Improving Accuracy of Wound Measurement in Clinical Practice

   Measurement of wounds is an important component of the wound assessment process and has the potential to provide baseline measurements, monitor healing rates, and differentiate between wounds that are static, deteriorating, or improving. However, measurement techniques currently available to clinicians are inaccurate and unreliable, so many clinicians consider wound measurement to be an optional aspect of wound assessment. Apart from serving as an outcome measure of clinical trials or within specialist centers, wound dimension is not normally plotted over time due to lack of time and resources.

   A variety of different wound measurement techniques are currently available and range from simple to sophisticated.¹ Selecting an appropriate method is dependent on availability of resources, wound type, and location. This review aims to determine means of improving the accuracy of non-invasive measurement techniques so that changes in wound dimension over time can be effectively monitored in clinical practice.

Literature Review

   To critically evaluate the efficacy of current wound measurement techniques, electronic databases, including the Cumulative Index to Nursing and Allied Health Literature (CINAHL), Medline, Embase, Best Evidence, and the Cochrane Database of Systematic Reviews, were searched from January 1965 to February 2003. Statistical pooling of results from trials was considered inappropriate due to inconsistencies between studies in relation to key variables (eg, sample characteristics and outcome measures). Studies appraising biochemical markers and other physical parameters (eg, blood flow, temperature, pH) were beyond the scope of this review because they are not currently widely available. Other criteria for exclusion of papers included: non-English papers, duplication within other sources, or references to unpublished work.

Current Wound Measurement Practice - Approximation of Wound Surface Area

   Current measurement of wound dimensions has many limitations. Simple approximations of surface area often are made in clinical practice for superficial, shallow wounds by multiplying the maximum perpendicular length and width measurements (diameter product measurement) (see Table 1). This technique has been shown to be imprecise, especially for large wounds, irregular shaped wounds, or cavities, because area is generally overestimated² but is used to provide a baseline for evaluation of healing. Reliability could be improved if the points from which measurements are taken are carefully documented (see Figure 1). Many different procedures have been recommended to improve accuracy of diameter product measurements, including use of perpendicular measurements and clockwise techniques,³ but precision has not been effectively demonstrated in clinical practice for either.⁴

   Alternatively, area can be estimated by tracing the wound outline onto a clear film using a fiber-tipped pen (see Figure 2) or by using photographic images. The circumference can then be transferred onto graph paper so that the surface area of the wound can be crudely estimated by counting each full square that falls within the perimeter of the wound tracing; half squares can be added up in the same way. Counting squares on graph paper is subjective, especially when adding up incomplete squares. This technique depends heavily on the clinical judgment of the user to determine the exact extent of the wound. Tracings made in this way do not record the three-dimensional aspect of the wound and provide no information relating to depth and volume.⁵

   Accurate identification of wound margins using either conventional contact (tracings) or non-contact (photographs) directly influences whichever method is used to determine wound area and is compromised by patient positioning, curvature of the body, and tapering of limbs.^6,7 In addition, tracing wound dimensions in circumferential wounds, small wounds, and wounds near skin folds will always be problematic and requires a high level of competency.^8,9

   In a study comparing inter-observer reliability of leg ulcer measurements using digital images and conventional contact tracing, Samad et al¹⁰ reported no significant difference in accuracy between the two methods. The results demonstrated that digital images need to be of high quality to accurately define epithelial growth at wound margins and that variation in camera angle can result in the reduction of the measured area by approximately 10%, especially in larger wounds.¹¹ These limitations can be overcome with use of sophisticated systems that allow projection of three-dimensional images (stereophotogrammetry), but these techniques are impractical in clinical settings.⁹

Precise Area Measurement

   Determining wound surface area involves a two-stage approach: identifying the wound margin using conventional contact tracing or digital tracing/photography and calculating surface area (approximate or precise). As such, two options are available to improve the accuracy of wound surface area measurement by minimizing the two stages of error that could result in tracing and/or calculation inaccuracies. Studies demonstrate that the greatest source of error occurs when identifying wound margins, rather than when determining the area traced.^12-14

