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DLCO Adjustment for Carboxyhemoglobin Concentration


Diffusing Capacity of the Lung for CO - Carbon Monoxide

DLCO (Diffusing Capacity of the Lung for Carbon Monoxide) is an indication of the rate of transfer of CO in the air of the lung into the  pulmonary capillary blood.
In Europe it is commonly referred to as TLCO (Transfer factor of the Lung for Carbon Monoxide). This transfer process requires several steps including:
  1. Bulk flow delivery of CO to the airways and alveolar spaces;
  2. Mixing and diffusion of CO in the alveolar ducts, air sacs and alveoli
  3. Transfer of CO across the gaseous to liquid interface of the alveolar membrane
  4. Mixing and diffusion of CO in the lung parenchyma and alveolar capillary plasma
  5. Diffusion across the red cell membrane and within the interior of the red blood cell and
  6. Chemical reaction with constituents of blood Hemoglobin (Hb)
The most common methodology to measure carbon monoxide (CO) uptake is with the single breath technique, although there are several other techniques that can be used (e.g., steady-state, intra-breath and rebreathing techniques). Equipment needed to measure DLCO is varies widely in complexity, but the basic principles are the same. All systems have a source of test gas (bag-in-box, spirometer, compressed gas cylinder), a method for measuring inspired and expired volume over time (spirometers with kymographs, pneumotachometers near the mouthpiece or near a bag-in-box), and gas analysers (single-sample analysers or continuous high-speed analysers).

A number of pathological states can affect DLCO (e.g., anemia, pumonary emboli, emphysema, Interstitial lung disease, etc.). Measurement of DLCO is indicated when any of these pathological processes are suspected or need to be ruled out. Moreover, measuring changes in DLCO over time in these processes is a useful way of following the course of disease.3

See an online calculator for DLCO predicted normal values

Units of measurement

The European Respiratory Society (ERS) recommends expressing DLCO in the SI units (mmol/(min kPa), while the ATS prefers the traditional units mL (standard temperature, pressure and dry (STPD))/(min·mmHg). Care must be taken to ensure the same set of units is used throughout all calculations. Values in SI units should be multiplied by 2.987 to obtain values in traditional units.


Adjustments to the measurement of DLCO prior to interpretation

DLCO depends upon a number of physiological factors. Besides varying with age, sex, height and possibly race, DLCO also changes with hemoglobin, lung volume, carboxyhaemoglobin (COHb), PIO2 - inspired oxygen partial pressure (e.g. altitude), exercise and body position. Although these effects may cause changes in DLCO in opposite directions  all should be considered in interpreting the observed CO uptake. Moreover, specific adjustments for three of these factors (Hb, COHb and PIO2) should always be made to ensure appropriate interpretation. The formula for the fully corrected DLCO is the multiple of the correction factors for Hb, COHb, and Pl, and altittde.

See an online calculator to correct for hemoglobin

See an online calculator to correct for altitude (PIO2).


Adjustment of DLCO due to Carboxyhemoglobin


The routine DLCO computation assumes that caron monoxide pressure in the blood is zero. However cigarette smoke as well as other environmental sources can produce measureable levels of CO backpressure. and carboxyhemoglobin (COHb). Carboxyhemoglobin is also endogenously produced as a byproduct of hemoglobin catabolism. Together they contribute to measured COHb levels of 1 to 2%, and this correction is already includeds in reference values for healthy non-smokers. Small increase in COHb also occur when CO is inspired during the DLCO test. Estimates from several studies showed that DLCO will decrease by 1% for every 1% increase in CoHb.4

See an online calculator for carboxyhemoglobin COHb predicted normal values


Carboxyhemoglobin concentrations can be easily measured using CO oximeters, and this will allow for estimating the impact on DLCO via the following formula:

DLCO adjusted for COHb = DCLOmeasured x (1+ (COHb/100)

where COHb is a percentage.

The online calculator here will calculate the DLCO adjusted for COHb based on the inputs of DLCO measured, and COHb percent via the formulas above.
These calculations are based on the method of Cotes1,2 as recommended by the American Thoracic Society.



CALCULATOR INPUT
DLCO adjustment for Carboxyhemoglobin (COHb)


Required inputs:

DLCO measured

-DLCO units

Percent COHb measured
%






References:
1.
J.E. Cotes 1993, Lung Function, 5th Edition., Blackwell Scientific Publications, London
2.
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).
3.
Macintyre N, Crapo RO, Viegi G et al. (October 2005). "Standardisation of the single-breath determination of carbon monoxide uptake in the lung". Eur. Respir. J. 26 (4): 720–35.
4.
American Thoracic society, Single Breath Carbon Monoxide Diffusing Capacity (transfer factor) Recomendations for a Standard Technique- 1995 Update, Am J Respir Crit Care Med. 152 pp 2185-2198 (1995).


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