Fractional exhaled nitric oxide concentration is a helpful tool that aids in asthma diagnosis. But it's not the only piece of the diagnostic puzzle.
Various tests-ie, bronchodilator response and positive results on bronchial challenge-are used to diagnose asthma. However, the diagnosis remains clinical, based on compatible symptoms and evidence of reversible airway obstruction. No single standard diagnostic test exists.
Fractional exhaled nitric oxide (FeNO) concentration has recently been added to the list of tests that may be used to diagnose asthma.
How accurate is FeNO?
To find out, researchers from the Mayo Clinic conducted a systematic review and meta-analysis of adult and pediatric patients with suspected asthma who received the FeNO test. The review included 43 studies with a total of 13,747 patients.
The researchers, who published their results in Mayo Clinic Proceedings, found that FeNO concentration has moderate accuracy for the diagnosis of asthma in patients aged 5 and older.
In adults, using FeNO cutoffs of less than 20, 20 to 29, 30 to 39, and 40 or more parts per billion, FeNO testing had sensitivities of 0.80, 0.69, 0.53, and 0.41, respectively, and specificities of 0.64, 0.78, 0.85, and 0.93, respectively.
In children, using FeNO cutoffs of less than 20 and 20 to 29 parts per billion, FeNO testing had sensitivities of 0.78 and 0.61, respectively, and specificities of 0.79 and 0.89, respectively.
Depending on the FeNO cutoff, the post-test odds of having asthma with a positive FeNO test result increased by 2.8-fold to 7-fold. Diagnostic accuracy was modestly better in corticosteroid-naive asthmatics, children, and nonsmokers than in the overall population.
“Asthma can sometimes be difficult to diagnose, and FeNO can be helpful to make therapeutic decisions more evidence-based,” said lead author M. Hassan Murad, MD, MPH, of the Mayo Clinic in Rochester, MN. “ In addition to a patient's history, the initial test is usually spirometry with an assessment of bronchodilator response. If this test does not confirm the diagnosis, but the index of suspicion for asthma is still high, measurement of FeNO may be helpful to rule in disease; although will still miss some patients with asthma.”
The researchers provide an example of a patient who has compatible symptoms and is clearly atopic. Elevated FeNO implies a diagnosis of asthma and that treatment with inhaled corticosteroids is indicated. Conversely, low FeNO implies compatible symptoms are not likely due to asthma.
The caveat is that low FeNO findings do not exclude asthma. “Such a scenario is common in pediatric practice,” the researchers stated, likely because most asthma in this population is atopic. “It is precisely in this population that overdiagnosis of asthma is a bigger clinical problem than underdiagnosis. In this case, patients with suspected asthma should have further testing to confirm the diagnosis,” the researchers stated.
“FeNO may be a helpful tool that aids in diagnosis. However, patients’ history, physical exam, response to treatment and other pulmonary function tests remain needed to complete the puzzle and make the diagnosis,” said Murad.
Future research is needed to determine how or whether FeNO can be used with other biomarkers with better accuracy to yield a more definitive diagnosis, he said.