cytometry and electrical impedance to count and size different cell populations. If significant abnormalities are detected, a manual blood film is prepared and examined by a haematologist to assess cell morphology.

Normal results show haemoglobin, white cell count, and platelet count within the laboratory's established reference ranges. Typically, this includes a haemoglobin of 130-170 g/L for men and 120-150 g/L for women. Mean cell volume (MCV) should be between 80-100 fL. The White Cell Count (WCC) usually ranges from 4.0-11.0 x 10^9/L, and platelets from 150-450 x 10^9/L. All parameters should be viewed alongside the patient's baseline and clinical context.

Interpretation must be contextual; for example, a mildly raised WCC is expected post-operatively or in smokers. Microcytic anaemia should prompt a search for occult GI bleeding via 'qFIT' or endoscopy in appropriate age groups. Macrocytosis without anaemia often relates to alcohol excess or hypothyroidism. A low platelet count must be cross-checked for 'pseudothrombocytopenia' caused by EDTA-induced clumping. Reticulocyte counts (though often separate) help determine if the marrow is responding appropriately to peripheral cell loss.

Anaemia is defined by low haemoglobin, with MCV categorising it into microcytic (e.g., iron deficiency), normocytic (e.g., chronic disease), or macrocytic (e.g., B12/folate deficiency) types. A high white cell count (leucocytosis) may indicate infection, inflammation, or malignancy, while leucopenia suggests marrow suppression or specific viral infections. Thrombocytopenia can result from consumption (e.g., DIC, ITP) or reduced production, whereas thrombocytosis often reflects an acute phase response or primary myeloproliferative disorder. Polycythaemia (raised haematocrit) requires differentiation between dehydration, smoking, or primary polycythaemia vera.

The FBC is the most common gateway investigation in clinical practice, providing immediate clues to underlying systemic illness. It is essential for monitoring patients on cytotoxic drugs (e.g., methotrexate), evaluating surgical fitness, and guiding the management of acute infections and bleeding. The presence of 'red flag' symptoms alongside cytopenias necessitates urgent haematological referral. Many findings on FBC are non-specific and must be correlated with the patient's hydration status and clinical presentation.

Dehydration can mask anaemia through haemoconcentration, giving a falsely normal Hb level. Conversely, intravenous fluid administration can cause an apparent drop in Hb due to dilution. Pregnancy naturally causes a lower Hb due to increased plasma volume. Failing to check the white cell differential can lead to missing significant lymphopenia or eosinophilia. Cold agglutinins can interfere with automated counts, leading to an erroneously high MCV and low RBC count.

FBC provides a quantitative assessment but often lacks qualitative detail; for instance, it cannot differentiate between types of white cells unless a differential is specifically requested. It may appear normal in the early stages of acute haemorrhage before haemodilution occurs. The FBC does not measure the functionality of cells, meaning a patient can have a normal platelet count but dysfunctional platelets due to antiplatelet medication. Individual baseline variations exist, particularly across different ethnic groups (e.g., benign ethnic neutropenia).

Understand the WHO criteria for anaemia (Hb <130 g/L in men, <120 g/L in women). Be aware of the 'rule of three' where the haematocrit is roughly three times the haemoglobin value. Candidates should recognise the implications of pancytopenia and the need for urgent peripheral films. Focus on the differentiation of microcytic anaemias, particularly iron deficiency versus thalassaemia trait using the Mentzer index.