Eosinophilia is typically associated with allergic disorders including drug sensitivity, skin diseases and parasitic infections. In most cases, the cause is indicated by the clinical history, which should include details of all medications and foreign travel, and by examination of the stool and urine for parasites, cysts and ova. A diagnosis of chronic eosinophilic leukaemia is made if there is dominant eosinophilia with an increase in blast cells in the blood or marrow, or cytogenetic or molecular evidence of an abnormal myeloid clone.⁵ If no other cause for eosinophilia is found it is important, because of the therapeutic implications (i.e. responsiveness to imatinib), to confirm or exclude a diagnosis of eosinophilic leukaemia related to rearrangement of PDGFRA or PDGFRB (see p. 559).⁶ The idiopathic hypereosinophilic syndrome is an unusual cause of eosinophilia in which release of the contents of eosinophil granules results in damage to the heart, lungs and other tissues. It is defined by the presence of a peripheral blood eosinophil count of 1.5 × 10⁹/l or greater for at least 6 months with resultant tissue damage. This is a diagnosis of exclusion, made only when detailed investigations exclude other possible causes of eosinophilia including systemic mastocytosis, eosinophilic leukaemia and eosinophilia associated with a phenotypically aberrant T-cell population or a neoplastic clone of T cells.

Basophilia

Basophilia as an isolated finding is unusual. However, it is a common feature of myeloproliferative neoplasms and basophils may be particularly prominent in chronic myelogenous leukaemia. In this condition, an increasing basophil count may be the first indication of accelerated phase disease.

Thrombocytosis

Thrombocytosis is often associated with infectious and inflammatory conditions such as osteomyelitis and rheumatoid arthritis. Haematological causes of thrombocytosis include chronic blood loss, red cell destruction, splenectomy and rebound following recovery from marrow suppression. Under these circumstances, a moderately increased platelet count (e.g. 400–800 × 10⁹/l) does not usually have any pathological implications. Primary thrombocythaemia is usually the result of a myeloproliferative neoplasm; rarely, it is an inherited condition. When there is isolated persistent thrombocytosis in a myeloproliferative neoplasm the diagnosis is essential thrombocythaemia (as long as a BCR–ABL1 fusion gene has been excluded). Thrombotic or haemorrhagic complications can occur but often the diagnosis is an incidental one.⁷ A significant proportion of individuals with essential thrombocythaemia have the JAK2 V617F mutation, which is associated with an increased risk of thrombosis.⁸ The criteria for this diagnosis are discussed on p. 559.

Reduced Numbers of Cells

Reductions in more than one cell line

A reduction in cell numbers occurs because of increased destruction, reduced production or increased pooling in the spleen or other organs. Reduced production of cells may be the result of aplastic anaemia, a lack of haematinics such as folate or vitamin B₁₂ or interference with normal haemopoiesis by infiltration (e.g. leukaemia, lymphoma, multiple myeloma, metastatic carcinoma – often with secondary myelofibrosis), infection (e.g. HIV infection, tuberculosis, leishmaniasis) or exposure to toxins (e.g. alcohol) or myelosuppressive drugs (e.g. hydroxycarbamide or methotrexate). Certain myeloid neoplasms, e.g. primary myelofibrosis and MDS, are characterized by cytopenias, which are at least in part the result of ineffective haemopoiesis. Cytopenia is also sometimes a feature of acute myeloid leukaemia (AML), when it is due both to ineffective haemopoiesis and to replacement of normal haemopoietic stem cells by leukaemic cells. A relatively common cause of a global reduction in circulating cells is pooling of the cells in a markedly enlarged spleen (hypersplenism), which may be secondary to conditions such as primary myelofibrosis and portal hypertension. Examination of a bone marrow aspirate and trephine biopsy specimen is often helpful in determining the cause of cytopenias for which no obvious cause is apparent

The mechanisms which result in anaemia are decreased production, reduced red cell lifespan, blood loss and splenic pooling. Anaemia is broadly divided into three types: microcytic (low MCV), macrocytic (high MCV) and normocytic (normal MCV). The choice of investigations is guided by the MCV and red cell morphology in addition to clinical features. Figures 23.1–23.3 are flow charts that provide an orderly sequence of investigations for the different types of anaemia on the basis of these indices. Examination of a blood film will usually suggest the quickest route to the diagnosis; confirmation may require the more specific tests, which are given in the text. The presence of basophilic stippling in a patient with microcytic red cells suggests thalassaemia trait or, much less often, lead poisoning. A dimorphic blood film is typical of congenital sideroblastic anaemia but is more often the result of iron deficiency responding to treatment. Pappenheimer bodies suggest that a microcytic anaemia is the result of sideroblastic erythropoiesis.

Figure 23.1 Investigation of a microcytic hypochromic anaemia. HbEP, Hb electrophoresis; HPLC, high-performance liquid chromatography.

