Pulmonary function tests are a broad range of tests that measure how well the lungs take in and exhale air and how efficiently they transfer oxygen into the blood. Spirometry measures how well the lungs exhale. The information gathered during this test is useful in diagnosing certain types of lung disorders, but is most useful when assessing for obstructive lung diseases (especially asthma and chronic obstructive pulmonary disease, COPD). Lung volume measurement detects restrictive lung diseases. In this set of diseases, a person cannot inhale a normal volume of air. Restrictive lung diseases may be caused by inflammation or scarring of the lung tissue (interstitial lung disease) or by abnormalities of the muscles or skeleton of the chest wall. Testing the diffusion capacity (also called the DLCO) permits an estimate of how efficiently the lungs transfer oxygen from the air into the bloodstream.