Bronchoalveolar lavage (BAL) is a meical procedure in which a bronchoscope is passed through the mouth or nose into the lungs and fluid is squirted into a small part of the lung and then recollected for examination. BAL is typically performed to diagnose lung disease. In particular, BAL is commonly used to diagnose infections in people with immune system problems, pneumonia in people on ventilators, some types of lung cancer, and scarring of the lung (interstitial lung disease). A major clinical limitation for the utility of bronchoalveolar lavage is the large range of normal values for each parameter, which makes BALF insensitive in detecting disease. Furthermore, abnormalities in BALF are rarely specific for any of the interstitial lung diseases. There are some subjects who have normal BALF constituents despite a definite disease and some without any evidence of disease despite abnormal BALF findings. There is large interindividual variation which may not be related to the disease, and the airspace cells and secretions may not reflect interstitial processes. The removal of BALF may preferentially select, activate or injure some cells, and the composition of the epithelial lining fluid (ELF) may change during the bronchoalveolar lavage. (Davis 1994).