Full Blood Count and Differential Count
Quick ED guide to common patterns on the FBC. Always interpret in context of the patient, smear / film report, and local reference ranges.
Red Cells, White Cells and Platelets
Hb, RBC and HCT usually move together. Start by asking: “Is there anaemia or polycythaemia? What is the MCV?”
| Parameter | Terminology for Increase | Terminology for Decrease | Differential Diagnosis for Increase | Differential Diagnosis for Decrease |
|---|---|---|---|---|
| Haemoglobin (Hb) | Polycythaemia | Anaemia | Dehydration (relative), chronic hypoxia (COPD, cyanotic heart disease), smoking, high altitude, erythrocytosis/polycythaemia vera | Acute or chronic blood loss, haemolysis, iron deficiency, B12/folate deficiency, anaemia of chronic disease, renal failure |
| Red Cell Count (RBC) | Polycythaemia | Anaemia | Same patterns as raised Hb: dehydration, hypoxia, polycythaemia vera, smoking | Same patterns as low Hb: blood loss, haemolysis, chronic disease, nutritional deficiency |
| Haematocrit (HCT) | Polycythaemia | Anaemia | Dehydration, hypoxia, smoking, polycythaemia vera | Blood loss, haemolysis, chronic disease, renal failure, pregnancy |
| White Blood Count (WBC) | Leukocytosis | Leukopenia | Bacterial infection, inflammation, stress, tissue necrosis, corticosteroids, myeloproliferative disease, acute/chronic leukaemia | Viral infection, severe sepsis, bone marrow suppression or infiltration, autoimmune disease, chemotherapy/other myelotoxic drugs |
| Platelet Count (Plt) | Thrombocytosis | Thrombocytopenia | Reactive (infection including TB, inflammation, iron deficiency, recent surgery/trauma), postsplenectomy, myeloproliferative disorders | Bone marrow failure or infiltration, sepsis/DIC, hypersplenism, immune thrombocytopenia (ITP), drugs (e.g. heparin, chemotherapy), viral infections (including HIV, hepatitis) |
| Mean Cell Volume (MCV) | Macrocytosis | Microcytosis | B12/folate deficiency, alcoholism, chronic liver disease, hypothyroidism, some drugs (e.g. hydroxyurea, zidovudine), myelodysplastic syndromes | Iron deficiency anaemia, thalassaemia, anaemia of chronic disease, sideroblastic anaemia, lead poisoning |
| Mean Cell Haemoglobin (MCH) | Hyperchromia (rare) | Hypochromia | Often parallels macrocytosis (e.g. B12/folate deficiency), hereditary spherocytosis | Usually parallels microcytosis: iron deficiency anaemia, thalassaemia, anaemia of chronic disease |
| Red Cell Distribution Width (RDW) | High RDW | — | Mixed population of red cells (e.g. early iron deficiency, recent transfusion, combined deficiency states). High RDW with low MCV suggests iron deficiency rather than thalassaemia trait. | Usually not clinically significant when low or normal. Normal RDW with microcytosis suggests thalassaemia trait. |
Differential White Cell Count
Always correlate with smear/film comments (left shift, blasts, atypical lymphocytes, toxic granulation, etc.).
| Parameter | Terminology for High Values | Differential Diagnosis for High Values | Terminology for Low Values | Differential Diagnosis for Low Values |
|---|---|---|---|---|
| Neutrophils | Neutrophilia | Bacterial infections, acute inflammation, tissue necrosis (MI, burns), stress, corticosteroids, smoking, myeloproliferative disease | Neutropenia | Viral infections, severe sepsis, bone marrow failure/infiltration, autoimmune neutropenia, chemotherapy and other myelotoxic drugs, hypersplenism |
| Lymphocytes | Lymphocytosis | Viral infections (EBV, CMV, hepatitis, HIV seroconversion), pertussis, chronic lymphocytic leukaemia and other lymphoid malignancies, some autoimmune diseases | Lymphopenia | Immunodeficiency (including advanced HIV), high-dose steroids, chemotherapy, severe sepsis, autoimmune disease, malnutrition |
| Monocytes | Monocytosis | Chronic infections (TB, endocarditis), autoimmune/connective tissue disease, recovery phase after neutropenia, myelodysplastic and myeloproliferative disorders | Monocytopenia | Often not clinically significant; can be seen with bone marrow suppression, high-dose steroids or severe sepsis |
| Eosinophils | Eosinophilia | Allergic disease (asthma, eczema, drug reactions), parasitic infections, some autoimmune diseases, Hodgkin lymphoma and other malignancies, hypereosinophilic syndromes | Eosinopenia | Often not clinically significant; may occur in acute infection, stress response and with corticosteroid therapy |
| Basophils | Basophilia | Chronic myeloid leukaemia and other myeloproliferative neoplasms, chronic inflammation, hypersensitivity reactions, hypothyroidism | Basopenia | Usually not clinically significant; may be seen with hyperthyroidism, pregnancy, stress and corticosteroids |
This guide is for education and rapid reference in the emergency department. Always correlate with clinical findings, peripheral smear/morphology and local haematology advice.