Introduction to Sub-Epidermal Moisture

“A PI/PU is defined as localised damage to the skin and/or underlying tissue, as a result of pressure or pressure in combination with shear. PI/PU usually occur over a bony prominence but also may be related to a medical device or other objects.”1

Inflammation is the body’s first response to tissue damage and a mechanism to combat injury. The inflammation increases vasodilation and permeability of blood vessels, leading to leakage of plasma and fluid creating localised oedema in the tissue. localised inflammatory oedema is described by the International Clinical Practice Guidelines (CPG)1 as one of the earliest signs of cell death in pressure injuries/ulcers.

As the level of tissue damage increases, so does the inflammatory response. Accordingly, the resulting level of localised tissue oedema or water in the skin and tissue, termed sub-epidermal moisture (SEM), increases.2 The CPG describe that this can be identified by a “biophysical marker called the Biocapacitance of the tissues.”1

These inflammatory changes in the skin and underlying tissues with the localised tissue oedema can occur from 3 to 10 days before damage and/or breakdown of skin is visible at the surface.3

Courtesy of Prof A Gefen

What is Biocapacitance?

A change in sub-epidermal moisture due to localised oedema or accumulation of interstitial fluid is now known to be an effective biophysical marker of the inflammatory response to injury.4

Biocapacitance is an electrical property of tissue that varies with the amount of interstitial moisture content. The greater the interstitial moisture, the higher the biocapacitance of the tissue. The higher biocapacitance is then interpreted by the Provizio® SEM Scanner.2

Professor Amit Gefen

“New damage cascade for why pressure injuries/ulcers develop.”

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Heel scan

The Provizio SEM Scanner

The Provizio SEM Scanner, is a CE-marked and FDA-authorised hand-held wireless device, indicated for use as an adjunct to the standard of care when assessing patients who are at increased risk of PI/PU, that objectively alerts clinicians to specific anatomical areas of a patient’s body at increased risk for developing pressure damage, 5 days* earlier than visual skin assessments,5 regardless of skin tone6

The device consists of a concentric sensor, an integrated pressure switch, and software that computes a “delta” (SEMΔ) value from a set of readings made around a common anatomical location.

When the sensor is pressed against an area on the skin, the Provizio SEM Scanner identifies the capacitance of the sensor and underlying tissue, which is affected by the moisture content within the tissue to a depth of approximately 4mm. The capacitance reading is converted into a unitless value (SEM) and displayed to the user. The device also computes a delta value for each set of measurements and displays this delta value (Δ) to the user.

A Δ < 0.6 at an anatomical site may suggest the tissue is at lower risk for PI/PUs

A Δ ≥ 0.6 at an anatomical site may suggest increased risk for PI/PUs

Current clinical decision making relies on assessing a patient’s overall risk for PI/PU development and then completing a subjective skin and tissue assessment.7 Both methods suffer from not being able to direct clinicians to where the risk is building until damage is visible at the skin’s surface. Once PI/PUs (also known as bed sores) become visible on the skin’s surface, tissue damage has already occurred.

Elevated readings from the Provizio SEM Scanner provide objective clinical information that directs clinicians to heels and sacrum’s at increased risk of developing PI/PU, even before the damage is visible.5 This data can facilitate earlier, anatomically specific interventions designed to reverse the damaging effects of pressure and shear and prevent the PI/PU from breaking through the skin.

International Clinical Practice Guideline 2019 (CPG)1

CPG recommendation 2.6 states that healthcare practitioners using their own qualified clinical judgement should, "Consider using a sub-epidermal moisture/edema measurement device as an adjunct to routine clinical skin assessment."1

CPG recommendation 2.7 states that HCPs using their own qualified clinical judgement when assessing darkly pigmented skin should, "Consider assessment of skin temperature and sub-epidermal moisture as important adjunct assessment strategies."1

1. European Pressure Ulcer Advisory Panel, National Pressure Injury Advisory Panel and Pan Pacific Pressure Injury Alliance. (2019). Prevention and Treatment of Pressure Ulcers/Injuries: Quick Reference Guide. Emily Hasler (Ed.). EPUAP/NPIAP/PPPIA
2. Gefen A. (2018). The Sub-Epidermal Moisture Scanner: the principles of pressure injury prevention using novel early detection technology. Wounds International, 9 (3)
3. Moore Z., et al. (2016). Subepidermal moisture (SEM) and bioimpedance: a literature review of a novel method for early detection of pressure-induced tissue damage (pressure ulcers). International Wound Journal, 14(2), pp.331-337
4. Bates-Jensen B. M., et al (2017). Subepidermal moisture detection of pressure induced tissue damage on the trunk: The pressure ulcer detection study outcomes. Wound Repair Regen. 2 May; 25(3):502-511.
5. Okonkwo H., et al. (2020). A blinded clinical study using a subepidermal moisture biocapacitance device for early detection of pressure injuries. Wound Repair Regen, (online) 1-11. Available at: (Accessed: 21 January 2020)
6. Bates-Jensen B., et al. (2009). Subepidermal Moisture Is Associated with Early Pressure Ulcer Damage in Nursing Home Residents With Dark Skin Tones. Journal of Wound, Ostomy and Continence Nursing, 36(3), pp.277-284
7. Moore Z., et al. (2019). Risk assessment tools for the prevention of pressure ulcers (Review) Cochrane Collaboration
* Median

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