When the patients leave, it is with corrected results in hand
With a high concentration of Hb for the CO molecules to bind to, the lung diffusion capacity will be seemingly high. Herman Groepenhoff, head of the Pulmonary function department at the VU University medical center in Amsterdam in the Netherlands and his colleagues always correct for their patients’ Hb values. “We want to measure lung function and not the function of their blood,” he says.
The recommendation is to use a recent Hb value. The question is; what is recent enough? Even a value that is no more than a few weeks old may not be accurate enough.
“The Hb value may not vary much in healthy individuals, but in a clinical setting, by definition, we don’t deal with healthy people. In hematology patients, the Hb value varies quite a lot and if a patient has just had a blood transfusion, the Hb value will be very different than just a week earlier”, says Herman Groepenhoff.
Before they had the ability to perform Hb tests themselves in their own department, he admits that his colleagues and he sometimes used Hb values that were more than a month old.
“The Hb values tended to get older and older just because we wanted to avoid the hassle of doing a venous puncture and having to go to the lab to have it analyzed.”
For two years Herman Groepenhoff and his colleagues have used the HemoCue® Hb 201 system and now they check the patients’ Hb values for every diffusion test they do. According to him, it is satisfying knowing that they follow the European Respiratory Society guidelines when they correct for an updated Hb value each time. Another advantage is that it saves time. The diffusion test is repeated at least twice. During the four minute break between tests, a finger stick can be performed for determination of the Hb value, which can be entered into the computer that is used for calculation of the diffusion capacity.
“The entire test takes about 20 minutes. If we have to do a venous puncture it takes twice as long. It takes me five minutes to walk to the lab and 15 minutes to have the analysis done. Everything has to be quick in this world and it is actually a great advantage that the patient is out of the waiting room directly after the test is finished, with the corrected diffusion values in hand.”
Since Herman Groepenhoff´s department started using the Hb 201 system, they have found that for Hb values within the normal range, correction for Hb may not be that critical. For values outside the normal range though, it is. There are also quite a few other things that can interfere with the outcome of a diffusion capacity test. If the patient instead of inhaling as much as he or she can, only inhales 90 percent, or holds his or her breath for eleven seconds instead of ten, the variation is already there.
“Therefore it is a good idea to try to minimize everything that could affect the result as much as possible and the Hb value is such a thing”.
Maras-Simunic, M. et al. The effect of hemoglobin levels in the blood in pulmonary diffusing capacity. Lijec Vjesn, 1994, 116; 22-25
Sansores, R. H. et al. The effect of menstruation on the pulmonary carbon monoxide diffusing capacity. Am J Respir Crit Care Med, 1995, 152: 381-384
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