The influence of dilution on the offline measurement of exhaled nitric oxide
Victoria MacBean, Dharmika Pooranampillai, Catherine Howard, Alan Lunt, and Anne Greenough
Published in the journal Physiological Measurement, February 2018
Asthma is a serious and growing health issue. Asthma UK say that 5.4 million people in the UK receive treatment and that, on average, three people die every day from asthma. People with asthma have inflamed airways and this causes them to breathe out higher-than-normal levels of a gas called nitric oxide (NO). By measuring the amount of NO in a person’s exhaled breath (which scientists call fractional exhaled nitric oxide or “FeNO”) asthma can be diagnosed and other health conditions that affect breathing can be ruled out. FeNO testing also allows the severity of asthma to be tracked so that the patient can know if the treatments they are using are helping or not.
The standard FeNO test requires a long, steady blow into a machine. This makes it unsuitable for young children and so an alternative test is often used, which involves them breathing normally into a bag and then analysing the exhaled air separately afterwards.
The authors of this study wanted to see whether the alternative FeNO test used for young children was accurate and, if not, whether it could be made accurate by adjusting the results using a mathematical equation. The authors devised an equation that took into account the amount of air already in the testing equipment and by how much that air ought to dilute a patient’s FeNO result.
Thirty-five adult participants underwent the standard and alternative FeNO tests using different numbers of exhalations. The alternative FeNO test was also conducted as a laboratory experiment using a precisely measured sample of gas instead of air exhaled by an adult participant.
The study showed that if a participant took multiple breaths during the alternative FeNO test then the FeNO measurements were very different from those provided by the standard test. The results provided by the laboratory experiment showed that the mathematical equation could accurately predict by how much the amount of FeNO detected would be reduced if multiple breaths were used. However, the mathematical formula did not provide accurate predictions when an adult participant undertook the alternative FeNO test. The authors suggested that this might be because the samples of exhaled air were becoming contaminated by air from the participants’ noses (where NO levels are much higher).
The authors of the study concluded that a patient’s true FeNO measurements could not be obtained using the alternative FeNO test if the patient took multiple breaths. This meant that the test is not suitable for young children.
This summary was produced by Joseph Henley, Year 12 student from JFS School, Harrow, as part of our departmental educational outreach programme.