FeNO test

FeNo stands for fractional concentration of exhaled nitric oxide. This test measures nitric oxide when you breathe out. Nitric oxide is released by white blood cells, called eosinophils. Eosinophils are part of a complex immune system response called inflammation.

Typically, inflammation helps fight disease. But with asthma, long-term inflammation is the underlying condition that leads to swelling, tightened muscles and excess fluids. These reactions narrow airways in the lungs and make it hard to breathe.

For a FeNO test, you breathe into a tube connected to a machine that measures the amount of nitric oxide gas in your breath. A high nitric oxide level is a sign of inflammation in the airways.