A 42-year-old man presents to the ER with fever, SOB, and non-productive cough, and night sweats for 10 days. He has a temp of 38.3C and results of P/E of the heart, lungs, and abdomen are normal. He has no skin lesions. Laboratory values are as follows: Hb 12 g/dL (11-13), WBC 8,400/uL (5000-10000) platelet count 150,000/uL (150,000-300,000), serum Cr 1.0 mg/dL (0.8-1.2). PO2 is 60 mmHg (65-120). HIV ELISA assay and Western blot positive, and CD4 cell count 105/uL (Normal >450). CXR bilateral symmetrical infiltrates. What is the next most appropriate diagnostic test?