A positive (abnormal) QSART test means the small nerve fibers that control your sweat glands aren’t working properly. Specifically, it shows that when those nerves are stimulated, one or more test sites on your body produce too little sweat, or the sweat response is delayed or absent. This points to damage in the thin, unmyelinated nerve fibers that regulate sweating, and it’s one of the key tools used to confirm conditions like small fiber neuropathy and autonomic neuropathy.
What the Test Actually Measures
QSART stands for Quantitative Sudomotor Axon Reflex Test. It evaluates a very specific part of your nervous system: the postganglionic sympathetic nerves that tell your sweat glands to activate. During the test, a small amount of acetylcholine (a chemical your nerves naturally use to signal sweat glands) is delivered into the skin using a mild electrical current at four sites: the forearm, proximal leg (near the knee), distal leg (near the ankle), and foot.
The test then measures how much sweat each site produces over about five minutes, along with how quickly the sweat response begins. A normal result means the nerves successfully relay the signal and the sweat glands respond within a predictable range. A positive result means the sweat volume falls below the expected minimum at one or more sites, or the response is sluggish or absent. The whole procedure takes roughly 45 minutes to an hour.
How Results Are Graded
Each test site has its own normal sweat volume threshold, adjusted for your age and sex. For example, the forearm typically needs to produce at least 0.3 microliters, while the foot needs at least 0.27 microliters. Results below these minimums are flagged as abnormal.
Doctors grade the severity on a scale:
- Mild (score of 1): A single site is abnormal but still produces at least 50% of the lower limit, or there’s a “length-dependent pattern” where sweat output drops off progressively from upper to lower body.
- Moderate (score of 2): A single site falls below 50% of the lower limit, or two or more sites are reduced but still above 50%.
- Severe (score of 3): Two or more sites fall below 50% of the lower limit.
The pattern of which sites are abnormal matters as much as the numbers themselves. A length-dependent pattern, where the foot is worst and the forearm is relatively spared, suggests nerve damage that starts at the longest nerve fibers and works its way up. This is the classic pattern seen in diabetic neuropathy. A patchy pattern, where abnormal sites are scattered rather than following a clear bottom-to-top gradient, suggests a different process.
Conditions Linked to Abnormal Results
The most common reason for a positive QSART is small fiber neuropathy. This is damage to the thinnest nerve fibers in your body, the ones responsible for sweating, temperature sensation, and pain perception. People with small fiber neuropathy often experience burning pain in the feet, unusual sensitivity to temperature, or episodes of excessive or absent sweating. QSART serves as a confirmatory test because it directly measures whether those small autonomic fibers are functioning.
Positive results also appear frequently in postural orthostatic tachycardia syndrome (POTS). In one study of 30 women with POTS, 56% had abnormal QSART results. The foot and proximal leg were the sites most likely to be affected, while the forearm was rarely abnormal. Notably, the pattern in POTS tends to be patchy rather than length-dependent, and patients rarely show complete loss of sweating. This distinguishes POTS-related nerve involvement from the progressive “dying-back” pattern of diabetic neuropathy.
Other conditions that can produce a positive QSART include autonomic neuropathies from autoimmune disorders, pure autonomic failure, and neuropathy related to prediabetes or impaired glucose tolerance.
QSART vs. Skin Biopsy
If you’ve been tested for small fiber neuropathy, you may have also had (or been offered) a skin biopsy. These two tests measure different things. QSART evaluates how well the autonomic nerve fibers function by testing sweat output. A skin biopsy counts the actual number of nerve fiber endings in a small sample of skin.
The two tests don’t always agree. In a study of 51 patients with suspected neuropathic POTS who underwent both tests, results were poorly correlated. This likely reflects the fact that each test captures a different subset of nerve damage. Someone with reduced sweat output may have selective damage to autonomic fibers without much sensory nerve loss, and vice versa. Because of this, having one test come back normal doesn’t necessarily rule out the problem if the other is abnormal.
What Can Cause a False Positive
A number of common medications suppress sweating and can make QSART results look abnormal even when the nerves are healthy. This is why your testing center will ask you to stop certain drugs well before the test. The categories most likely to interfere include:
- Antidepressants and ADHD medications: SSRIs, SNRIs, and tricyclic antidepressants generally need to be held for 5 to 14 days. Stimulant medications like methylphenidate require 3 to 5 days off.
- Antihistamines: Over-the-counter allergy medications like diphenhydramine (Benadryl), loratadine (Claritin), and cetirizine (Zyrtec) need to be stopped 5 to 7 days prior.
- Anticholinergic medications: Drugs used for overactive bladder, motion sickness, or certain GI conditions directly block the chemical signal QSART relies on. These typically need 3 to 7 days off.
If you were still taking any of these medications at the time of testing, your results may not be reliable, and your doctor may want to repeat the test after a proper washout period.
Limitations to Keep in Mind
QSART is good at confirming autonomic small fiber involvement, but it doesn’t catch everything. Its sensitivity for small fiber neuropathy is moderate, roughly in the range of 50 to 70%, depending on the specific device and protocol used. That means a normal QSART doesn’t rule out small fiber neuropathy. Some people with genuine nerve damage will still produce enough sweat to pass the test.
Results also vary with age, sex, and body site. Normative values are adjusted for these factors, but individual variation still exists. The test is most useful as one piece of a larger diagnostic picture that may include skin biopsy, sensory testing, and a thorough clinical evaluation of your symptoms.