The Diffusing Capacity of the Lung for Carbon Monoxide (DLCO) test measures how effectively the lungs transfer gas from the air sacs into the bloodstream. This measurement reflects the efficiency of the membrane separating the air in the lungs from the blood in the capillaries. However, the raw DLCO result is easily skewed by the amount of oxygen-carrying protein in the blood, known as hemoglobin. To accurately reflect the true health of the lung tissue, the result must be mathematically adjusted, or corrected, based on the patient’s individual hemoglobin level. This correction ensures that a diagnosis is based on the lung’s function, not the blood’s capacity to carry gas.
Measuring Gas Exchange Efficiency
The DLCO test indirectly assesses the lungs’ ability to transfer gas to the blood. It specifically measures the uptake of a small, non-toxic amount of carbon monoxide (CO) from the air sacs (alveoli) into the pulmonary capillaries. CO is used because it has a very high affinity for hemoglobin, binding 200 to 250 times more readily than oxygen does.
The test involves the patient inhaling a gas mixture containing a tracer gas and a tiny concentration of CO, holding their breath for about ten seconds, and then exhaling. By comparing the concentration of CO inhaled with the amount exhaled, the test calculates how much CO was absorbed into the bloodstream. This absorbed volume is used to derive the raw DLCO value, which reflects the efficiency of the alveolar-capillary membrane.
The Role of Hemoglobin in DLCO
Hemoglobin is the protein within red blood cells responsible for binding and transporting gas throughout the body. The measured DLCO value depends directly on this protein because CO gas must bind to it to be cleared from the lungs. Therefore, the measured DLCO reflects two components: the physical ability of the gas to cross the membrane, and the blood’s capacity to accept and carry the gas away.
If a person has anemia (a lower-than-normal concentration of hemoglobin), the raw DLCO result will be artificially low. This happens because fewer hemoglobin molecules are available to quickly bind the inhaled CO, making the lungs appear less efficient, even if the lung tissue is healthy. Conversely, polycythemia (an abnormally high concentration of hemoglobin) makes the raw DLCO result seem artificially high. This overestimation occurs because the greater number of hemoglobin molecules rapidly clears the CO. The correction isolates the true function of the lung tissue from the confounding influence of the blood’s carrying capacity.
How the Hemoglobin Correction is Calculated
The hemoglobin correction adjusts the raw DLCO score to reflect what it would be if the patient had a standardized, normal hemoglobin level. To perform this adjustment, the patient’s actual hemoglobin concentration is measured via a blood sample. This measured value is compared against a standard reference value, typically \(14.6\) grams per deciliter for men and \(13.4\) grams per deciliter for women.
A mathematical formula, often developed by Cotes and colleagues, is applied to the raw DLCO result using the patient’s measured hemoglobin level. If the patient’s hemoglobin is below the standard reference, the formula increases the raw DLCO value. This upward adjustment compensates for the lack of red blood cells, estimating the gas transfer rate for a person with healthy blood. If the patient’s hemoglobin is high, the formula decreases the raw DLCO value, accounting for the increased gas uptake capacity. The resulting figure is the corrected DLCO, which provides a more accurate measure of the lung’s physical diffusing capacity.
What Corrected DLCO Results Reveal
The final, corrected DLCO result is a diagnostic tool because it isolates true lung impairment from blood disorders. A low corrected DLCO suggests a genuine physical problem in the lungs that impairs gas transfer across the alveolar-capillary membrane. This impairment can be caused by conditions that thicken the membrane, such as pulmonary fibrosis, or those that destroy the surface area, like emphysema.
A low corrected DLCO can also point to issues in the pulmonary blood vessels, such as pulmonary hypertension, which reduces the amount of blood available to pick up the CO gas. Conversely, a higher-than-predicted corrected DLCO can signal conditions that increase the pulmonary capillary blood volume, such as a left-to-right cardiac shunt or acute pulmonary hemorrhage. Physicians rely on this corrected value to accurately diagnose and monitor lung diseases, ensuring treatment decisions are based on the health of the respiratory system.