Autonomic Testing: Every Test Your Doctor Might Order and What They Measure

Getting a dysautonomia diagnosis often requires a battery of specialized tests that most patients have never heard of before. The autonomic nervous system controls heart rate, blood pressure, sweating, digestion, bladder function, and dozens of other involuntary processes — and testing it requires equally specialized tools. This guide covers every major autonomic test you might encounter, what each one measures, how to prepare, and what the results mean.

The Tilt Table Test (TTT)

The tilt table test is the gold standard for diagnosing POTS and other forms of orthostatic intolerance. The patient lies flat on a motorized table, is secured with straps, and is tilted to 70–80 degrees upright over 10–15 seconds. Heart rate and blood pressure are monitored continuously for 45 minutes.

What it diagnoses:

• POTS: heart rate increase ≥ 30 bpm (or ≥ 40 bpm under age 19) within 10 minutes of tilting, without significant hypotension
• Orthostatic hypotension: blood pressure drop ≥ 20/10 mmHg within 3 minutes
• Vasovagal syncope: sudden drop in heart rate and blood pressure with near-syncope or syncope
• Initial orthostatic hypotension: transient blood pressure drop in the first 15–30 seconds

Preparation: Hold beta-blockers, fludrocortisone, midodrine, and other cardiovascular medications for 48–72 hours (confirm with your doctor). Fast for 4 hours. Hydrate normally.

The NASA Lean Test (At-Home Orthostatic Test)

The NASA Lean Test is a simplified orthostatic test that can be performed at home without specialized equipment. The patient lies flat for 10 minutes, then stands with their back against a wall (leaning at about 75 degrees) for 10 minutes. Heart rate is measured with a pulse oximeter or smartwatch at 1-minute intervals.

What it diagnoses: POTS (using the same ≥ 30 bpm criterion as the tilt table test). Studies have shown the NASA Lean Test has reasonable sensitivity and specificity for POTS compared to the formal tilt table test.

Limitations: Does not measure blood pressure, cannot diagnose orthostatic hypotension, and is less standardized than the formal tilt table test. A positive NASA Lean Test should be confirmed with a formal tilt table test.

Plasma Norepinephrine Testing

Blood drawn supine (after 20–30 minutes lying flat) and again after 10 minutes of standing. Measures the sympathetic nervous system's catecholamine output.

What it diagnoses: Hyperadrenergic POTS (standing norepinephrine ≥ 600 pg/mL). Also used to rule out pheochromocytoma when norepinephrine is very high (≥ 1,000 pg/mL).

Preparation: Hold SNRIs, beta-blockers, and stimulants for 48–72 hours. Fast for 4 hours. Avoid caffeine for 24 hours.

QSART (Quantitative Sudomotor Axon Reflex Test)

Acetylcholine is applied to four standardized skin sites using mild electrical current (iontophoresis). The sweat response is measured precisely. Tests postganglionic sympathetic sudomotor nerve function.

What it diagnoses: Small fiber neuropathy (length-dependent pattern), neuropathic POTS (reduced leg sweating), diabetic autonomic neuropathy.

Preparation: No lotions on test sites. Hold anticholinergics and antihistamines. Avoid caffeine for 48 hours.

Thermoregulatory Sweat Test (TST)

The patient is placed in a heated chamber (45–50°C) while coated with a powder that changes color when wet. The resulting sweat pattern is photographed and analyzed. Tests the entire sudomotor pathway from the hypothalamus to the sweat gland.

What it diagnoses: Anhidrosis (absence of sweating), hyperhidrosis, and the distribution of sudomotor dysfunction. Complements the QSART — the QSART tests the peripheral nerve, while the TST tests the complete pathway including central connections.

Preparation: No lotions. Avoid antiperspirants for 48 hours. Wear only a hospital gown.

Skin Punch Biopsy for Small Fiber Neuropathy

A 3mm punch biopsy of skin (typically from the calf and thigh) is taken under local anesthesia. The biopsy is stained for PGP9.5, a nerve fiber marker, and the density of intraepidermal nerve fibers (IENFD) is counted per millimeter.

