Looking inside the airways
Bronchoscopy is the act of passing a camera into the airways. The modern flexible bronchoscope is a slim, steerable tube with a light, a camera chip, and a working channel for instruments. Under local anesthetic and light sedation, it travels through the nose or mouth, past the vocal cords, down the trachea, over the carina, and into the bronchi. The operator directly inspects the airway walls for inflammation, narrowing, bleeding, or a visible tumor.
But looking is only half of it; the working channel lets the doctor take samples. A transbronchial biopsy passes tiny forceps through the bronchial wall to grab a piece of lung tissue, useful in diffuse disease and many tumors. To reach lymph nodes and masses beside — not inside — the airway, endobronchial ultrasound (EBUS) adds an ultrasound probe at the tip: it sees through the airway wall and guides a needle into a node in real time, the cornerstone of modern mediastinal staging for lung cancer.
Washing and sampling
A gentler sampling method is bronchoalveolar lavage (BAL). The scope is wedged into a small bronchus, warm saline is instilled and then suctioned back, and that returned fluid carries a sample of cells and microbes from deep in the alveoli. The cell make-up of BAL fluid helps sort interstitial diseases, and culturing it can identify infections — invaluable when a patient is too immunocompromised to risk a bigger procedure.
When the problem is fluid in the chest wall lining rather than the airways, the procedure is thoracentesis: a needle drawn into the pleural space to remove pleural fluid. It serves two purposes — relieving breathlessness by draining a large effusion, and, just as importantly, sending fluid to the lab. Almost always it is done under pleural ultrasound guidance, which shows the fluid pocket in real time and dramatically lowers the risk of puncturing the lung and causing a pneumothorax.
The first big question about pleural fluid is whether it is a transudate (a thin filtrate from systemic forces like heart failure) or an exudate (a protein-rich fluid from local trouble like infection or cancer). Light's criteria compare protein and LDH levels in the fluid against the blood to make that call, steering the rest of the work-up.
Light's criteria — classifying a pleural effusion Fluid is an EXUDATE if ANY one is true: 1) Pleural protein / serum protein > 0.5 2) Pleural LDH / serum LDH > 0.6 3) Pleural LDH > 2/3 upper limit of normal serum LDH Worked example: Pleural protein 4.2 / serum protein 6.0 = 0.70 ( > 0.5 -> exudate ) Pleural LDH 320 / serum LDH 180 = 1.78 ( > 0.6 -> exudate ) Either ratio alone meets a criterion -> EXUDATE Next step: pursue a local cause (infection, malignancy, etc.) (If ALL three were below cutoff -> transudate; look for heart failure, etc.)