A sniff test is an imaging test that checks whether your diaphragm is working properly. A provider takes real-time X-ray images of your chest while you breathe in, breathe out, and sniff sharply through your nose. The images reveal whether the muscle and nerves responsible for breathing are functioning normally or showing signs of weakness or paralysis. The test is also called chest fluoroscopy or diaphragm fluoroscopy, and appointments typically last about 30 minutes.
The term “sniff test” can also refer to standardized smell tests used to evaluate your sense of smell, or to the “whiff test” used in gynecological exams. This article covers all three, starting with the most common meaning.
How the Diaphragm Sniff Test Works
Your diaphragm is a dome-shaped muscle that sits beneath your lungs. When you breathe in, it contracts and moves downward, pulling air into your lungs. When you breathe out, it relaxes and rises back up. Both halves of the diaphragm (left and right) should move in sync.
During a sniff test, you stand in front of a fluoroscopy machine, which captures continuous X-ray images in real time, almost like a video. You’ll be asked to breathe normally, take deep breaths, and then sniff quickly and forcefully through your nose. The sniffing motion is key because it creates a rapid, sharp contraction of the diaphragm that makes any abnormal movement easier to spot.
You’ll wear a hospital gown and need to remove jewelry, watches, eyeglasses, and other metal objects. No fasting or special preparation is required.
What the Results Mean
In a normal result, both sides of the diaphragm move downward when you inhale and sniff. If one side moves upward while the other moves downward, that opposite motion is called paradoxical movement. This is the hallmark of a positive sniff test and strongly suggests that one half of the diaphragm is paralyzed.
The sniffing portion of the exam is especially useful because it exaggerates any paradoxical movement, making even subtle dysfunction more obvious. To qualify as truly paradoxical, the affected side must move in the completely opposite direction from the healthy side, a full 180-degree phase difference in motion.
A positive result typically points to a problem with the phrenic nerve, which is the nerve that signals your diaphragm to contract. Phrenic nerve damage can result from neurological disease, surgical injury, tumors pressing on the nerve, or conditions that affect nerve function more broadly. Your provider may order this test if you’re experiencing unexplained shortness of breath, difficulty breathing when lying flat, or if a routine chest X-ray shows one side of the diaphragm sitting higher than expected.
Fluoroscopy vs. Ultrasound
Fluoroscopy has long been considered the gold standard for diagnosing one-sided diaphragm paralysis. More recently, ultrasound has emerged as a viable alternative. Ultrasound can visualize diaphragm movement without any radiation exposure, and it’s more portable and accessible, meaning it can be done at the bedside rather than requiring a trip to the radiology suite.
Research comparing the two methods has found that ultrasound measurements of diaphragm movement correlate well with fluoroscopy and can serve as the primary evaluation tool. In practice, many providers now use ultrasound first and reserve fluoroscopy for cases where results are unclear.
Smell-Based Sniff Tests
In a completely different context, “sniff test” can refer to standardized tests that measure your ability to detect and identify odors. These are used to diagnose conditions like anosmia (complete loss of smell) or hyposmia (reduced smell), which can be early signs of neurological conditions or can follow head injuries, sinus disease, or viral infections.
Several standardized versions exist. The University of Pennsylvania Smell Identification Test (UPSIT) is among the most widely known. It uses scratch-and-sniff cards containing microencapsulated odors, and you choose from a list of options for each one. The Connecticut Chemosensory Clinical Research Center (CCCRC) test takes a two-part approach: first measuring how faint a concentration of a chemical you can detect, then testing whether you can identify common household smells like peanut butter, coffee, cinnamon, chocolate, soap, and baby powder. Each nostril is tested separately, and the scores are averaged. Another common option, the Sniffin’ Sticks Test, uses felt-tip pen-like devices that release odors when uncapped.
These smell tests are simple, painless, and take only a few minutes. They’re often used by neurologists evaluating early cognitive decline, since reduced smell ability can precede other symptoms by years in certain conditions.
The Whiff Test for Vaginal Infections
The “whiff test” (sometimes casually called a sniff test) is a quick diagnostic tool used during gynecological exams to help identify bacterial vaginosis. A provider mixes a drop of potassium hydroxide with a small sample of vaginal discharge. If the mixture produces a distinct fishy odor, the test is positive. The smell comes from certain chemical compounds released by the bacteria responsible for the infection when exposed to an alkaline solution.
A positive whiff test alone doesn’t confirm bacterial vaginosis. Diagnosis requires meeting at least three of four criteria: the characteristic fishy odor on the whiff test, a thin and homogeneous discharge, the presence of “clue cells” (vaginal cells coated in bacteria visible under a microscope), and a vaginal pH above 4.5.