Diffusion capacity

# Overview
In biology, diffusion capacity is a measurement of the lung's ability to transfer gases. Oxygen uptake may be limited by diffusion in circumstances low ambient oxygen or high pulmonary blood flow.  Carbon dioxide is not limited by diffusion under most circumstances.
The "diffusing capacity" or "diffusion capacity" is part of a comprehensive test series of lung function called pulmonary function testing.  In Europe, this is often called the "transfer factor".

# Calculation
The rate of flow is directly proportional to the oxygen gradient (per Fick's law of diffusion), and the diffusion capacity (DLOO2) can be calculated as the ratio of oxygen flux (J^*) across the alveolar-capillary membrane over the oxygen partial pressure gradient between the alveoli ("A") and the alveolar capillaries ("c")
(The asterisk should be read as a dot over the letter, which is used to denote rate in respiratory physiology.)
J*O2 is the uptake of oxygen (ml/min).  PAO2 is the alveolar partial pressure of oxygen.  PcO2 is the partial pressure of oxygen in the alveolar capillary.  Thus, the higher the diffusing capacity DL, the more gas will be transferred across the alveolar-capillary membrane for a given gradient in partial pressure (or concentration) of the gas.
Because it is difficult to measure the capillary partial pressure of oxygen, carbon monoxide (CO) is used as the test gas to measure the diffusing capacity (Dlco), in Europe this is called transfer factor (TLco).  Because CO is so tightly bound to hemoglobin in the red blood cells, the partial pressure of CO in the capillaries is assumed to be zero.  Thus,

# Test Performance
In practice, the test is performed by having the test subject blow out all of the air that they can to reach residual volume.  The person then takes a full vital capacity inhalation of a test gas mixture that contains a small amount of carbon monoxide (usually 0.3%) and some helium or other non-absorbed tracer gas.  The test gas is held in the lung for about 10 seconds and then is exhaled from the lung.  The first part of the expired gas is discarded and the next portion which represents gas from the alveoli is collected.  By analyzing the concentrations of carbon monoxide and helium in the inspired gas and in the exhaled gas, it is possible to calculate how much carbon monoxide was taken up during the breath hold, and the partial pressure of carbon monoxide was during the breath hold.  This method is known as the single-breath diffusing capacity test.
Other methods that are not so widely used at present can measure the diffusing capacity.  These include the steady state diffusing capacity that is performed during regular tidal breathing, or the rebreathing method that requires rebreathing from a reservoir of gas mixtures.

# Interpretation
The diffusing capacity (DLCO) is a test of the integrity of the alveolar-capillary surface area for gas transfer.  It may be reduced in disorders that damage the alveolar walls (septa) such as emphysema, which leads to a loss of effective surface area.  The DLCO is also reduced in disorders that thicken or damage the alveolar walls such as pulmonary fibrosis.  The DLCO is also reduced in interstitial lung diseases such as hypersensitivity pneumonitis (e.g bird fanciers lung, farmers lung) or dust-inhalation diseases such as asbestosis. Because the transfer of CO depends upon how much blood is present in the lung capillaries, the DLCO can be reduced due to anemia and raised in polycythemia.  Some laboratories adjust or correct the DLCO for a serum hemoglobin to take into account the effect of anemia.  Disorders of the pulmonary blood vessels such as chronic thromboembolic disease or pulmonary hypertension also reduce the diffusing capacity, as can any condition/disease which results in reduction in pulmonary capillary blood volume.  The DLCO is not, however, a sensitive test for acute pulmonary embolism.  Cigarette smokers tend to have lower DLCO, which is partly due to damage to the lungs (mild emphysema) and partly due to the retention of carbon monoxide from the cigarettes, which causes back pressure (decreasing the CO gradient).  For these reasons, the DLCO is a sensitive test for disorders of the lung, but does not point toward a specific diagnosis.  By calculating the dilution of the tracer gas, the test can also be used to estimate the total lung capacity.  In the presence of poor gas mixing from obstructive lung disease the Punjabi correction is used to measure the single breath total lung capacity.