   Once the wound margin has been identified accurately, the area contained within the traced perimeter can either be approximated by simple techniques (eg, square counting) or precisely measured by planimetry. Wound planimetry may be defined as the precise measurement of the area contained within the wound tracing or the traced outline of a digital image of the wound (where both of these represent a plane or flat figure). Planimetry performs area measurement on a plane (ie, a flat figure that could be either a tracing of wound perimeter or a two-dimensional photograph or digital image). Precise measurement of the area of the wound tracing can be achieved with a mechanical planimeter, but this instrument is not routinely used to measure wounds as use is time-consuming and involves specialist skills.¹⁵ Wound area also can be measured from a digital image of the wound using a computer or other electronic device. The digital image can be generated in two ways: 1) downloaded from a digital camera and delineated by the use of the mouse, or 2) digitized from a manual tracing or a digital tablet.

   Computer analysis has been used for some time in an attempt to improve accuracy of wound measurement.¹³ Today, many computer-based programs are available for determining wound area that are easy to use and demonstrate high precision.^16,17 However, computer planimetry is not widely available and is more suitable for specialist settings. The difficulties associated with photo-based planimetry (eg, curvature, angular contours) are not inherent to "computer planimetry" but are a result of using photography.

   Planimetric measurement has been shown to be more precise than the diameter product measurements and the square counting approximations of larger chronic wounds.^1,7,12,18 Cutler et al¹ compared various methods of calculating the area of pressure ulcers and found that results obtained from length and width measurements overestimated ulcer area compared with areas calculated by computer-planimetry. Öien et al¹⁵ compared four methods of measuring leg ulcer area (n = 50) using mechanical planimetry, digital planimetry, square counting approximations, and diameter product measurements. Ulcer area was determined using each method from transparency tracings, and inter-rater reliability was tested using an independent researcher. All methods demonstrated a high degree of agreement for smaller wounds but differences occurred as size increased. Diameter product measurements were found to be the least reliable method of measuring ulcer size, followed by square counting approximations and mechanical planimetry.

Measuring Wound Volume and Depth

   Measuring depth, especially of irregularly shaped, deep wounds, has always been imprecise in clinical practice as it involves three-dimensional measurements that are affected by body curvature and soft tissue distortion.¹⁹ Probes, sterile swabs, forceps, and gloved fingers all have been used with varying degrees of accuracy to measure wound depth (see Figure 3). The evaluation of wound depth with a probe has been shown to consistently underestimate the volume of larger, more irregularly shaped wounds.⁵ Ultrasound, structured light techniques, and stereophotogrammetry are usually reserved for research purposes and are not generally suitable for everyday use.²⁰

   Kundin²¹ attempted to improve accuracy of volume and area measurement using a device and a mathematical formula to approximate the length, depth, and width of a wound. However, studies have demonstrated that this approximation underestimates wound volume.⁶ Melhuish, Plassmann, and Harding¹⁹ evaluated healing at weekly intervals in 14 surgical wounds using a structured light technique to measure area, volume, and depth in order to investigate whether wound circumference measurement could be used instead of volume and area. This study demonstrated a direct correlation between wound circumference and wound area (0.90, P <0.001) and circumference and wound volume (0.70, P <0.001). Recent evidence suggests that calculating the area and volume is unnecessary, as percentage of area reduction alone is a valid outcome measure.^22-24
Melhuish, Plassmann, and Harding¹⁹ demonstrated that decreases in wound circumference, area, and volume exhibited a similar pattern in all surgical wounds in their sample. Volume decreased most rapidly as healing took place, followed by a slower decrease in circumference and area. Wound infection increased measurements of volume, area, and circumference, demonstrating the clinical significance of calculating the percentage of area change over time. This study provides evidence that accurately monitoring wound healing is possible by measuring circumference alone because this parameter is directly related to both volume and area. This suggests that accurate identification of the wound margin is sufficient in order to determine wound area and that routine volume measurement should be stopped.

