Sniff Test (Chest Fluoroscopy)

What is a sniff test?

A sniff test is an imaging test that checks how well your diaphragm is working. Your diaphragm is a large muscle that allows you to breathe. A provider takes X-ray images of your chest while you breathe in and out. The images can show whether you have diaphragm weakness or paralysis.

A sniff test is also called chest fluoroscopy or diaphragm fluoroscopy.

When do you need a sniff test?

Your healthcare provider might perform a sniff test if you have a condition that could affect the nerves that control your breathing. These include:

• Amyotrophic lateral sclerosis (ALS).
• Chronic inflammatory demyelinating polyneuropathy (CIPD).
• Dermatomyositis.
• Guillain–Barré syndrome.
• Lambert-Eaton syndrome.
• Multiple sclerosis (MS).
• Muscular dystrophy.
• Myasthenia gravis.
• Pompe disease.
• Post-polio syndrome.
• Spinal cord injury.

A provider might also use a sniff test to diagnose issues with your diaphragm if you have symptoms like unexplained shortness of breath (dyspnea) or to evaluate the motion of your diaphragm before or after surgery.

How does a sniff test work?

A provider notes how your diaphragm is moving on images from an X-ray camera while you breathe in and out for a sniff test. They may ask you to:

• Relax and breathe normally.
• Inhale deeply.
• Exhale completely.
• Inhale quickly through your nose (sniff).

If your diaphragm is working as expected, it will pull down towards your belly button and flatten as you breathe in. It’ll pull upward into an arch when you breathe out. If your diaphragm is arched upward (elevated) when you breathe in, there might be something wrong with your diaphragm or the phrenic nerve, which makes your diaphragm work.

How do I prepare for a sniff test?

There’s nothing specific you need to do to prepare for a sniff test.

What can I expect during a sniff test?

During a sniff test (chest fluoroscopy):

1. A provider will help you get into position, standing in front of the X-ray camera. If you aren’t able to stand, you can sit or they’ll have you lie on a table at an incline. Straps across your arms and legs secure you, and your feet have a place to rest so you won’t slide down.
2. Instruct you to take various types of breaths (for instance, breathing normally, deeply inhaling or exhaling, or breathing in quickly). Follow their directions as closely as possible.
3. Take X-ray images throughout the test.

How long does a sniff test take?

A sniff test takes around 15 minutes.

Does a sniff test have risks or side effects?

There aren’t any side effects of a sniff test. X-rays expose you to a very low dose of radiation. Your provider weighs the benefits of getting the test against the risk of this very small radiation exposure.

What is a normal sniff test report?

Sniff test results are either positive or negative. A positive sniff test means that the images showed that your diaphragm was elevated when you inhaled, which could mean that at least part of your diaphragm is paralyzed. A normal, or negative, sniff test report shows that your diaphragm is moving as it should.

When should I know the results of a sniff test?

Your provider might be able to tell you the results of a sniff test right away. Or a radiologist may interpret your results and send them to your provider, which could take a few days.

If the results are abnormal what are the next steps?

If a sniff test is positive, your provider will talk to you about what that means for your specific situation. Sometimes, sniff test results have false positives (where your diaphragm was elevated during the sniff test but the nerves aren’t paralyzed) or false negatives (where your diaphragm appears to be working properly but there’s something wrong with your nerves). Your provider will use the results of your sniff test and other information — like other test results and any symptoms you have — to determine next steps.

When should I call my doctor?

Contact your healthcare provider if you have any questions about the test or its results.