What it diagnoses: Small fiber neuropathy (reduced IENFD below age- and sex-matched normative values). Can detect SFN even when QSART is normal, as different small fiber populations may be affected.

Preparation: No blood thinners for 5–7 days (confirm with doctor). The procedure takes about 15 minutes and leaves a small scar that heals within 2 weeks.

Deep Breathing Test (Respiratory Sinus Arrhythmia)

The patient breathes at exactly 6 breaths per minute (5 seconds in, 5 seconds out) for 1 minute while heart rate is recorded. The variation in heart rate between inspiration and expiration (the E:I ratio) is calculated.

What it diagnoses: Cardiac parasympathetic (vagal) dysfunction. Reduced heart rate variability during deep breathing is an early and sensitive marker of autonomic neuropathy, particularly diabetic autonomic neuropathy.

Preparation: No caffeine for 24 hours. No beta-blockers (they suppress heart rate variability). Breathe at the specified rate — too fast or too slow significantly affects the result.

Valsalva Maneuver

The patient blows into a tube to maintain a pressure of 40 mmHg for 15 seconds while heart rate and blood pressure are recorded continuously. The test has four phases that assess different aspects of autonomic function.

What it diagnoses: The Valsalva ratio (ratio of maximum to minimum heart rate during the maneuver) assesses cardiac parasympathetic function. The blood pressure response during phases II and IV assesses sympathetic adrenergic function. Abnormal responses indicate autonomic neuropathy.

Preparation: No caffeine. No beta-blockers. Avoid if you have a history of retinal detachment, recent eye surgery, or severe hypertension.

24-Hour Urine Catecholamines and Metanephrines

A 24-hour urine collection measures the total output of epinephrine, norepinephrine, dopamine, and their metabolites (metanephrines, normetanephrines, vanillylmandelic acid).

What it diagnoses: Pheochromocytoma and paraganglioma (rare catecholamine-secreting tumors). Ordered when plasma norepinephrine is very high or when the clinical picture raises concern for a tumor. Also used to assess overall sympathetic activity.

Preparation: Avoid vanilla, caffeine, alcohol, and certain medications for 48 hours before and during the collection. Keep the urine collection container refrigerated.

Plasma Metanephrines

A single blood draw that measures plasma metanephrine and normetanephrine — the stable metabolites of epinephrine and norepinephrine. More sensitive than 24-hour urine for pheochromocytoma detection.

What it diagnoses: Pheochromocytoma and paraganglioma. Ordered when plasma norepinephrine is ≥ 1,000 pg/mL or when clinical suspicion for a catecholamine-secreting tumor is present.

Preparation: Supine for 20–30 minutes before the blood draw. Avoid caffeine and certain medications.

Ambulatory Blood Pressure Monitoring (ABPM)

A blood pressure cuff worn for 24 hours that inflates automatically every 15–30 minutes during the day and every 30–60 minutes at night. Provides a complete picture of blood pressure variation over 24 hours.

What it diagnoses: Orthostatic hypotension (when combined with activity logging), nocturnal hypertension (a marker of autonomic dysfunction), non-dipping blood pressure pattern (failure of normal nocturnal blood pressure decrease), and white coat hypertension.

Preparation: Wear the cuff on the non-dominant arm. Log activities and posture changes. Keep the arm still during measurements.

Gastric Emptying Study (Scintigraphy)

The patient eats a standardized meal (typically scrambled eggs) labeled with a radioactive tracer. Gamma camera images are taken at 0, 1, 2, and 4 hours to measure how quickly the stomach empties.

What it diagnoses: Gastroparesis (delayed gastric emptying, defined as > 10% retention at 4 hours). Relevant to dysautonomia because vagal nerve dysfunction is a common cause of gastroparesis in POTS and other autonomic disorders.

Preparation: Hold opioids, prokinetics, and anticholinergics for 48–72 hours. Fast for 4 hours. Avoid smoking on the day of the test.

Putting It All Together: The Typical Autonomic Evaluation

A comprehensive autonomic evaluation at a specialized center typically includes:

Not every patient needs every test. The selection depends on the clinical picture, the suspected diagnosis, and what questions need to be answered to guide treatment. Your autonomic specialist will tailor the evaluation to your specific situation.