Need for Percent Area Reduction and Percent Volume Reduction Measurements in Diagnostic Wound Imaging: A Statement From the Wound Care Collaborative Community—Part 1

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With a multitude of options available, health care professionals often grapple with inconsistent image quality and measurement accuracy. In this article, we discuss the role and importance of PAR and PVR in wound care, highlighting their importance in wound assessments, clinical endpoints, and use for early intervention in wound healing. 

Every day, health care professionals face the challenge of assessing wounds, monitoring their healing progress, and determining the most effective treatment strategies. Thus, the ability to accurately measure wound parameters is paramount. Because wounds are not formed in a rectangular or square fashion, ruler measurements of length and width are typically inaccurate in assessing the wounds’ true dimensions.¹ The ruler method to measure wounds has poor intrarater and interrater reliability, which calls in to question the ability to accurately measure a PAR, time to wound healing, or time to complete wound healing utilizing this antiquated method.² Wound square surface area is measured by length, where L is the length of the long axis and W is the widest width of the wound. Wound volume can be calculated using clinical measurements and the formula for volume of an ellipsoid; the volume is V = 4/3 LWDπ, where L is the length of the long axis, W is the width, and D is the depth. In a study analyzing PVR, wound area and volume reduction were calculated as percentage change from baseline wound size.³ Although using the greatest length by the greatest width improves the reliability, size alone is not the only indicator of wound progression. Wound assessment can be improved by the MEASURE tool: Measure (length, width, depth, and area), Exudate (quality and quantity), Appearance (tissue type), Suffering (pain and level), Undermining, Reevaluate (continue to monitor parameters), and Edge of the wound.⁴ Ideally, the assessment should include the use of these parameters.

An extensive retrospective analysis involving 2768 wounds has shed light on a significant concern with the traditional L × W method for wound area measurement—it tends to yield an overestimation of the wound area, surpassing the actual size by a substantial 44%.⁵ This concern finds reinforcement in several other studies that have also highlighted the propensity of L × W measurements to inaccurately magnify wound dimensions.^5,6 The issue is further exacerbated when dealing with large wounds or those with irregular shapes, where the inaccuracy of this method becomes even more pronounced.⁷ When using a 3D scanner as a control, acetate tracings can improve accuracy in comparison to ruler measurement: 41% difference in measurement versus 75%.⁸ Digital planimetry and computer-assisted techniques provide greater accuracy.⁵

PAR and PVR are known indicators and important clinical endpoints with robust evidence for use in predicting wound healing. In an initial study performed by Sheehan et al, wound area measurements performed in 203 patients over a 12-week prospective randomized trial of patients with diabetic foot ulcers demonstrated a 53% percent or greater change in wound area at 4 weeks correlated with those patients who healed at 12 weeks, whereas less than that percent change in area correlated to those who failed to heal over the 12 weeks. Therefore, the percent change in foot ulcer area after 4 weeks of observation is a robust predictor of healing at 12 weeks.⁹ Similar findings concurred in a cohort study of 704 patients with diabetic wounds.¹⁰ In a recent systematic review of diabetic foot ulcers and venous leg ulcers, 6 studies reported in favor of using PAR while 5 studies demonstrated the lack of having discriminatory ability.¹¹ Some of the challenges faced when analyzing these studies was the risk of bias, heterogeneity of research design, and report of post hoc analysis of patients in randomized trials. A recent study that surveyed 628 interdisciplinary wound experts assessed 28 literature-based wound care endpoints for their relevance and importance to clinical practice, clinical research, and/or if they were patient-centered. Findings included 22 endpoints with CVIs greater than or equal to  0.75, with 15 having CVIs of greater than or equal to 0.85; those 15 were selected for further research as primary endpoints for FDA approval of future wound care interventions.¹² Additional research included a search of 485 references involving over 462 000 participants supporting FDA-required parameters for all 15 of these endpoints.¹³ More than 50 references for PAR alone and hundreds more supported FDA-required parameters qualifying the following outcomes for use in clinical trials supporting interventions for FDA clearance. These parameters include Pain reduction, Physical function and ambulation, Infection reduction, Time to heal, and Percent wound area reduction in 4 to 8 weeks.¹³ Despite these parameters, only time to heal is currently recognized by the FDA as a primary wound outcome in clinical trials.¹³

As part of the research conducted for these 15 primary endpoints, The Opinion Survey from People with Wounds¹⁴ addressed the gap between clinicians’ and patients’ clinical endpoints. Results from 438 patients identified their highest priorities were reduced infection, recurrence, and amputation. The main endpoints in terms of usefulness for measuring clinical trial success were time to heal, wound size, infection, recurrence, and pain.¹⁴ The most valuable quality-of-life outcomes were increased independence, reduced social isolation, and pain.¹⁴ Narrative responses from wound patients emphasized that the most important endpoints are those reflecting pain and the inability to perform activities of daily living.¹⁴

Support for the validity and reliability of monitoring tools to evaluate outcomes has been reported in the literature. The Spinal Cord Injury Pressure Ulcer Monitoring Tool is a validated and reliable tool developed from previously published pressure ulcer healing tools and specific to spinal cord injury.¹⁵ Similar validated monitoring tools such as the Bates-Jensen Wound Assessment Tool and PUSH tool are commonly utilized for pressure injury evaluation. Characteristics of the PUSH tool include the surface area constituting 58% of the total score when the measurements are greater than 24 cm² and the tissue-type score is highest for pressure injuries with slough or eschar. However, the tool omits undermining, tunneling, and depth, which are common clinical findings in pressure injuries.¹⁵ Subsequently, a study using the PUSH tool for measurements in venous leg ulcers has been reported.¹⁶ A prospective, multicenter study validated the use of the PUSH tool in patients with pressure injuries, diabetic ulcers, and/or venous ulcers.¹⁷ However, 3D wound measurements in comparison to other modalities demonstrated increased accuracy. Interrater reliability has been demonstrated with multiple 3D imaging devices.^18,19

