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Interpreting changes in FeNO results for better outcomes

FeNO is a great asthma management tool and, to optimise treatment, it’s helpful to understand the clinical context for each patient so they can reach the right FeNO level for them. The American Thoracic Society (ATS) defined variations in FeNO value that help monitor patients over time. This article looks at how to interpret those changes so that everyone can achieve better outcomes when it comes to asthma control.

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Usually, the change in a patient’s FeNO value after treatment is of more help to physicians than the person’s absolute FeNO value. This is because the direction in which the FeNO level is moving can reveal how well treatment is working so physicians can optimize steroid dosage and understand patient behaviours. Watching the changes and sharing results with patients can also be a way to open up discussion and motivate them to get involved in managing their condition.

How does FeNO monitoring work?

The ATS strongly recommends using FeNO to monitor airway inflammation in patients with asthma.1 For patients with a FeNO value >50 ppb, an increase of at least 20% between check-ups is considered significant. For patients whose FeNO was last measured at <50 ppb, an increase of 10 ppb or more is significant and could indicate a loss of control. Most FeNO measurement devices give results at the point-of-care so clinicians can take immediate action.

In patients for whom FeNO is decreasing between visits, the ATS suggests a reduction of at least 20% to be significant in those whose FeNO was >50 ppb and a decrease of more than 10 ppb for patients whose FeNO was previously <50 ppb. These changes indicate a significant response to treatment.

FeNO monitoring is also useful for patients whose asthma is thought to be under control. For example, in asymptomatic patients with stable asthma, the chance of relapse if inhaled corticosteroid (ICS) therapy is withdrawn is highest in patients whose FeNO increases to above 49 ppb during the four weeks after withdrawal.1 Spotting these signs is a simple way to reduce the risk of exacerbation but follow-up and repeat FeNO measurements are vital.

Achieving a personal best

Serial FeNO measurements obtained when a patient’s asthma is both stable and unstable allow each patient to act as their own control for the assessment of future measurements. Working from a patient’s individual baseline FeNO like this also contributes to the goal of the new era of personalized medicine.

The ATS says using a “personal best” can be more helpful than aiming for a standardized “normal” to see how treatment is working. When FeNO is persistently high, knowing a patient’s personal baseline can uncover non-adherence, poor inhaler technique or continued exposure to an allergen. In some cases, allergens can cause FeNO levels to increase beyond 50 ppb – even in those with no respiratory symptoms.1

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Changes in FeNO levels between appointments might also be particularly useful in understanding asthma patients in whom additional factors are contributing to symptoms, such as obesity or anxiety.

Are results always comparable?

The ATS points out that FeNO values can be affected by several factors, including measurement technique, exhalation flow rate, nasal nitric oxide (NO) contamination and the NO analyzer used. It’s therefore important to use a FeNO measurement device that will produce results that are truly robust and genuinely comparable between every visit.

NIOX VERO®, the gold standard FeNO device, ensures reliability and repeatability of results. It does this by eliminating the impact of environmental NO using an NO scrubber in the breathing handle. In addition, when exhalation is outside the standard of 50ml/s with a 5ml/s allowance, FeNO levels can vary widely and cannot be reasonably interpreted using the usual cut points. NIOX VERO® has a specific mechanism called Flow Rate Control™ to prevent this, as recommended by the ATS/European Respiratory Society.2 Further, NIOX VERO® uses sensor technology that detects small differences across a large range (5-300 ppb).

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1. 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.
2. ATS/ERS recommendations for standardized procedures for the online and offline measurement of exhaled lower respiratory nitric oxide and nasal nitric oxide. Am J Respir Crit Care Med. 2005;171(8):912-30.