Carbon monoxide diffusing capacity and membrane diffusion capacity form a part of complete pulmonary function tests that also include spirometry, lung volume(s), flow volume loop and airway resistance. Carbon monoxide diffusing capacity is a pulmonary function test that helps measure the available surface of the lung for diffusion. It also gives an idea about the volume of blood in the pulmonary capillaries, as well as finding the thickness of the alveolar capillary barriers. The test also helps understand the ability of the lungs to exchange oxygen and carbon dioxide to the bloodstream. "The test can diagnose certain lung diseases, and repeat testing can also identify improvement or disease progression for patients with established lung disease," says Carol Pohlig, BSN, RN, CPC, ACS, senior coding and education specialist at the University of Pennsylvania Department of Medicine in Philadelphia. The values of DLCO are usually reduced in many of these lung conditions although in conditions such as pulmonary hemorrhage, asthma, and polycythemia you may see an increase in the value. "Testing is contraindicated in patients with mental confusion, muscular incoordination or previous symptoms triggered by an associated Valsalva or Muller maneuver which may slow the heart rate" says Pohlig.