At a Glance

Basophilia is defined as an absolute increase in the number of basophils. Reference values vary from laboratory to laboratory, but an absolute count of basophils greater than 0.2 X 109/L is considered a true basophilia.

Basophilia is most commonly associated with neoplastic conditions, such as myeloproliferative neoplasms, the prototype being chronic myelogenous leukemia, BCR-ABL1 positive (CML). A rising basophil count in a patient with CML, defined as more than 20% basophils in the peripheral blood, often indicates a transition to blast crisis. The myeloproliferative neoplasm polycythemia vera is also associated with a mild basophilia. Finally, a small subset of acute myeloid leukemias are associated with basophilia, including acute myeloid leukemia with t(6;9)(p23;q34); DEK-NUP214 and acute basophilic leukemia.

Conditions associated with a reactive basophilia include allergic and hypersensitivity reactions, collagen vascular disorders, renal disease, endocrinopathy, irradiation, myxedema, diabetes mellitus, infections, such as tuberculosis and certain viral infections, rare carcinomas, and some medications.

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What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

Most hematology analyzers do a very poor job of enumerating basophils. Hence, a complete blood count (CBC) with manual differential is important in confirming basophilia. The percentage of basophils in the differential should be multiplied by the white blood cell count to obtain the absolute basophil count; basophil percentages are not considered an adequate substitute for the absolute basophil count. Morphologic evaluation of basophils is important. Basophils have a unique morphologic appearance of a mature myeloid cell with a segmented nucleus that is obscured by multiple coarse, dense purple-black cytoplasmic granules that overlay the nucleus.

Other mature myeloid cells can mimic basophils, such as neutrophils with toxic granulation, mast cells, and rarely eosinophils. Basophils may also show partial degranulation. A peripheral blood smear should easily distinguish basophils from toxic neutrophils and eosinophils. Abnormal eosinophils may show basophilic granules, such as in acute myeloid leukemia with inv(16)(p13.1q22) or t(16;16)(p13.1;q22), but the presence of blasts and abnormal eosinophils leads to that diagnosis. Distinction from mast cells can be more problematic; the circulating mast cell is neoplastic and may show a variety of appearances. Typically, the mast cell has smaller and finer purple granules compared to the basophil; the nucleus can vary from round to oval to bilobed. Some circulating mast cells resemble blast cells and are termed “metachromatic blasts” with smooth chromatin and scattered large basophilic type granules; these cells are more often confused with blasts rather than basophils.

As part of the morphologic review of the smear, all cell lines should be assessed for evidence of a myeloproliferative neoplasm. In CML, the most common condition associated with basophilia, a high white blood cell count with many immature granulocytes, a basophilia, eosinophilia, and thrombocytosis is typically seen. If the blood smear is suspicious for CML and clinical features appear compatible (i.e., hepatosplenomegaly, fatigue, malaise), assessing for BCR-ABL1 on the peripheral blood using polymerase chain reaction (PCR) or fluorescence in situ hybridization (FISH) and a bone marrow study with cytogenetic analysis can be performed. If, instead, the morphology suggests circulating mast cells, a serum tryptase is recommended. Flow cytometry can also readily distinguish basophils from mast cells, but specialized laboratories familiar with mast cells should be used.

Conditions associated with reactive basophilia may present with varied clinical findings requiring a variety of different laboratory tests.

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?

Medications, including estrogen and antithyroid agents, have been associated with a reactive basophilia.

Transient elevations in basophil counts may be observed at the onset of menses and in blood smears collected in the afternoon and evening.

What Lab Results Are Absolutely Confirmatory?

A CBC with manual differential is essential for determining the absolute basophil count and, thus, a true basophilia.