Thanks for your question on HCM. Ideally we need to drain the fluid and ger done culture of the fluid to know the type of infection. But since it is not tappable, we need to rule out type of infection by other tests like 1. Sputum routine microscopy and culture. 2. Bronchoscopy and BAL ( Broncho Alveolar Lavage)analysis. 3. Blood reports like ESR, CRP and PCT. Amongst these PCT is highly specific for bacterial infection. And most common cause for pleural effusion is bacterial infection only. So discuss about these tests with your treating doctor.