What is the Minor's starch-iodine test?

A simple bedside procedure in which an iodine solution is painted onto the skin, allowed to dry, and then dusted with starch. Wherever sweat appears, the starch and iodine react to turn a dark blue-purple, mapping active sweat glands in real time.

Is the Minor's test used to diagnose hyperhidrosis?

Not on its own. Primary hyperhidrosis is diagnosed by symptom-based criteria (Hornberger 2004). The Minor's test is used mainly to map the affected area before botulinum-toxin injections and to visualize sudomotor deficits (for example in Horner's syndrome).

Why is the mapping important before botulinum toxin?

Injected botulinum toxin diffuses only about 2 cm around each site. Mapping ensures injection points cover the whole hyperhidrotic area, which often extends beyond the hair-bearing zone, and are spaced (typically 1.5 cm staggered) so their effects overlap.

Can the test measure how much a person is sweating?

No. It is qualitative. It shows where sweat is being produced but does not quantify volume. For quantitative measurement, trials use gravimetric weighing or sudorometry.

What can make the test unreliable?

Diluted iodine (povidone-iodine has only about 1% free iodine), too much starch, anesthetic cream applied before the test, residual antiperspirant on the skin, and patient movement or sweating too little during the observation window. Iodine sensitivity is a contraindication; alizarin red or Ponceau red with starch can be substituted.

Research

The Minor's Starch-Iodine Test for Mapping Sweat

The Minor's starch-iodine test is the oldest still-used tool for visualizing where a person is sweating. Described by the German neurologist Victor Minor in 1928, it uses two cheap ingredients (an iodine solution and starch powder) and a simple chemical reaction: when sweat wets both, the mixture turns a dark blue-purple that traces active sweat glands on the skin. The test is qualitative, not quantitative; it maps where sweat appears rather than how much. Today it is used most often to outline the hyperhidrotic area before botulinum toxin injections, to confirm anhidrosis in conditions like Horner's syndrome, and to demonstrate gustatory sweating (Frey's syndrome). This page summarizes how it works, how it is used clinically, and its documented limitations. It is educational; it is not medical advice.

By the Sweat Explained Editorial Team · Published 2026-07-19 · Last reviewed 2026-07-19 · Educational information, not medical advice.

Key statistics at a glance

1928
year Victor Minor published the original description in Deutsche Zeitschrift für Nervenheilkunde
Minor 1928
3-5%
iodine solution concentration used before starch application in standard protocols
Trindade 2014
10-15 min
wait time after starch application for the color reaction to develop
IHhS onabotulinumtoxinA injection protocol
1.5 cm
recommended spacing of staggered botulinum-toxin injection points within the mapped area
IHhS; Allergan dosing guide

Where the test came from

Victor Minor published Ein neues Verfahren zu der klinischen Untersuchung der Schweißabsonderung ("A new procedure for the clinical study of sweat secretion") in the journal Deutsche Zeitschrift für Nervenheilkunde in 1928. His interest was neurological: he needed a way to see, at the bedside, which parts of a patient's skin were sweating and which were not. That let him map sudomotor lesions from stroke, spinal-cord injury, and sympathetic-chain disorders.

The chemistry is straightforward. Iodine forms a deep blue-black complex with the amylose in starch, but only in the presence of moisture. Painted iodine on dry skin does very little; add starch and moisture (sweat), and the color develops on contact. Minor's insight was that the skin itself could be the reaction surface, giving a real-time map of active sweat glands. Nearly a century later, the technique is essentially unchanged.

How the test is performed

The standard axillary protocol used to plan botulinum-toxin injections, as described by the International Hyperhidrosis Society and industry dosing guides:

The patient sits at rest, without antiperspirants or deodorants for 24 hours, and without exercise, hot drinks, or heavy meals for about 30 minutes before the test.
The clinician dries the axilla (or other target area) thoroughly.
A 3-5% iodine solution (tincture of iodine, povidone-iodine, or Betadine) is painted onto the skin and allowed to dry, about 30-60 seconds.
The area is dusted lightly and evenly with fine cornstarch or potato flour; excess is brushed off.
The clinician waits about 10 to 15 minutes. Where sweat is being produced, the mixture turns dark blue to purple-black, often with a speckled appearance corresponding to individual duct openings.
The clinician traces the border of the sweating zone with a surgical marker and places staggered injection dots (about 1.5 cm apart for onabotulinumtoxinA) inside it.

What the test is used for

Modern uses fall into three groups. In all of them, the test answers a yes-no or where-question, not a how-much question.

Modern clinical uses of the Minor's starch-iodine test
Use	What the test shows	Why it matters
Botulinum toxin injection mapping	The exact borders of the hyperhidrotic axilla, palm, sole, or scalp region	Botulinum toxin diffuses only about 2 cm; mapping ensures the whole affected area is covered and no toxin is wasted on quiet skin
Confirming anhidrosis	Absence of color change on one side of the face or body	Objectively documents a sudomotor deficit in Horner's syndrome, Wallenberg syndrome, or after nerve injury
Frey's syndrome (gustatory sweating)	Blue-purple color appearing over the parotid area during a chewing challenge	Confirms auriculotemporal-nerve misdirection after parotid surgery
Post-treatment reassessment	Residual sweating islands after botulinum toxin or ETS	Guides top-up injections or documents areas of compensatory sweating

The test is qualitative. It shows where sweat is happening; it does not measure how much sweat.

Why mapping matters for botulinum toxin

Botulinum toxin injected into the dermis diffuses roughly 2 cm in diameter around each injection point. If the injections are placed only under the hair, they may miss sweat glands at the periphery of the hyperhidrotic area, which often extends beyond the hair-bearing zone. The Minor's test avoids that mistake by making the boundary visible before the syringe comes out.

