Peripheral blood eosinopenia is an uncommon finding, diagnosed when the absolute eosinophil count is less than the normal range of the testing laboratory (Table 7.1) (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29). In our laboratory, the lower limit of normal is 0.050 × 10⁹‘L.

Eosinopenia has been reported in acute inflammation; adult respiratory distress syndrome; corticosteroid excess, as seen in corticosteroid therapy and Cushing syndrome; drug reactions (e.g., to diethylcarbamazine, ivermectin, and theophylline); dysbarism; hemodialysis, within the first 15 minutes; immune disorders, including asthma and vitiligo; infectious disease, especially bacterial infections, Campylobacter and human immunodeficiency virus infections, hyperinfective strongyloidiasis, malaria, schistosomiasis, and typhoid fever; major depression; niacin deficiency (pellagra); stress, as seen in extreme exercise and heat exhaustion; ovarian carcinoma; and thymoma.

Peripheral blood eosinophilia is diagnosed when the absolute eosinophil count exceeds the normal range of the testing laboratory (Table 7.2) (30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89). In our laboratory, the upper limit of normal is 0.550 × 10⁹‘L. In eosinophilia, the absolute eosinophil count may exceed 20.0 × 10⁹‘L.

Eosinophilia is usually classified into three groups: neoplastic (or primary), reactive (or secondary), and idiopathic hypereosinophilic syndrome (30, 31, 32). Reactive and idiopathic hypereosinophilia are discussed here. Neoplastic eosinophilia is discussed with the myeloproliferative neoplasms, myelodysplastic syndromes, and acute myeloid leukemia.

Persistent eosinophilia, whatever the cause, has serious clinical consequences, including thromboembolic disease, organ damage, and increased overall mortality (33,34).