Editing Diffusing capacity (section)

{{Short description|Measure of the transfer of gas from the lung to red blood cells}}
{{Infobox diagnostic
|  Name        = Diffusing capacity
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|  Caption     =
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|  MeshID      = D011653
|  OPS301      =
|  OtherCodes  = CPT: 94720
}}
'''Diffusing capacity''' of the lung (D<sub>L</sub>) (also known as ''transfer factor'') measures the transfer of gas from air in the lung, to the [[red blood cell]]s in lung blood vessels.  It is part of a comprehensive series of [[pulmonary function testing|pulmonary function tests]] to determine the overall ability of the [[lung]] to transport gas into and out of the blood.  D<sub>L</sub>, especially [[DLCO|D<sub>LCO</sub>]], is reduced in certain diseases of the lung and heart. D<sub>LCO</sub> measurement has been standardized according to a position paper<ref name="multiple">{{cite journal  |vauthors=Macintyre N, Crapo RO, Viegi G, etal | year = 2005 | title = Standardisation of the single-breath determination of carbon monoxide uptake in the lung | journal = Eur Respir J | volume = 26 | issue = 4| pages = 720–35 | doi = 10.1183/09031936.05.00034905 | pmid = 16204605 | s2cid = 18177228 | doi-access = free }}</ref> by a task force of the [[European Respiratory Society|European Respiratory]] and [[American Thoracic Society|American Thoracic]] Societies.

In [[respiratory physiology]], the diffusing capacity has a long history of great utility, representing [[Electrical resistance and conductance|conductance]] of gas across the alveolar-capillary membrane and also takes into account factors affecting the behaviour of a given gas with hemoglobin.{{Citation needed|reason=uncited definition|date=March 2014}}

The term may be considered a misnomer as it represents neither [[diffusion]] nor a [[Battery (electricity)|capacity]] (as it is typically measured under submaximal conditions) nor [[capacitance]]. In addition, gas transport is only diffusion limited in extreme cases, such as for oxygen uptake at very low ambient oxygen or very high pulmonary blood flow.{{Citation needed|reason=unproved statement|date=August 2015}}

The diffusing capacity does not directly measure the primary cause of [[hypoxemia]], or low blood oxygen, namely mismatch of [[Ventilation/perfusion ratio|ventilation to perfusion]]:<ref>West, J. 2011. Respiratory Physiology: The Essentials. 9e. {{ISBN|978-1-60913-640-6}}</ref>
* Not all pulmonary arterial blood goes to areas of the lung where gas exchange can occur (the anatomic or physiologic shunts), and this poorly oxygenated blood rejoins the well oxygenated blood from healthy lung in the pulmonary vein.  Together, the mixture has less oxygen than that blood from the healthy lung alone, and so is hypoxemic.
* Similarly, not all inspired air goes to areas of the lung where gas exchange can occur (the [[Dead space (physiology)|anatomic and the physiological dead spaces]]), and so is wasted.