Abnormal Blood Counts — Summary

See also: Neutropenia/Lymphocytopenia, Neutrophilia/Lymphocytosis, Thrombocytosis/Thrombocytopenia.

Interpreting a full blood count

The complete blood count (CBC/FBC) is one of the most commonly performed laboratory investigations in medicine and reports circulating blood cell size and quantity.

Reference intervals are lab-specific — local laboratory guidelines apply when interpreting a result.

Any abnormal count should prompt review of a peripheral blood smear and, where the picture is unclear, a repeat count and comparison with the patient’s previous results.

Typical adult reference intervals (lab-dependent): white cell count 4500–11,000 cells/µL; neutrophils 40–60% or 1500–8000 cells/µL; lymphocytes 20–40% or 1000–4000 cells/µL; monocytes 2–8% or 200–1000 cells/µL; eosinophils 0–4% or 0–500 cells/µL; basophils 0.5–1% or 0–200 cells/µL; platelets 150,000–400,000/µL.

General approach to an abnormal count

• Confirm the abnormality is genuine before acting on it.
  • Spurious leukopenia can arise from white cell clumping.
  • Spurious leukocytosis can arise from nucleated red cells, platelet clumping, fibrin clumps or cryoproteinaemia.
  • Platelet clumping with EDTA causes pseudo-thrombocytopenia — repeat in a citrate tube.
  • The platelet count can be spuriously high in bacteraemia, fungaemia or malaria (organisms miscounted as platelets) and with red cell fragments in haemolysis.
• Identify which line is affected on the differential, reported as a percentage or as an absolute count per cell type.
• Grade the degree and the tempo. The higher a white cell count — particularly approaching 100,000 cells/µL — the more likely a leukaemia or myeloproliferative disorder; distinguish acute from chronic, and a single reading from a trend.
• Examine a peripheral smear, with a manual differential when the automated result is unreliable. Blasts and other immature precursors point to leukaemia. A predominance of mature neutrophils with a marked count above 50,000 cells/µL but no myeloproliferative neoplasm is a leukemoid reaction, which resolves as its underlying cause is treated.
• Screen for red flags — unwellness out of proportion, rapid rate of change, lymphadenopathy or hepatosplenomegaly, pallor, petechiae, bruising, bleeding or recurrent infection — and refer to haematology-oncology if malignancy is a concern.

Cell-by-cell causes

Cell	High	Low
Neutrophil	Neutrophilia: infection, trauma, tissue necrosis, physical or emotional stress, heavy exercise, smoking, myeloproliferative disease (e.g. CML), chronic inflammatory disease (IBD, SLE, rheumatoid arthritis), vasculitis, diabetic ketoacidosis, acute gout, blood loss, haemolytic anaemia, obesity, pregnancy, medications, congenital causes. Red flags: person particularly unwell, severity of neutrophilia, rate of change, presence of left shift	Neutropenia significant if <1.0×10⁹/L (high infection risk). Causes: viral infection (CMV, measles, rubella, HIV, dengue, EBV), bacterial (typhoid, septicaemia), autoimmune (SLE, rheumatoid arthritis), bone marrow failure (myelo-/lymphoproliferative disease, metastatic malignancy, myelodysplasia, irradiation), megaloblastic anaemia, hypersplenism, hereditary, benign ethnic neutropenia, medications. Red flags: person particularly unwell, severity/rate of change, lymphadenopathy/hepatosplenomegaly
Lymphocyte	Lymphocytosis: viral infection, pertussis, toxoplasmosis, tuberculosis, brucellosis, hyposplenism, acute stress response, autoimmune thyroiditis, lymphoproliferative disease (CLL). Reactive lymphocytosis can reach 20,000–30,000 cells/µL and resolves over 1–2 months; refer if absolute count >50,000 cells/µL with no cause found	Lymphocytopenia: viral or bacterial infection, autoimmune disease (SLE, rheumatoid arthritis, Sjögren, IBD), malignancy, myelo-/lymphoproliferative disease, primary immunodeficiency, medications. Not usually clinically significant in isolation
Eosinophil	Eosinophilia (>500 cells/µL): atopy/allergy (allergic rhinitis, atopic dermatitis — usually mild), asthma, parasitic (helminth) infection, drug hypersensitivity, connective tissue disease, skin disorders (psoriasis, scabies), vasculitis (especially eosinophilic granulomatosis with polyangiitis), Hodgkin lymphoma, myelo-/lymphoproliferative disease, hypereosinophilic syndrome. Severe eosinophilia (≥20,000 cells/µL) is more likely myeloid neoplasm	Eosinopenia: acute stress, inflammation, glucocorticoids. No real cause for concern
Monocyte	Monocytosis (>1000 cells/µL): chronic infection and inflammation (IBD, endocarditis, tuberculosis, malaria, typhoid, syphilis, brucellosis), autoimmune disease, granulomatous disease, haemolytic anaemia, cigarette smoking, pregnancy, asplenia, leukaemia and lymphoma, medications	Monocytopenia: infection, SLE, aplastic anaemia, hairy cell leukaemia, medications. Not clinically significant in isolation
Basophil	Basophilia: myeloproliferative disorders, hypersensitivity/inflammatory reactions, myxoedema, viral infection, tuberculosis, chronic inflammatory disease, allergy, anaphylaxis, drugs. Isolated basophilia is uncommon and warrants workup to exclude a myeloproliferative process — persistent basophilia beyond ~8 weeks raises concern for malignancy	Basopenia: acute infection, hyperthyroidism, ovulation, medications. Difficult to demonstrate
Platelet	Thrombocytosis: count >450,000/µL, rarely symptomatic unless >1,000,000/µL. Reactive causes (infection, inflammation), pregnancy, iron deficiency, post-splenectomy (reduced sequestration), bone marrow stimulation, primary myeloproliferative disorders (essential thrombocythaemia)	Thrombocytopenia: count <150,000/µL, but major bleeding is unusual above 50,000/µL; spontaneous major bleeding rises steeply below 20,000/µL and cerebral bleeding risk below 10,000/µL. Causes: decreased production, splenic sequestration, or increased peripheral utilisation — viral infection, immune thrombocytopenic purpura, liver disease, drugs, hypersplenism, autoimmune disease, pregnancy. Red flags: bruising, petechiae, signs of bleeding