Merck Manual

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Overview of Eosinophilic Pulmonary Diseases


Joyce Lee

, MD, MAS, University of Colorado School of Medicine

Reviewed/Revised Jul 2023

Eosinophilic pulmonary diseases are a heterogeneous group of disorders characterized by the accumulation of eosinophils in alveolar spaces, the interstitium, or both. Peripheral blood eosinophilia Eosinophilia Eosinophilia is defined as a peripheral blood eosinophil count > 500/mcL (> 0.5 × 109/L). Causes and associated disorders are myriad but often represent an allergic reaction or... read more Eosinophilia is also common. Known causes of eosinophilic pulmonary disease include

Often the cause is unknown.

The two primary eosinophilic pulmonary diseases of unknown etiology are

Diagnosis of Eosinophilic Pulmonary Diseases

  • Chest-x-ray or computed tomography scan of the chest

  • Demonstrating eosinophilia in peripheral blood, bronchoalveolar lavage fluid, or lung tissue

Diagnosis is based on demonstration of opacities on chest imaging and identification of eosinophilia (> 450/microL [0.45 × 109/L]) in peripheral blood, bronchoalveolar lavage fluid, or lung biopsy tissue. However, pulmonary eosinophilia may occur in the absence of peripheral eosinophilia. Pulmonary opacities on chest imaging associated with blood eosinophilia are sometimes called PIE (pulmonary infiltrates with eosinophilia) syndrome.

Eosinophils are primarily tissue-dwelling and are several hundred–fold more abundant in tissues than in blood. Consequently, blood eosinophil numbers do not necessarily indicate the extent of eosinophilic involvement in affected tissues. Eosinophils are most numerous in tissues with a mucosal epithelial interface with the environment, such as the respiratory, gastrointestinal, and lower genitourinary tracts. Eosinophils are not present in the lungs of healthy people, so their presence in tissue or bronchoalveolar lavage fluid (> 5% of differential count) identifies a pathologic process.

Eosinophils are exquisitely sensitive to corticosteroids and completely disappear from the bloodstream within a few hours after administration of corticosteroids. This rapid disappearance from the blood may obscure the diagnosis in patients who receive corticosteroids before the diagnostic assessment is instituted.

Drugs Mentioned In This Article

Drug Name Select Trade
Dilantin, Dilantin Infatabs, Dilantin-125, Phenytek
5-HTP, 5-HTP Maximum Strength
NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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