One of the key issues facing wound imaging in both clinical practice and in designing clinical wound healing studies is the lack of standardized protocols and guidelines across imaging devices and applications. The US FDA CDRH oversees the safety and regulates the approval of medical devices for marketing. These regulators examine the technical and design applications of the product, considering the safety of the medical device. CDRH is focused on digital medicine developers building SaMD products.²⁰ SaMD provides manufacturers links to techniques, definitions, and sources that are available to help SaMD generate appropriate evidence. Valid clinical association, analytical, and clinical validation are provided by the SaMD manufacturers. It is recommended that manufacturers perform ongoing data analysis and modify software as needed to meet these requirements. SaMD may leverage connectivity between devices and people to continuously monitor the safety, effectiveness, and performance.²⁰ The FDA does not regulate what the product does but what the organization claims the product does. The FDA classifies and regulates medical devices based on risk, whereas class I devices are subject to the lowest level of regulatory controls while class III devices are subject to the highest level of regulation. Class I devices include simple medical supplies and some devices. Class I and some class II devices do not require premarket notification; however, most class II devices require premarket notification, known as 510(k). The 510(k)-clearance process is the path for a new medical device if equivalent to legally marketed devices that predicates it. However, the FDA does allow the manufacturer of a low- to moderate-risk device without a predicate to submit a De Novo request to the FDA to make a risk-based classification of the device into class I or II. Once a De Novo request is granted, this device may then serve as a predicate for 510(k) premarket approval of similar devices in the future.²¹ Subtle differences in terminology used to describe regulatory status have underlying meaning: FDA-approved indicates successful completion of an FDA Premarket Approval process evaluating the safety and efficacy of Class III high-risk products; while FDA-cleared suggests that the device completed a 510(k) pathway for lower-risk products. Drug development tools are recognized separately from medical device development tools.

Additionally, regulation between the European Commission and the FDA may differ. Although they share a similar process for approval, every marketed medical device in the European Union (EU) holds a Conformité Européenne (CE) mark indicating that it conforms to directives set in the EC Medical Device Directives of the EU.²² Similar to SaMD, the National Institute for Health and Care Excellence in England has developed an evidence standards framework for digital health technologies through different evidence tiers the manufacturers provide. Data generated that is accurate, reproducible, and relevant to the range of values expected in the target population undergoes a second-tier review when there is also a commitment to ongoing data collection.²³

In 2016, the FDA and National Institutes of Health created outcome measures. BEST (Biomarkers, EndpointS, and Tools) defines outcomes related to measurable characteristics affected by an individual’s baseline state or/and intervention in a clinical trial.²⁴ The validation of the tool is not based on the item’s usefulness but rather its technical performance. Additionally, the listed biomarkers for diagnostic, monitoring, response, and predictive endpoints are limited. Digital biomarkers could include any of the BEST categories. However, a systematic approach to assessing the quality and usefulness of digital biomarkers is needed.²⁵ Biomarkers are a characteristic measured as an indicator of a normal or pathogenic process or responses to an intervention. An endpoint is an outcome that can be measured objectively. A clinical outcome describes how an individual feels.²⁶ Thus, digital biomarkers may help serve as an endpoint or tool.

As the WCCC TWG strives to address concerns and enhance wound imaging tools, it becomes evident that these tools, when meeting established criteria and undergoing proper evaluation, could be deemed as ‘digital biomarkers.’ These digital biomarkers, with their potential to provide consistent and accurate measurements, hold promise in advancing the field of wound care and improving patient outcomes. By promoting the use of PAR and PVR as primary endpoints accepted by the FDA, the WCCC aims to elevate the standards of wound assessment in clinical trials.

Authors: Alisha Oropallo, MD, FSVS, FACS^1,2,3,4; Peggy Dotson, RN, BS^5,6; Tod Brindle, PhD, MSN, RN, ET, CWCN^7,8; Vickie R Driver, DPM, MS^9,10,11; and Lisa Gould, MD, PhD, FACS^8,12

Affiliations: ¹Chair, Tools Work Group, Wound Care Collaborative Community (WCCC); ²Director, Department of Vascular Surgery, Northwell Health; ³Professor, Zucker School of Medicine, Hofstra University/Northwell Health; ⁴Professor, Feinstein Institutes of Medical Research, Northwell Health; ⁵President Healthcare Reimbursement Strategy; ⁶Officer, WCCC; ⁷Global Medical Affairs Director, Ostomy Care, ConvaTec Group; ⁸Co-Chair, Tools Work Group, WCCC; ⁹Chair, WCCC; ¹⁰Professor, Washington State University, School of Medicine; ¹¹Professor, Barry University, affiliate; ¹²Attending Plastic Surgeon, South Shore Health 

Disclosure: The authors disclose no financial or other conflicts of interest.

Disclaimer: The opinions and statements expressed herein are specific to the respective author(s) and not necessarily those of Wounds or HMP Global. This article was not subject to the Wounds peer-review process.