Q: How strikingly different is pleural effusion finding in Acute Eosinophilic Pneumonia (AEP)?

Answer:  The presence of eosinophils in the pleural effusion is usually considered nondiagnostic. But if the pleural fluid is exudative with an increased percentage of eosinophils, AEP should be strongly considered. It makes diagnosis very susceptible with hypoxemia, pulmonary infiltrates, eosinophils in bronchoalveolar lavage fluid, and prompt response to steroid therapy.

Other reasons for eosinophillia in pleural fluid is previous thoracentesis with air or blood  in contact with the effusion.

Reference:

1. Pope-Harman AL, Davis WB, Allen ED, Christoforidis AJ, Allen JN. Acute eosinophilic pneumonia. A summary of 15 cases and review of the literature. Medicine (Baltimore) 1996; 75 (6) 334–342

2. Fitzgerald DJ1, Chaudhary BA, Davis WB. - Eosinophilic pleural effusion: is it always nondiagnostic? - J Fam Pract. 1996 Apr;42(4):405-7.