A woman is presenting with low-grade fever since 6 weeks. Chest X ray shows bilateral hilar adenopathy with clear lung fields. All of the following investigations are useful in making the diagnosis except:

Patient is showing features of sarcoidosis. Lymphocyte markers such as CD4 and CD8 are used to determine the CD4/CD8 ratio in the BAL fluid. A ratio of >3.5 is strongly suppoive of sarcoidosis. Sarcoidosis is an inflammatory disease characterized by the presence of non caseating granulomas. Sarcoidosis can affect viually every organ of the body, lung is most commonly affected. Other organs commonly affected are the liver, skin, and eye. Cough and dyspnea are the most common presenting symptoms. Nonspecific symptoms include fever, fatigue, night sweats, and weight loss. Investigations: Chest X-ray shows bilateral hilar adenopathy and is the most commonly used tool to asses lung involvement. PET (positive emission tomography) scan has replaced gallium 67 scan. Serum ACE levels is helpful in diagnosis, but has low sensitivity and specificity. CT of chest is rarely helpful, it may identify early fibrosis and active alveolitis. CD4/CD8 ratio >3.5 in BAL fluid is strongly suggestive of sarcoidosis. Ref: Harrison's Internal Medicine, 18th Edition, Chapter 329