Eosinophilic Asthma Sputum Induction Test: What to Know

Medically Reviewed by Hansa D. Bhargava, MD on February 03, 2020

The best way to tell if you have eosinophilic asthma vs. another type is to check your mucus for certain kinds of cells. Sputum is a fancy word for the gunk trapped in your lungs.

But simple coughing usually won’t bring up the sputum deep inside your chest. That’s where the “induction” part of the test comes in.

How is Sputum Induced?

Your doctor will collect your sample by giving you an inhaler filled with a type of medication called fast-acting bronchodilator. The drug will relax and widen your airway. Then you’ll breathe in saline mist for 5-20 minutes to loosen up anything that’s clogging the air path.

You’ll then take turns coughing into a cup. You may need to huff hard and to clear your throat to spit out enough mucus, saliva, and cells. The mixture might be thinner and more watery than you’re used to.

It’s not uncommon if you can’t give a good sample or if you need to stop coughing. If your doctor asks you retake the test, it’s best to wait a couple of days to try again, since the test could irritate you airway.

What Happens to My Sample?

Your specimen is shipped to a laboratory for analysis, so you’ll need to wait for the results.

The technician will count the number of various types of immune cells. They include such white blood cells as eosinophils, neutrophils, macrophages, and lymphocytes.

The cells in a healthy sputum sample are usually less than 1% eosinophils. If your count is higher than that, your doctor may diagnose you with eosinophilic asthma. You also can detect eosinophils faster with blood samples. But the sputum induction test is considered the gold standard for confirming that you have this condition.

What to Ask

If your doctor has ordered a sputum induction test, you’ll want to understand how it can help you. You may want to ask your doctor:

  1. Why do I need this test?
  2. What are you looking for specifically?
  3. How long will I need to wait for the results?
  4. What does it mean if my sputum counts are high?
  5. Will I need this and other tests in the future?
  6. Are there other things I can do to help keep my asthma under control?


WebMD Medical Reference



Breathe: “Induced sputum analysis: step by step.”

European Respiratory Journal: “Sputum Induction.”

Journal of Visualized Experiments: “Methodology for Sputum Induction and Laboratory Processing.”

Journal of Asthma and Allergy: “Diagnosis and management of eosinophilic asthma: A U.S. perspective.”

National Jewish Health: “Sputum Induction."

© 2020 WebMD, LLC. All rights reserved.