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The '''single-breath diffusing capacity test''' is the most common way to determine <math>D_L</math>.<ref name="multiple" /> The test is performed by having the subject blow out all of the air that he/she can, leaving only the [[Lung volumes|residual lung volume]] of gas. The person then inhales a test gas mixture rapidly and completely, reaching the [[Lung volumes|total lung capacity]] as nearly as possible. This test gas mixture contains a small amount of carbon monoxide (usually 0.3%) and a ''tracer gas'' that is freely distributed throughout the alveolar space but which doesn't cross the alveolar-capillary membrane. Helium and methane are two such gasses. The test gas is held in the lung for about 10 seconds during which time the CO (but ''not'' the tracer gas) continuously moves from the alveoli into the blood. Then the subject exhales.
The anatomy of the airways brings with it complications, since the inspired air must pass through the mouth, trachea, bronchi and bronchioles before it gets to the alveoli where gas exchange will occur; on exhalation, alveolar gas must return along the same path, and so the exhaled sample will be purely alveolar only after a 500 to 1,000 ml of gas has left the subject. While it is algebraically possible to approximate the effects of anatomy (the ''three-equation method''<ref>{{cite journal | author vauthors= Graham BL, Mink JT, Cotton DJ | year = 1981 | title = Improved accuracy and precision of single-breath CO diffusing capacity measurements | url = | journal = J Appl Physiol | volume = 51 | issue = 5| pages = 1306–13 | pmid = 7298468 }}</ref>), disease states introduce considerable uncertainty to this approach. Instead, the first 500 to 1,000 ml of the expired gas is disregarded and the next portion which contain gas that has been in the alveoli is analyzed.<ref name="multiple" /> By analyzing the concentrations of carbon monoxide and inert gas in the inspired gas and in the exhaled gas, it is possible to calculate <math>(D_{L_{CO}})</math> according to Equation {{EquationRef|2}}. First, the ''rate'' at which CO is taken up by the lung is calculated according to:
{{NumBlk|::|<math>\dot{V}_{CO} =\frac {\Delta{[CO]} * V_A} {\Delta{t}} </math> . | {{EquationRef|4}} }}
::::The pulmonary function equipment monitors the change in the concentration of CO that occurred during the breath hold, <math>\Delta{[CO]}</math>, and also records the time <math>\Delta{t}</math>.
== Interpretation ==
In general, a healthy individual has a value of <math>D_{L_{CO}}</math> between 75% and 125% of the average.<ref name=uppsala>LUNGFUNKTION - Practice compendium for semester 6. Department of Medical Sciences, Clinical Physiology, Academic Hospital, Uppsala, Sweden. Retrieved 2010.</ref> However, individuals vary according to age, sex, height and a variety of other parameters. For this reason, reference values have been published, based on populations of healthy subjects<ref>{{cite journal | author vauthors= Miller A, Thornton JC, Warshaw R, Anderson H, Teirstein AS, Selikoff IJ | year = 1983 | title = Single breath diffusing capacity in a representative sample of the population of Michigan, a large industrial state. Predicted values, lower limits of normal, and frequencies of abnormality by smoking history | url = | journal = Am Rev Respir Dis | volume = 127 | issue = 3| pages = 270–7 | pmid = 6830050 }}</ref><ref>{{cite journal | author vauthors= Knudson RJ, Kaltenborn WT, Knudson DE, Burrows B | year = 1987 | title = The single-breath carbon monoxide diffusing capacity. Reference equations derived from a healthy nonsmoking population and effects of hematocrit | url = | journal = Am Rev Respir Dis | volume = 135 | issue = 4| pages = 805–11 | pmid = 