The International Hyperhidrosis Society's onabotulinumtoxinA axillary protocol specifies staggered injection points 1.5 cm apart within the mapped area, giving overlapping fields of effect across the whole outlined region. Industry dosing guidance is essentially the same and adds the practical detail that the marker ink used to mark points should be placed between, not on, injection dots to avoid a permanent tattoo effect from ink pushed into the dermis.

What the test cannot do

First, it does not quantify. A darkly staining area could represent very heavy sweat production or simply many active glands with modest per-gland output. Trials therefore pair Minor's mapping with gravimetric weighing (the actual milligrams of sweat over 5 minutes).

Second, it can be fooled. Povidone-iodine solutions labeled 10% contain only about 1% free iodine, and results are less reliable than with 3-5% tincture; anesthetic creams applied before the test can dampen the reaction; excess starch obscures the speckled pattern; and residual antiperspirant can produce false-negative patches. Sensitivity to iodine is a contraindication (alizarin red or Ponceau red with starch has been used as a substitute).

Third, it is a snapshot. The map reflects sweating during the observation window; a nervous patient at 9 a.m. and the same patient two hours later can produce different maps. That variability is why the test is a guide, not a diagnostic verdict, and why symptom-based criteria (Hornberger 2004) remain central to hyperhidrosis diagnosis.

Methodology and limitations

This page draws on the original 1928 Minor publication (as cited in the modern secondary literature), the Wikipedia entry which reproduces the original citation, an Indian Journal of Otolaryngology & Head-Neck Surgery case report describing modern use, a JAAD Notes & Comments article on dermoscopic viewing of the test, the International Hyperhidrosis Society step-by-step onabotulinumtoxinA axillary protocol, the Allergan BOTOX ONE physician dosing guidance, and the Trindade et al. Facial Plastic Surgery Clinics of North America chapter. Procedural details (iodine concentration 3-5%; 10-15 minute wait; 1.5 cm staggered injection spacing; 30-gauge intradermal injection) come from those cited protocols.

Limitations: the test is qualitative and technique-dependent. Reported diffusion distances, wait times, and injection-point spacing come from industry and society protocols that reflect expert consensus rather than randomized head-to-head comparison. No numeric sensitivity/specificity of the Minor's test as a standalone diagnostic tool is published (it is used as a mapping aid, not a diagnostic test), so no such numbers are cited here. Nothing on this page is medical advice.

Frequently asked questions

What is the Minor's starch-iodine test?: A simple bedside procedure in which an iodine solution is painted onto the skin, allowed to dry, and then dusted with starch. Wherever sweat appears, the starch and iodine react to turn a dark blue-purple, mapping active sweat glands in real time.
Who invented it?: Victor Minor, a German neurologist, described the procedure in 1928 in the journal Deutsche Zeitschrift für Nervenheilkunde. The chemistry and the basic technique have not meaningfully changed since.
Is the Minor's test used to diagnose hyperhidrosis?: Not on its own. Primary hyperhidrosis is diagnosed by symptom-based criteria (Hornberger 2004). The Minor's test is used mainly to map the affected area before botulinum-toxin injections and to visualize sudomotor deficits (for example in Horner's syndrome).
Why is the mapping important before botulinum toxin?: Injected botulinum toxin diffuses only about 2 cm around each site. Mapping ensures injection points cover the whole hyperhidrotic area, which often extends beyond the hair-bearing zone, and are spaced (typically 1.5 cm staggered) so their effects overlap.
Can the test measure how much a person is sweating?: No. It is qualitative. It shows where sweat is being produced but does not quantify volume. For quantitative measurement, trials use gravimetric weighing or sudorometry.
What can make the test unreliable?: Diluted iodine (povidone-iodine has only about 1% free iodine), too much starch, anesthetic cream applied before the test, residual antiperspirant on the skin, and patient movement or sweating too little during the observation window. Iodine sensitivity is a contraindication; alizarin red or Ponceau red with starch can be substituted.

Sources

Primary peer-reviewed studies and official sources first, then reviews and institutional framing (secondary).

Minor V. Ein neues Verfahren zu der klinischen Untersuchung der Schweißabsonderung. Dtsch Z Nervenheilkd. 1928;101:302-308. DOI
Naumann M, Glaser DA, Lowe NJ, et al. Treatment of hyperhidrosis with Botox (onabotulinumtoxinA): development, insights, and impact (Minor's test in pivotal trials). Medicine (Baltimore). 2023;102(S1):e32764. Full text
Trindade de Almeida AR, Montagner S. Botulinum toxin for axillary hyperhidrosis. Dermatol Clin. 2014;32(4):495-504. Full text (IHhS mirror)
Zangrilli A, Diluvio L, Bianchi L, et al. Starch-iodine test viewed through a dermoscope. J Am Acad Dermatol. 2020;82(6):e195-e196. Full text
Bhattacharjee A, Krishnappa S. Minor's test: objective demonstration of Horner's syndrome. Indian J Otolaryngol Head Neck Surg. 2013;65(Suppl 3):649-650. Full text
International Hyperhidrosis Society. Starch-iodine + onabotulinumtoxinA injection protocol for axillary treatment. IHhS clinician resource. (secondary) IHhS
Allergan. BOTOX ONE physician dosing and administration guide: primary axillary hyperhidrosis. AbbVie/Allergan clinician resource. (secondary) Physician guide

How to cite this page

Sweat Explained. The Minor's Starch-Iodine Test for Mapping Sweat. Published 2026-07-19; last reviewed 2026-07-19. Available at: https://sweatexplained.com/research/minors-starch-iodine-test

Please cite the original studies for the underlying figures. Journalists are welcome to link to this page; the charts are original renderings of the cited data.