QSART Test: What It Is, What to Expect, and What the Results Mean

The Quantitative Sudomotor Axon Reflex Test (QSART) is one of the most important diagnostic tests for autonomic small fiber neuropathy and is a key component of the comprehensive autonomic testing battery used to evaluate POTS, dysautonomia, and related conditions. If you have been referred for QSART testing, this guide explains exactly what to expect and how to interpret your results.

What QSART Measures

QSART measures the function of the postganglionic sympathetic sudomotor (sweat-producing) nerves — the small, unmyelinated C-fibers that innervate sweat glands. These are the same type of nerve fibers that innervate blood vessels and are damaged in neuropathic POTS and small fiber neuropathy.

The test works by applying acetylcholine (a neurotransmitter) to the skin using a small electrode. Acetylcholine stimulates the axon reflex in the sudomotor nerve: the signal travels up the nerve fiber to a branch point, then back down to the sweat gland, causing sweating. The amount of sweat produced is measured by a small capsule placed over the skin.

Reduced sweat output indicates damage to or dysfunction of the sudomotor nerve fibers — a direct measure of autonomic small fiber neuropathy.

Where Is QSART Tested?

QSART is typically measured at four standardized sites:

1. Forearm (medial surface, 10 cm proximal to the wrist)
2. Proximal leg (lateral surface of the thigh)
3. Distal leg (lateral surface of the lower leg, 10 cm proximal to the lateral malleolus)
4. Foot (dorsum of the foot)

The distal-to-proximal gradient of sweat output is diagnostically important: a length-dependent neuropathy (like diabetic neuropathy) shows the greatest reduction at the foot, while patchy autonomic neuropathy (as in autoimmune POTS) may show abnormalities at non-distal sites.

How to Prepare for QSART

Stop these medications before the test (confirm timing with your doctor):

• Anticholinergic medications (bladder medications, some antihistamines): 48 hours before
• Sympathomimetics (decongestants, stimulants): 24 hours before
• Beta-blockers: discuss with your doctor (some labs ask you to hold them)

Day of the test:

• Do not apply lotions, creams, or powders to the test sites
• Avoid caffeine on the day of the test
• Wear comfortable clothing that allows easy access to the forearm, thigh, lower leg, and foot
• Stay well-hydrated (dehydration can reduce sweat output and affect results)
• Avoid vigorous exercise for 24 hours before the test

Temperature matters: QSART results are temperature-dependent. Most labs maintain a controlled room temperature of 22–24°C. If you are tested in a cold room, sweat output may be artificially reduced. Mention if you feel cold during the test.

What Happens During the Test

The test takes approximately 45–60 minutes. You will lie on an examination table. A technician will place small capsules over each of the four test sites and secure them with tape. Each capsule contains an electrode that delivers acetylcholine through the skin (iontophoresis) for 5 minutes, then measures sweat output for an additional 5 minutes.

The acetylcholine application produces a mild tingling or burning sensation at the test site — this is normal and expected. The sensation is brief and resolves when the stimulation stops.

Interpreting QSART Results

Results are reported as the volume of sweat produced (in microliters) at each site, compared to age- and sex-matched normative values.

Normal: Sweat output within the normal range at all four sites.

Reduced (hypohidrosis): Sweat output below the lower limit of normal at one or more sites, indicating sudomotor nerve damage. The pattern of abnormality helps identify the type of neuropathy.

Absent (anhidrosis): No measurable sweat output at a site, indicating severe nerve damage or complete denervation.

Increased (hyperhidrosis): Sweat output above the upper limit of normal, which can indicate autonomic hyperreactivity (sometimes seen in hyperadrenergic POTS or early neuropathy with compensatory upregulation).

Persistent sweating (post-stimulus sweating): Sweating that continues after the acetylcholine stimulation stops, indicating autonomic instability.

What Abnormal QSART Results Mean for POTS

In the context of POTS evaluation, an abnormal QSART (reduced sweat output, particularly at the distal leg and foot) supports a diagnosis of neuropathic POTS — the subtype caused by autonomic small fiber damage. This has important treatment implications: neuropathic POTS is more likely to respond to midodrine and pyridostigmine, and if an autoimmune cause is identified, immunotherapy (IVIG) may be appropriate.

A normal QSART does not rule out dysautonomia — it simply means the sudomotor fibers are intact. Other autonomic fiber populations (cardiovascular, gastrointestinal) may still be affected.