Diagnostic Utility of the Sudomotor Function Test Neuropad in Patients with Diabetes

A pair of moist feet

This peer-reviewed study, conducted by researchers affiliated with the University of Manchester and Weill Cornell Medical College in Qatar, was published in the Journal of Diabetes Research and made accessible via the U.S. National Institutes of Health’s PubMed Central.

The aim was to assess the diagnostic performance of Neuropad®, a visual screening test for diabetic peripheral neuropathy (DPN) that evaluates sudomotor function via colour change. The researchers compared Neuropad’s results with gold-standard assessments for small- and large-fibre neuropathy, including corneal confocal microscopy (CCM), intraepidermal nerve fibre density (IENFD), vibration perception threshold (VPT), and nerve conduction studies (NCS).

Methods

The study enrolled 110 subjects with type 1 or type 2 diabetes, who underwent detailed neurological evaluations. These included:

  • Neuropad test: A cobalt chloride-based patch placed on the sole of the foot; normal function results in a full colour change from blue to pink within 10 minutes.
  • CCM: Quantitative assessment of corneal nerve fibre density and length.
  • IENFD: Biopsy-based measure of small nerve fibre density.
  • NCS and VPT: Established tools for assessing large-fibre function.

Both categorical (normal/intermediate/abnormal) and continuous (image-analysis) readings of Neuropad were used, allowing the team to compare both simple and more quantitative approaches.

Results

Key findings include:

  • High sensitivity of Neuropad for detecting early neuropathic changes
  • 91% sensitivity and 83% specificity for abnormal autonomic function (based on the expiration-to-inspiration ratio).
  • 88% sensitivity and 78% specificity for detecting reduced IENFD.
  • 79% sensitivity and 61% specificity for identifying abnormal CCM parameters.
  • The categorical reading of Neuropad (based on timing and completeness of the colour change) was easy to use, but slightly less precise than continuous, image-based analysis – which improved reproducibility and objectivity.
  • Strong correlation was observed between Neuropad results and small-fibre damage, confirming its utility as a marker of early DPN, particularly sudomotor and small-fibre dysfunction, which typically precede symptoms or large-fibre loss.

Conclusions

The team concluded that Neuropad is a valid, sensitive, and reproducible tool for detecting early diabetic neuropathy. Its strongest performance was in identifying autonomic and small-fibre dysfunction, which are difficult to detect with standard large-fibre tests like monofilament or vibration perception.

Importantly, Neuropad’s non-invasive nature, simplicity, and affordability make it highly suitable for first-line screening-especially in primary care or community settings where more advanced neurodiagnostic tests are impractical. The study also recommended that automated image-based scoring of Neuropad be used wherever possible to enhance objectivity and reduce variability.

Key Strengths of Neuropad Highlighted by the Study:

  • Detects early-stage neuropathy before clinical symptoms arise.
  • Easy to use with no need for specialist equipment or training.
  • 10-minute visual test – suitable for routine diabetes check-ups.
  • Strong correlation with small-fibre diagnostic gold standards (IENFD, CCM).
  • Well-tolerated and low-cost, enhancing accessibility.

Link to full study

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