A full blood count (FBC) is one of the most frequently requested laboratory tests in UK clinical practice. Whether you are a foundation doctor, practice nurse, or biomedical scientist, interpreting the FBC accurately can transform patient care. This guide walks through the key components, common abnormalities, and how to avoid common pitfalls.

Red Cell Parameters: Anaemia and Beyond

The red cell portion of the FBC includes haemoglobin (Hb), haematocrit (Hct), red cell count (RBC), mean cell volume (MCV), mean cell haemoglobin (MCH), and red cell distribution width (RDW). Anaemia is defined by a haemoglobin below the laboratory's reference range—typically <130 g/L for men and <120 g/L for non-pregnant women. But the MCV is your first clue to the underlying cause.

• Microcytic anaemia (MCV <80 fL): Think iron deficiency, thalassaemia trait, or chronic disease. Iron deficiency is the most common in the UK, often due to menstrual loss or gastrointestinal bleeding. Check ferritin—but remember it rises in inflammation.
• Normocytic anaemia (MCV 80–100 fL): Consider anaemia of chronic disease, acute blood loss, renal impairment, or early iron deficiency. A high RDW suggests a mixed picture.
• Macrocytic anaemia (MCV >100 fL): B12 or folate deficiency, alcohol excess, hypothyroidism, or myelodysplasia. Always check B12 and folate before starting replacement.

Polycythaemia (raised Hb or Hct) requires investigation. It may be relative (dehydration) or absolute (polycythaemia vera, chronic hypoxia). A raised RBC with normal MCV in a smoker often points to hypoxic drive.

White Cell Count and Differential: Infection, Inflammation, and Malignancy

The total white cell count (WCC) and differential (neutrophils, lymphocytes, monocytes, eosinophils, basophils) provide a window into the immune response. Reference ranges vary, but a typical adult WCC is 4.0–11.0 x 10⁹/L. Neutrophilia (>7.5 x 10⁹/L) suggests bacterial infection, inflammation, steroids, or stress. Neutropenia (<1.5 x 10⁹/L) increases infection risk; causes include viral infections, drugs (e.g., carbimazole, clozapine), and haematological malignancy. Lymphocytosis (>4.0 x 10⁹/L) is common in viral infections, but persistent lymphocytosis in older adults may indicate chronic lymphocytic leukaemia. Eosinophilia (>0.5 x 10⁹/L) points to allergy, parasites, or drug reactions.

• Left shift: The presence of immature neutrophils (bands, metamyelocytes) suggests acute infection or inflammation.
• Leucocytosis with blasts: Urgent haematology referral—possible acute leukaemia.
• Pancytopenia: Low WCC, Hb, and platelets together require urgent investigation for marrow failure.

Platelet Count: Bleeding and Thrombotic Risks

Platelets (normal range 150–400 x 10⁹/L) are essential for haemostasis. Thrombocytopenia (<150 x 10⁹/L) may be due to reduced production (marrow failure, B12 deficiency), increased destruction (immune thrombocytopenia, DIC, drugs), or sequestration (hypersplenism). A count below 50 x 10⁹/L significantly increases bleeding risk; below 20 x 10⁹/L is a medical emergency. Thrombocytosis (>450 x 10⁹/L) is often reactive (infection, inflammation, iron deficiency) but can be primary (essential thrombocythaemia). A raised MPV (mean platelet volume) suggests younger, more active platelets.

Patterns and Pitfalls in FBC Interpretation

Never interpret a single parameter in isolation. A low Hb with a high MCV and low B12 tells a different story from a low Hb with a low MCV and low ferritin. Always consider the clinical context: age, sex, ethnicity, medications, and recent illness. Analytical pitfalls include clumping (EDTA-dependent pseudothrombocytopenia), cold agglutinins (falsely low RBC), and lipaemia. If results do not fit the clinical picture, repeat the sample or request a blood film.

• Delta checks: Compare with previous results—a sudden drop in Hb or platelets is more concerning than a stable mild abnormality.
• Reference ranges: Always use the ranges provided by your local laboratory—they vary by analyser and population.
• Blood film: Request one when you see unexplained cytopenias, blasts, or suspicious flags.

Practical Tips for the Busy Clinician

Start with the clinical question. Is this a screening test, a diagnostic workup, or monitoring? For suspected iron deficiency, check ferritin and CRP together. For macrocytosis, check B12, folate, and thyroid function. For neutropenia, review drugs and ask about recurrent infections. Always document your interpretation and communicate urgent findings—such as a platelet count below 20 x 10⁹/L or a WCC above 30 x 10⁹/L—directly to the responsible clinician. And remember: the FBC is a guide, not a diagnosis. Combine it with history, examination, and other tests to reach the right conclusion.

By understanding the red cell, white cell, and platelet parameters—and their common patterns—you can make faster, safer decisions for your patients. If in doubt, discuss with a haematologist or clinical scientist; they are there to help.