3565929 }}</ref><ref>{{cite journal | author vauthors= Cotes JE, Chinn DJ, Quanjer PH, Roca J, Yernault JC | year = 1993 | title = Standardization of the measurement of transfer factor (Diffusing capacity) | url = | journal = Eur Respir J Suppl | volume = 16 | issue = | pages = 41–52 | pmid = 8499053 }}</ref> as well as measurements made at altitude,<ref>{{cite journal | author vauthors= Crapo RO, Morris AH, Gardner RM | year = 1982 | title = Reference values for pulmonary tissue volume, membrane diffusing capacity, and pulmonary capillary blood volume | url = | journal = Bull Eur Physiopathol Respir | volume = 18 | issue = 6| pages = 893–9 | pmid = 6927541 }}</ref> for children<ref>{{cite journal | author vauthors= Koopman M, Zanen P, Kruitwagen CL, van der Ent CK, Arets HG | year = 2011 | title = Reference values for paediatric pulmonary function testing: The Utrecht dataset | url = | journal = Respir Med. | volume = 105 | issue = | pages = 15–23 }} Erratum in: ''Respir Med.'' 2011 105:1970-1. PMID 20889322.</ref> and some specific population groups.<ref>{{cite journal | author vauthors= Chin NK, Ng TP, Hui KP, Tan WC | date = Jun 1997 | title = Population based standards for pulmonary function in non-smoking adults in Singapore | url = | journal = Respirology | volume = 2 | issue = 2| pages = 143–9 | pmid = 9441128 | doi=10.1111/j.1440-1843.1997.tb00070.x}}</ref><ref>{{cite journal | author vauthors= Piirilä P, Seikkula T, Välimäki P | year = 2007 | title = Differences between Finnish and European reference values for pulmonary diffusing capacity | url = | journal = Int J Circumpolar Health | volume = 66 | issue = 5| pages = 449–57 | pmid = 18274210 }}</ref><ref>{{cite journal |vauthors=Ip MS, Lam WK, Lai AY, etal | year = | title = Hong Kong Thoracic Society. Reference values of diffusing capacity of non-smoking Chinese in Hong Kong | url = | journal = Respirology | volume = 12 | issue = 4| pages = 599–606 | doi = 10.1111/j.1440-1843.2007.01084.x | pmid = 17587430 | date=July 2007}}</ref>
===Blood CO levels may not be negligible===
====The two components of <math>D_{L_{CO}}</math>====
While <math>(D_L)</math> is of great practical importance, being the overall measure of gas transport, the interpretation of this measurement is complicated by the fact that it does not measure any one part of a multi-step process. So as a conceptual aid in interpreting the results of this test, the time needed to transfer CO from the air to the blood can be divided into two parts. First CO crosses the alveolar capillary membrane (represented by <math>D_M</math> ) and then CO combines with the hemoglobin in capillary red blood cells at a rate <math>\theta</math> times the volume of capillary blood present (<math>V_c</math>).<ref>{{cite journal | author vauthors= Roughton FJ, Forster RE | year = 1957 | title = Relative importance of diffusion and chemical reaction rates in determining rate of exchange of gases in the human lung, with special reference to true diffusing capacity of pulmonary membrane and volume of blood in the lung capillaries | url = | journal = J Appl Physiol | volume = 11 | issue = 2| pages = 290–302 | pmid = 13475180 }}</ref> Since the steps are in series, the conductances add as the sum of the reciprocals:
{{NumBlk|::|<math>\frac {1} {D_{L_{CO}}} =\frac {1} {D_M} + \frac {1} {\theta * V_c}</math> . | {{EquationRef|3}} }}
==Further reading==
* Mason RJ, Broaddus VC, Martin T, King T, Jr., Schraufnagel D, Murray JF, Nadel JA. (2010) Textbook of Respiratory Medicine. 5e. ISBN 978-1-4160-4710-0.
* Ruppel, G. L. (2008) Manual of Pulmonary Function Testing. 9e. ISBN 978-0-323-05212-2.
* West, J. (2011) Respiratory Physiology: The Essentials. 9e. ISBN 978-1-60913-640-6.
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