This test involves measuring the partial pressure difference between inspired and expired carbon monoxide. It relies on the strong affinity and large absorption capacity of red blood cells for carbon monoxide and thus demonstrates gas uptake by the capillaries that are less dependent on cardiac output. The measurement of DLCO is affected by atmospheric pressure and/or altitude and correction factors can be calculated using the method recommended by the American Thoracic Society. Expected DLCO is also affected by the amount of hemoglobin, carboxyhemoglobin, age and sex. The correction for hemoglobin is based on the method of Cotes as recommended by the American Thoracic Society. Online resources exist to facilitate these corrections for altitude, hemoglobin, and carboxyhemoglobin.
DLCO vs TLCO
Generally DLCO is measured in "ml/min/kPa" and TLCO is measured in "ml/min/mmHg".
Factors affecting DLCO
DLCO is decreased in any condition which affects the effective alveolar surface area:
Amiodarone high cumulative dose; more than 400 milligrams per day
However, many modern devices compensate for the hemoglobin value of the patient (taken by blood test), and excludes it as a factor in the DLCO interpretation.
Factors that can increase the DLCO include polycythaemia, asthma (can also have normal DLCO) and increased pulmonary blood volume as occurs in exercise. Other factors are left to right intracardiac shunting, mild left heart failure (increased blood volume) and alveolar hemorrhage (increased blood available for which CO does not have to cross a barrier to enter). 
Significance of results
A DLCO of less than 60% predicted portends a poor prognosis for lung cancer resection. FEV1 is of lesser prognostic value for lung resection survival.
^Hughes J, Bates D (2003). "Historical review: the carbon monoxide diffusing capacity (DLCO) and its membrane (DM) and red cell (Theta.Vc) components". Respir Physiol Neurobiol. 138 (2–3): 115–42. doi:10.1016/j.resp.2003.08.004. PMID14609505.
^Sue, DY; Oren, A; Hansen, JE; Wasserman, K (1987). "Diffusing capacity for carbon monoxide as a predictor of gas exchange during exercise". N. Engl. J. Med. 316 (21): 1301–1306. doi:10.1056/nejm198705213162103.
^American Thoracic society, Single Breath Carbon Monoxide Diffusing Capacity (transfer factor) Recommendations for a Standard Technique- 1995 Update, Am J Respir Crit Care Med. 152 pp 2185-2198 (1995).
^J.E. Cotes, J.M. Dabbs, P.C. Elwood, A.M. Hall, A. McDonald, and M.J. Saunders. Iron-deficiency anaemia: its effects on transfer factor for the lung (diffusing capacity) and ventilation and cardiac frequency during submaximal exercise. Clin. Sci. 42:325-33 (1972).
^Diffusion lung capacity for carbon monoxide (DLCO) is an independent prognostic factor for long-term survival after curative lung resection for cancer (p n/a)
Michael J. Liptay, Sanjib Basu, Michael C. Hoaglin, Neil Freedman, L. Penfield Faber, William H. Warren, Zane T. Hammoud, Anthony W. Kim. Journal of Surgical Oncology.
Published Online: Oct 1 2009 8:20AM doi:10.1002/jso.21407