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The importance of accuracy and reproducibility in FeNO testing for asthma.

FeNO (fractional exhaled nitric oxide) testing has become a common complement to traditional asthma diagnostics such as spirometry. Considering that several factors may cause variation in FeNO levels, how reliable are such tests? Let’s look at recent reproducibility studies and what they tell us regarding the accuracy of FeNO testing.

The guidelines

Several global asthma organisations now recommend FeNO testing to support asthma management.1,2,3

GINA 2018 recommends FeNO testing in guiding treatment in severe asthmatics1.

NICE 2017 (NG80) recommends FeNO testing as a first line pulmonary function test in diagnosing adults with suspected asthma2. This was based on cost analysis (NICE DG12 in 2014) and the evidence base4.

The American Thoracic Society (ATS) even has separate guidelines dedicated to FeNO testing and how the results should be interpreted.3 Their recommendations say that FeNO greater than 50 ppb (35 ppb in children) can be used to indicate that eosinophilic inflammation is likely. Conversely, they recommend that FeNO lower than 25 ppb (20 ppb in children) indicates that eosinophilic inflammation is less likely.

Most of the studies used to create the ATS guidelines were carried out using NIOX® FeNO devices. Below we will look into results from recent studies with a focus on NIOX®.

Factors that influence FeNO levels

An important part of FeNO testing is to contribute to more confident and accurate asthma diagnoses and treatment decisions. However, it has been shown that several factors may affect a person’s FeNO levels, including smoking habits, diet, medication used, and ambient NO levels.5

NIOX VERO testing a Child’s FeNO with Mother and Doctor present.

NIOX VERO testing a Child’s FeNO with Mother and Doctor present.

The impact of ambient NO levels can be eliminated by using a FeNO device which eliminates ambient NO in the environment. The NIOX VERO® has a scrubber in the breathing handle for this purpose which plays a significant part in the accuracy of the test results.

Demographic and biologic factors on the other hand cannot be eliminated prior to testing. With this in mind, how reliable is a FeNO test? One way to determine the accuracy of FeNO testing devices is through reproducibility studies. Over 600 studies have been carried out in the last few years – let’s look at the results of some of them.

Reproducibility testing of NIOX® testing devices

In 2016, a comprehensive systematic analysis with 4,518 participants investigated the diagnostic accuracy of FeNO measurement. The study concluded that FeNO testing appears to give results of fair accuracy for asthma diagnosis, in particular for ruling in as opposed to ruling out asthma. The most frequently used devices for measuring FeNO were NIOX MINO® (12 studies) and NIOX FLEX® (5 studies).6

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Randomized, controlled studies have also investigated the reproducibility of NIOX VERO® against its predecessor, NIOX MINO®, across the spectrum of expected FeNO values for patients with diagnosed and symptomatic asthma. Based on the clinically acceptable agreement between paired FeNO readings taken with both devices, it was demonstrated that the portable instrument NIOX VERO® is clinically equivalent to NIOX MINO® when used in an asthma population.7


Considering the factors that may influence FeNO levels, accuracy and the ability to reproduce the same result are vital to optimise asthma treatment decisions. According to the studies discussed above, FeNO testing can yield results with fair, good or excellent accuracy and repeatability.

Several asthma organisations with global reach include FeNO testing in their recommendations to improve overall asthma diagnosis and management. ATS has even created a separate set of guidelines on how to approach and interpret FeNO testing results. The most common device used in the studies they refer to is a NIOX® device, which comes with many advantages; such as a breathing handle that filters out ambient NO levels, proven accuracy and reproducibility of results, and sensor technology that detects small differences across a large range (5 – 300ppb)7.

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1. Global Initiative for Asthma (GINA). Global strategy for asthma management and prevention, 2018 Update. 2018.
2. National Institute for Health and Clinical Excellence (NICE). Asthma: diagnosis, monitoring and chronic asthma management. NICE guideline [NG80]. 2017.
3. Dweik RA et al. An official ATS clinical practice guideline: interpretation of exhaled nitric oxide levels (FENO) for clinical applications. Am J Respir Crit Care Med. 2011;184(5):602-15.
4. National Institute for Health and Clinical Excellence (NICE). Measuring fractional exhaled nitric oxide concentration in asthma: NIOX MINO, NIOX VERO and NObreath. Diagnostics guidance [DG12]. 2014.
5. Abba AA. Exhaled nitric oxide in diagnosis and management of respiratory diseases. Ann Thorac Med. 2009;4(4):173-81.
6. Karrasch S et al. Accuracy of FENO for diagnosing asthma: a systematic review. Thorax. 2017;72(2):109-116.
7. Alving K et al. Validation of a new portable exhaled nitric oxide analyzer, NIOX VERO®: randomized studies in asthma. Pulm Ther. 2017;3(1):207-218.