Hello! Rheumatoid arthritis is diagnosed based on clinical presentation of inflammatory arthritis involving three or more joints in your body. The blood tests which help in diagnosis are Rheumatoid factor , Anti - CCP (cyclic citrullinated peptide), ESR (Erythrocyte sedimentation Rate) and CRP (C reactive protein). Blood test that helps in diagnosing sarcoidosis is Serum ACE (Angiotensin Converting Enzyme)level . Rheumatoid Arthritis is an autoimmune disease (the body reacts against it s own tissues and damages them )so it can affect lungs as well and not just be restricted to your joints. The various common manifestations of rheumatoid lung are interestial lung disease (ILD) , pleural effusion or thickening , inflammation of airways or blood vessels of the lung , rheumatoid nodule . There is also increased risk of respiratory infection due to immunosuppressant drugs prescribed for Rheumatoid.. Sometimes the drugs used for treatment of rheumatoid themselves can be toxic to your lung and cause damage. Hence this needs careful evaluation . So the lung lesion can indicate any of the problems mentioned above. If on clinic radiological correlation , it does seem obvious that the lesson is due to your rheumatoid, then treatment of rheumatoid will help in the resolution of your lung lesion as well . If however there is doubt regarding the cause for your lung lesion then it would be better to sample the tissue from the affected area in the lung and study it under the microscope and arrive at the diagnosis. For tissue sampling open lung biopsy , though the best , is not the only option .U can discuss with your Consultant in person regarding feasibility of a bronchoscopy guided biopsy or CT guided biopsy instead of open lung biopsy .