Figure 23.2 Investigation of a macrocytic anaemia. MDS, myelodysplastic syndrome.

Figure 23.3 Investigation of a normocytic, normochromic anaemia.

Microcytic Anaemia

The most common cause of anaemia worldwide is iron deficiency, which can be suspected from a low MCV (Fig. 23.1) and the presence of hypochromic, microcytic red cells. Laboratory confirmation of iron deficiency may be based on measurements of serum ferritin, serum iron plus either total iron-binding capacity or transferrin assay, red cell protoporphyrin and staining of bone marrow aspirates for iron (see Chapter 4). Assay of soluble transferrin receptors has good sensitivity and specificity for iron deficiency and may be useful in the presence of inflammation when interpretation of ferritin levels is difficult.⁹ A diagnosis of iron deficiency must be followed by a search for the cause. This should include specific questions relating to blood loss and dietary insufficiency and may require stool examination for parasites and occult blood, endoscopic examination of the gastrointestinal tract to exclude occult malignancy and tests for coeliac disease. The differential diagnosis of iron deficiency anaemia includes anaemia of chronic disease. Clinical and laboratory features of inflammation or chronic infection may suggest this diagnosis, which is confirmed by demonstration of normal or high serum ferritin and reduced serum iron, transferrin and iron-binding capacity.

The thalassaemias also cause microcytosis, but both α and β thalassaemia trait are usually associated with an increased red blood cell count (RBC) and a normal or near-normal Hb despite a considerable reduction of the MCV and MCH. In contrast, in iron deficiency the MCV and MCH do not fall until the Hb is significantly reduced. Further investigations, such as high-performance liquid chromatography (HPLC) or haemoglobin electrophoresis supplemented by measurement of Hb A₂ and Hb F usually confirm the diagnosis of β thalassaemia trait. The diagnosis of α thalassaemia trait is more difficult; detection of infrequent Hb H inclusions is usually possible in α⁰ thalassaemia trait, but definitive diagnosis requires DNA analysis.¹⁰ A diagnosis of α⁰ thalassaemia heterozygosity can be clinically important for prediction of haemoglobin Bart’s hydrops fetalis since, if both parents have α⁰ thalassaemia, this very serious condition can occur in a fetus. DNA analysis is therefore indicated when a pregnant woman of appropriate ethnic origin has an MCH of <25 pg. Hb H inclusions may not be detected in α⁺ thalassaemia trait but this diagnosis is of less clinical importance and confirmation is thus not usually required.

Macrocytic Anaemia

A high MCV (Fig. 23.2) with oval macrocytes and hypersegmented neutrophils suggests folate or vitamin B₁₂ deficiency and is an indication for assays of these vitamins (see Chapter 10); subsequent investigations could include malabsorption studies, tests for coeliac disease and tests for intrinsic factor and gastric parietal cell antibodies. In patients with these blood film findings and normal vitamin assays, haematinic deficiency is not completely excluded and further investigation is indicated. A Schilling test permits a definitive diagnosis of pernicious anaemia but currently this test is not available due to a lack of reagents. In the absence of intrinsic factor antibodies, the diagnosis of pernicious anaemia may be presumptive. Pernicious anaemia is commonly associated with autoimmune thyroid disease and other autoimmune disorders, such as diabetes mellitus. A high MCV may also be associated with alcohol excess and liver disease or use of drugs such as hydroxycarbamide or zidovudine. Macrocytosis resulting from chronic haemolysis is associated with increased numbers of immature red cells, which appear slightly larger and bluer than normal red cells (polychromatic macrocytes) on a Romanowsky-stained peripheral blood film. Supravital staining of blood films (see p. 33) or an automated reticulocyte count can be used to confirm reticulocytosis. Untreated anaemia associated with polychromasia is likely to indicate blood loss or haemolysis. The combination of red cell fragments, thrombocytopenia and polychromasia indicates a microangiopathic haemolytic anaemia and should trigger further tests such as a platelet count, coagulation studies, assessment of renal function and a search for infection or neoplastic disease. This further assessment is urgent because these may be features of thrombotic thrombocytopenic purpura, which requires immediate treatment by plasma exchange.

Normocytic Anaemia

Normochromic, normocytic anaemia (Fig. 23.3) is frequently the result of an underlying chronic, non-haematological disease. Investigations should include screening for renal insufficiency, subclinical infections, autoimmune diseases and neoplasia. In the presence of anaemia, a lack of polychromasia, confirmed by reticulocytopenia, points toward a primary failure of erythropoiesis or lack of compensatory increased red cell production in blood loss or haemolysis. Examination of the bone marrow may be helpful in demonstrating haematological causes for a normochromic, normocytic anaemia such as aplastic anaemia or MDS.¹¹ Staining for iron may also show that there is a block in iron metabolism suggestive of anaemia associated with chronic inflammatory disease.