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Measuring FeNO – The technology behind NIOX VERO®

Fractional exhaled nitric oxide (FeNO) was discovered as a biomarker of airway inflammation in the 1990s. Ever since, FeNO measurements have proved to be a helpful tool in asthma diagnosis and management. In this article we will focus on what is important when measuring FeNO levels and how the technology behind NIOX VERO® addresses these needs.

There are various tests to help diagnose asthma, mainly looking at airflow limitation. However, results may be inconclusive as obstruction is not always present, especially in mild asthma.1 On the other hand, airway inflammation, the key characteristic of asthma, is not routinely measured and this can lead to incorrect diagnosis and suboptimal management of the disease.

FeNO testing is the most convenient way to assess airway inflammation. It is simple, immediate and non-invasive. In addition, performing FeNO testing with NIOX VERO® has been proven not to be an aerosol-generating procedure.2

In order to achieve the best patient outcomes, it is vital to ensure that the equipment used to perform FeNO testing provides accurate and reproducible results.

Different technologies to help diagnose asthma and other respiratory conditions

In the past few decades, respiratory authorities such as the European Respiratory Society (ERS) and the American Thoracic Society (ATS) have defined guidelines on how to measure and interpret FeNO levels.3,4

A General Practioner explaining how the NIOX VERO® works before a test.

First NIOX Device - NIOX FLEX - © Circassia

FeNO devices are equipped with a sensor, a piece of technology which measures the nitric oxide (NO) level in the exhaled breath of the user. Two examples of technologies that can provide these measurements are chemiluminescence and laser. Unfortunately, these are both very expensive and non-portable. They may also need frequent calibration and maintenance, which makes these technologies more suitable for research labs or clinical studies.5

NIOX® trivia: The original NIOX® device was launched in 1999 and was actually the first ever commercially available FeNO device!

In the early 2000s, the company behind the NIOX® brand evolved the technology and launched a new device with an electrochemical sensor, which allowed FeNO devices to go portable for the first time. It was rapidly shown that the NIOX® electrochemical sensor provided equivalent readings to those obtained with chemiluminescence.6

Key aspects of electrochemical devices for FeNO measurement

Devices that measure FeNO with electrochemical technology, such as NIOX VERO®, have sensors that convert the gas concentration of exhaled breath into electrical signals. Unlike chemiluminescence and laser technology, they are small devices that are easy to use at the point-of-care. They are also relatively inexpensive to buy and maintain. They usually provide results within a minute or so.

FeNO devices are non-invasive and allow for the measurement of FeNO in children. To guide the patient through the test, most devices have an interface with helpful animations.

Interpreting FeNO test results depends on the device

Many studies have shown a high degree of correlation between the measurements of different devices, although there are discrepancies between device readings, especially for higher levels of inflammation.7-9 Studies have concluded that FeNO devices are not interchangeable.5,10,11

The cut-off points to determine the degree of airway inflammation may be different across devices. The most used cut-off points for the interpretation of FeNO levels were published by the ATS in 2011.4 These thresholds were derived from studies where 83% of patients performed FeNO testing on NIOX® devices.12 Based on studies comparing NIOX® readings with other devices, it seems that NIOX® better reflects the ATS cut-off points.13 Some authors have concluded that separate cut-off points should be established for different devices.5,7

The ATS/ERS recommendations on the standardisation of the measurement of FeNO are very clear on the need to remove ambient NO from the exhaled breath, to avoid contamination by exogeneous NO.3 High levels of ambient NO can affect FeNO results.14 NIOX VERO® is the only portable device available outside China that eliminates the impact of environmental NO by using an NO scrubber in the breathing handle.15 During testing, the patient inhales through the handle and the scrubber filters out NO that is already present in the air. As the exhaled air only contains NO from the patient’s airways, the FeNO value obtained is an accurate picture of the level of inflammation in the lungs.

When exhalation is outside the standard of 50ml/s with a 5ml/s allowance, FeNO levels can vary widely and cannot be interpreted using the usual cut-off points.3,4 NIOX VERO® has a specific mechanism called Flow Rate Control to prevent this.

NIOX® FeNO testing devices

For more information about FeNO and the NIOX VERO® testing device, go to You can also contact us directly with any questions using the details below or complete the contact form:

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1. Global Initiative for Asthma (GINA). Global strategy for asthma management and prevention. 2021 update.
2. Sheikh S et al. Are aerosols generated during lung function testing in patients and healthy volunteers? Results from the AERATOR study. Thorax Published Online First: 02 November 2021. doi: 10.1136/thoraxjnl-2021-217671
3. American Thoracic Society; European Respiratory Society. ATS/ERS recommendations for standardized procedures for the online and offline measurement of exhaled lower respiratory nitric oxide and nasal nitric oxide, 2005. Am J Respir Crit Care Med. 2005;171(8):912-30.
4. 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.
5. Maniscalco M et al. Fractional exhaled nitric oxide-measuring devices: technology update. Med Devices (Auckl). 2016;9:151-60.
6. Boot JD et al. Comparison of exhaled nitric oxide measurements between NIOX MINO electrochemical and Ecomedics chemiluminescence analyzer. Respir Med. 2008;102(11):1667-71.
7. Huang T et al. Fractional exhaled nitric oxide measurement: Comparison between the Sunvou-CA2122 analyzer and the NIOX VERO analyzer. J Asthma. 2019:1-8.
8. Korn S et al. Measurement of fractional exhaled nitric oxide:comparison of three different analysers. Respiration. 2020;99(1):1-8.
9. Silkoff PE et al. Clinical precision, accuracy, number and durations of exhalations for a novel electrochemical monitor for exhaled nitric oxide. J Breath Res. 2019;14(1):016011.
10. Saito J et al. Comparison of fractional exhaled nitric oxide levels measured by different analyzers produced by different manufacturers. J Asthma. 2020;57(11):1216-26.
11. Molino A et al. Comparison of three different exhaled nitric oxide analyzers in chronic respiratory disorders. J Breath Res. 2019;13(2):021002.
12. Data on file C-NIOX-0001. © Circassia Group PLC.
13. Data on file C-NIOX-0008. © Circassia Group PLC.
14. Corradi M et al. Influence of atmospheric nitric oxide concentration on the measurement of nitric oxide in exhaled air. Thorax. 1998;53(8):673-6.
15. Data on file C-NIOX-0010. © Circassia Group PLC.