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Inflammatory Biomarkers

Asthma is an inflammatory disease of the airways.1 Inflammation is always present, even when you do not have asthma symptoms. The inflammation makes your airways sensitive (also called “hyperresponsive” or “irritable”). Sensitive airways react to allergens and irritants by becoming more narrow (“obstructed”), which limits airflow.

Eosinophils are inflammatory cells with an important role in asthma. About half of people with asthma have high eosinophils.2 People with higher levels of eosinophils tend to have more severe asthma.1 Inhaled corticosteroids are helpful for most people with high eosinophils.2

Biomarkers are measurable laboratory values that indicate something about a disease. Examples of biomarkers of inflammation for asthma include:

  • Exhaled nitric oxide, a simple breathing test to measure the amount of nitric oxide in your breath.
  • The percentage of eosinophils in the mucus (sputum) from your lungs.
  • The percentage of eosinophils in a sample of blood.

Some asthma specialists measure exhaled nitric oxide.1,4 Blood and mucus eosinophils are mainly measured for research purposes. This information is used for studies that are trying to define different types of asthma. It is useful for studies of new treatments, such as anti-IL5 medications.3 More information is needed about how to use the results of these tests in everyday practice.1

Exhaled nitric oxide

Many people with asthma have high levels of nitric oxide in their breath.4 Increased nitric oxide is linked with increased eosinophils in the airway.1 When people with high levels of nitric oxide take inhaled corticosteroids, their asthma usually gets better. For this reason, the American Thoracic Society recommends measuring FeNO, which stands for “fractional exhaled nitric oxide.”

How is FeNO tested?
The US Food and Drug Administration has approved two devices for measuring FeNO.5 The devices have a monitor and a mouthpiece. First, you inhale filtered air through the mouthpiece until your lungs are full.6 Then you exhale into mouthpiece at a constant rate for ten seconds. The test does not work if you blow air out of your lungs forcefully. The results are available in one minute. This test can be done in the office of your allergist or other health care provider.

What are the results of FeNO testing?
The results of the test are given in parts per billion (abbreviated ppb). People with a FeNO higher than 50 ppb will probably have a good response to taking inhaled corticosteroids.4 Inhaled corticosteroids do not work as well for people with a FeNO less than 25 ppb (or 20 ppb for children).

More research is about FeNO testing is needed. Researchers want to figure out how factors like age, high, smoking, and use of medications influence FeNO test results.4 Additional studies are needed of people with and without asthma to come up with normal ranges.

How are the results of FeNO testing used?
FeNO may be used is to find out if you have eosinophilic inflammation.4 This information will help your provider recommend an effective medication. It can also be used to measure changes in inflammation over time. Some—but not all—studies have shown that FeNO measurements can help your provider know whether you need more or less medication.1,7,8

FeNO cannot be used alone to diagnose asthma. In one study, 47% of patients with mild to moderate asthma did not have high levels of eosinophils, which means their FeNO probably would be low.2,4 Besides asthma, high FeNO may be a sign of bronchitis, hay fever, and other allergic diseases.9 However, if a person has other signs and symptoms of asthma, high FeNO supports the diagnosis.

Sputum (mucus) eosinophils

The number of eosinophils in mucus (sputum) from the lungs is linked with asthma severity and response to inhaled corticosteroids.1

How are mucus eosinophils tested?
The test basically involves coughing up mucus from your lungs. You inhale a salt (saline) spray, which draws water into your airways. The extra water in your airways thins out the mucus, and makes it easier to cough up.1 The test takes between five and 20 minutes.

This test is done research studies more often than in everyday practice. The test requires highly trained staff to perform. Few labs are able to process the samples.1

What are the results of mucus eosinophil testing?
The proportion of eosinophils is measured. If 3% or more of the cells in the sample are eosinophils, you have high levels of eosinophils.3

How are the results of mucus eosinophil testing used?
More research is needed about how to use the results of this test in everyday practice.1 In studies, adjusting treatment based on eosinophil levels has been helpful. Patients who were treated until their mucus eosinophil levels were normal had better control of their asthma, fewer asthma attacks, and fewer hospitalizations.1

Blood eosinophils

Blood samples are easier to collect than lung mucus samples.3 The level of eosinophils in your blood is linked to the levels in your lungs. More than 3% or 220/mm3 of blood eosinophils is considered high.

Written by: Sarah O'Brien | Last Reviewed: May 2016.
  1. National Heart, Lung, and Blood Institute. Expert panel report 3 (EPR-3): Guidelines for the diagnosis and management of asthma - Full Report 2007. Accessed 11/12/14 at:
  2. McGrath KW, Icitovic N, Boushey HA, et al; Asthma Clinical Research Network of the National Heart, Lung, and Blood Institute. A large subgroup of mild-to-moderate asthma is persistently noneosinophilic. Am J Respir Crit Care Med. 2012;185:612-619.
  3. Schleich FN, Manise M, Sele J, et al. Distribution of sputum cellular phenotype in a large asthma cohort: predicting factors for eosinophilic vs neutrophilic inflammation. BMC Pulm Med. 2013;13:11.
  4. Dweik RA, Boggs PB, Erzurum SC, et al; American Thoracic Society Committee on Interpretation of Exhaled Nitric Oxide Levels (FENO) for Clinical Applications. An official ATS clinical practice guideline: interpretation of exhaled nitric oxide levels (FENO) for clinical applications. Am J Respir Crit Care Med. 2011;184:602-615.
  5. Aerocrine AB receives US FDA market clearance for Niox Vero [press release]. November 6, 2014. Accessed 1/29/15 at:
  6. National Health and Nutrition Examination Survey. Respiratory Health ENO Procedures Manual. Accessed 1/29/15 at:
  7. Smith AD, Cowan JO, Brassett KP, Herbison GP, Taylor DR. Use of exhaled nitric oxide measurements to guide treatment in chronic asthma. N Engl J Med. 2005;352:2163-2173.
  8. Calhoun WJ, Ameredes BT, King TS, et al; Asthma Clinical Research Network of the National Heart, Lung, and Blood Institute. Comparison of physician-, biomarker-, and symptom-based strategies for adjustment of inhaled corticosteroid therapy in adults with asthma: the BASALT randomized controlled trial. JAMA. 2012;308:987-997.
  9. Global Initiative for Asthma. Global Strategy for Asthma Management and Prevention 2014. Accessed 11/12/14 at: