Full blood count - Investigations and procedures

Investigations and procedures

Full blood count

The full blood count is the most commonly performed investigation in medicine. It measures the five types of white blood cell (neutrophil, lymphocyte, monocyte, eosinophil and basophil), the red blood cells and the platelets. The mean cell volume (MCV), packed cell volume (PCV), mean cell haemoglobin content (MCH) and the mean cell haemoglobin concentration (MCHC) are also either measured or calculated. Further details about cellular morphology can be obtained by examining the blood film.

Neutrophils

A neutrophilia is seen in bacterial infections, tissue necrosis, inflammation, myeloproliferative diseases and corticosteroid therapy. A leukaemoid reaction is when overproduction of white cells leads to the release of immature cells. It occurs in severe infections, tuber-culosis or malignant infiltration of the bone marrow. A leucoerythroblastic anaemia occurs when nucleated red cells and immature white cells are released into the circulation. This may result from marrow infiltration or myelofibrosis.

A neutropenia may occur in viral infections and severe bacterial infections. It occurs with any cause of pancytopenia, in association with rheumatoid arthritis (Felty’s syndrome). There is a recognised racial neutropaenia in Afro-Carribbeans.

Lymphocytes

A lymphocytosis is seen in viral infections particularly Epstein Barr virus and cytomegalovirus. Chronic inflammation including tuberculosis and toxoplasma may cause a rise in lymphocytes. Malignant proliferation may result from leukaemias and lymphomas.

Monocytes

Monocytes are the blood and bone marrow located precursors of tissue macrophages (including liver Kupffer cells, pulmonary alveolar macrophages and Langerhan cells in the skin) and dendritic antigen presenting cells. They are phagocytic and are involved in antigen processing and presentation.

A monocytosis may be seen in viral infections such as glandular fever and in chronic bacterial infections such as endocarditis, tuberculosis and myelodysplasia. In cyclical neutropenia the monocytes rise as the neutrophil count falls.

Eosinophils

Eosinophils are phagocytic, with a particular affinity for antigen–antibody complexes and are involved in allergic reactions and parasitic infections.

An eosinophilia may arise in parasitic infections, allergic disorders (hay fever, hypersensitivity), skin disor-ders (urticaria, eczema), pulmonary disorders (asthma, allergic aspergillosis, Churg Strauss syndrome) and in hyper eosinophilic syndrome.

Basophils

Basophils are thought to be the circulating equivalent of tissue mast cells, the granules contain proteoglycans, heparin, histamine. They have surface IgE receptors and are increased in myeloproliferative disorders particularly chronic myelogenous leukaemia.

Red blood cells

Polycythaemia (increased red cell count) may result from reduced plasma volume (diarrhoea, vomiting, diuretics). True polycythaemia may be primary or secondary. Secondary polycythaemia may be due to

Hypoxia, which may be physiological (neonates, altitude), or due to respiratory disease (smoking, COAD) or cyanotic heart disease.

Inappropriate erythropoetin production: Renal cell carcinoma, renal cysts or following renal transplant.

Tumour secretion: Hepatoma, cellular haemangioma. Severe polycythaemia may result in hyperviscosity of the blood, which prediposes to thrombosis, haemorrhage and cardiac failure.

Platelets

Thrombocytopenia (reduced platelet count) may be due to failure of bone marrow production or excess destruction of platelets. Bone marrow platelet production may fail due to aplasia, marrow infiltration or the effects of drugs. Peripheral platelet destruction may result from immune mechanisms, from excess coag ulation in disseminated intravascular coagulation  or thrombotic thrombocytopenic purpura.