Dryness of Foot Skin Assessed by the Visual Indicator Test and Risk of Diabetic Foot Ulceration: A Prospective Observational Study

A sandy footprint on a beach

Introduction

Diabetic foot ulcers (DFUs) remain one of the most devastating complications of diabetes mellitus (DM), often leading to amputation, reduced quality of life, and increased mortality. Central to the development of DFUs are diabetic peripheral neuropathy (DPN) and peripheral arterial disease (PAD), both of which impair foot sensation and vascular function. Identifying patients at risk of ulceration is vital in reducing DFU incidence.

Sudomotor dysfunction, an early manifestation of autonomic neuropathy affecting sweat gland function, contributes to foot skin dryness and potentially to ulcer formation. The Indicator Plaster Method (IPM), a simple, visual, colorimetric test, detects foot skin dryness and hence sudomotor dysfunction. While earlier cross-sectional studies established a link between IPM-detected dryness and existing DFUs, this study uniquely investigates whether the IPM can prospectively predict future DFU development.

Study Design and Methods

This prospective, multicentre observational study followed 308 diabetic individuals over six years (2012–2017) across seven outpatient clinics in four European countries. Key inclusion criteria were adults with diabetes but no history of prior DFU, amputation, or critical limb ischaemia.

Participants underwent:

  • IPM testing: Plasters applied to the soles changed colour based on skin moisture. Normal = pink; abnormal = blue or patchy.
  • Neurological assessments:
    • Neuropathy Symptom Score (NSS): Patient-reported symptoms.
    • Neuropathy Disability Score (NDS): Clinician-assessed signs including reflexes, vibration, temperature, and pinprick.
    • Vibration Perception Threshold (VPT): Measured with a biothesiometer in a subset of 210 patients.

Patients were monitored every 3–6 months, with ulcers defined as full-thickness skin lesions below the malleoli.

Results

Over the follow-up period, 55 participants (17.9%) developed a DFU, translating to an annual incidence of 2.97%. Significant findings include:

  • Abnormal IPM strongly predicted future DFUs. Participants with an abnormal IPM had over a threefold increased risk (HR = 3.319).
  • High NDS (≥6) and high VPT (≥25V) were also independently predictive, with hazard ratios of 2.782 and 2.587, respectively.
  • The IPM showed high sensitivity (86%) but modest specificity (49%)—meaning it was effective at ruling out those not at risk, but also flagged some low-risk individuals.
  • In contrast, high NDS and VPT had low sensitivity (40% and 39%) but high specificity (87% and 89%)—better for confirming risk than excluding it.
  • Combining IPM with NDS or VPT only marginally improved predictive value.

Ulcers occurred most frequently on toes and the plantar area, reinforcing the role of both pressure and sensation loss.

Discussion

The study robustly demonstrates that IPM can prospectively predict DFU risk, making it a valuable screening tool. Unlike other modalities that require specialized equipment or clinician administration, IPM is cheap, simple, non-invasive, and suitable for self-testing.

While established neurological tools like NDS and VPT remain important, they predominantly assess large fibre function and may miss early small fibre dysfunction—a gap that IPM helps fill. In fact, since sudomotor (sweat gland) impairment often precedes other neuropathic signs, the IPM may detect risk earlier in the disease course.

Despite lower specificity, the IPM’s high negative predictive value (>90%) means a normal result reliably rules out high DFU risk, which is essential in primary care settings. False positives, while a limitation, lead to non-invasive further monitoring—not harmful interventions.

The IPM also holds promise in public health settings. Given the poor uptake of diabetic foot screening across Europe, empowering patients with a self-administered, visual tool may bridge the gap between guidelines and practice. Studies have shown >90% agreement between patient and clinician interpretations of the IPM.

Conclusion

This is the first large-scale prospective study to validate the IPM as a predictor of diabetic foot ulceration. Its ease of use, sensitivity, and suitability for self-application make it a valuable addition to routine diabetes care.

In summary, the Indicator Plaster Method is a practical, cost-effective, and powerful screening tool. By identifying patients at risk for DFUs early—especially those with undetected small fibre neuropathy—it offers a simple route to prevention. As diabetic foot care continues to evolve, integrating tools like the IPM into standard protocols could help reduce the global burden of diabetic foot disease.

References

Dryness of Foot Skin Assessed by the Visual Indicator Test and Risk of Diabetic Foot Ulceration: A Prospective Observational Study (original PDF).