Although millions of people worldwide are diagnosed with
asthma every year, there is no single test that can diagnose the disease.
Common symptoms, such as shortness of breath, wheezing, and cough, are
relatively nonspecific, and physicians may use multiple tests and observations
to reach an accurate diagnosis. In order to assess the accuracy and reliability
of one of these tests that can be added to the clinician’s toolbox to diagnose
asthma – fractional exhaled nitric oxide (FeNO) concentration – researchers
gathered and analysed data from previous peer-reviewed studies. They conclude
that the FeNO test has moderate accuracy for patients aged 5 and older.
Their results are reported in Mayo Clinic Proceedings.

Researchers from the AHRQ-funded Evidence-based Practice
Center at Mayo Clinic identified 43 studies comprising a total of almost 14 000
adult and paediatric patients with suspected asthma who received the FeNO test.

“Asthma can sometimes be difficult to diagnose, and FeNO can
be helpful to make therapeutic decisions more evidence based,” explained lead
investigator M. Hassan Murad, MD, MPH, of the Mayo Clinic. “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.”

Investigators evaluated FeNO data from the selected studies
and categorised the results into four cut-off values, less than 20 ppb, 20 to
29 ppb, 30 to 39 ppb, and 40 ppb and over. Patients were also divided into two
age groups, those older than 18 and those aged between 5 and 18.

For patients over 18, sensitivity decreased from 0.80 at
<20 ppb, 0.69 at 20-29 ppb, 0.53 at 30-39 ppb to 0.41 at >40 ppb.
Specificity values were 0.64, 0.78, 0.85, and 0.93, respectively. Cut-off
values change the sensitivity and specificity of the FeNO test. Lower cut-offs
capture more people with disease but also cause more false positive results.

According to Dr Murad, “There is no single test that can
diagnose asthma. 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.
Future research is needed to determine how FeNO can be used with other
biomarkers with hopefully better accuracy that can provide a more definitive


 Reference:  Murad MH, et al. The Diagnostic Accuracy of
Fractional Exhaled Nitric Oxide Testing in Asthma: A Systematic Review and
Meta-analyse. Mayo Clinic Proceedings, volume 93, issue 2, February 2018.