   However, general assessment of wound depth, including presence of undermining and sinus formation, is clinically useful for planning realistic treatment goals,²⁵ because full-thickness wounds generally take longer to heal than partial-thickness wounds.^26,27 Wound staging systems were originally developed to classify wounds based on maximum anatomic depth of soft tissue damage.²⁸ However, these classification systems only describe the status of the wound at the time of assessment^29-31 and were not intended to assess the progress of wound healing over time.²⁵ In some health settings, clinicians use pressure ulcer staging systems in reverse order in an attempt to document the progress of healing in pressure ulcers. Such practices are extremely inaccurate, as the repair process produces granulation tissue rather than muscle and subcutaneous fat. The clinical relevance of this is highlighted in the National Pressure Ulcer Advisory Panel's position statement (available at: www.npuap.org/positn5.htm).

Monitoring Wound Healing Using True Area

   A decrease in wound size is an important indicator of healing. The relationship between initial wound size and healing has been established for both acute and chronic wounds, but healing rates are not always constant during this process.^19,27,32 Initial wound dimensions are not reliable for predicting chronic wound closure if subsequent changes are not monitored - factors delaying healing over time are unpredictable. Stacy et al,³³ in a clinical trial involving 78 venous leg ulcers, found that initial ulcer size did not influence the length of time to healing. This is supported by the earlier work of McGrath and Simon,³⁴ who suggested that the relative rate of healing is independent of initial wound size. Stacy concluded that the percentage of area reduction is a useful parameter to consider as part of routine wound assessment to help establish whether the wound is responding to treatment. Although various techniques can be used to measure initial wound area accurately, comparing measurements over time is more valuable.

Predicting Healing Rates

   Studies have confirmed that the size of diabetic foot ulcers (DFU) correlates with time required for healing.^24,35,36 As the healing rates for DFU in these studies were broadly comparable, the authors suggest that the use of a DFU healing time curve could be used to predict healing. In a study by Robson,³⁵ healing time curves differentiated clearly between healing and non-healing wounds at 20 weeks, suggesting that plotting the percentage of area reduction over time could be useful in clinical practice.

   Leg ulcer studies have demonstrated that reduction in ulcer area between 2 and 4 weeks of treatment is predictive of time to healing.^22,37-39 In a multicenter, randomized controlled trial of 90 venous leg ulcers over 10 weeks, Arnold et al³⁷ demonstrated that the size of ulcer at baseline was associated with treatment response and time to healing (P = 0.002). This study showed that percentage reduction in wound area after 2 weeks correlated well with treatment outcome (P = 0.004), and time to healing (P = 0.002). When outcome measures were analyzed, only the percentage reduction in wound area after 2 weeks remained statistically significant (P = 0.002). A percentage reduction of >30% during the first 2 weeks of treatment was found to be an accurate predictor of healing.

   These findings were confirmed by van Rijswijk,⁴⁰ who studied 72 leg ulcers, the majority of which were venous. All ulcers were followed-up for a period of 26 months. She also found that a >30% reduction in ulcer area after 2 weeks of treatment was a predictor of treatment outcome (P = 0.016) and time to healing (P = 0.004) and that ulcers exhibiting at least a 30% reduction in wound area were twice as likely to heal (P = 0.0001). However, unlike Arnold et al,³⁷ van Rijswijk⁴⁰ did not find that initial ulcer area significantly influenced treatment outcomes, although a trend was noted. As in previous studies, Philips et al²³ showed that initial ulcer area and duration of ulceration were predictive of leg ulcer healing. This prospective, randomized controlled trial of 165 patients with venous leg ulcers over 12 weeks demonstrated that a reduction in area of greater than 44% at week 3 correctly predicted healing at 12 weeks for 77% of all cases.

   In a study of 104 leg ulcer patients, Kanter and Margolis²⁴ provided evidence that a percentage of change in leg ulcer area determined by digital planimetry after 4 weeks of initial treatment was able to distinguish between healing and non-healing wounds at 24 weeks (P = <0.05). In contrast to other studies, this study found that patients had similar rates of healing during the first 4 weeks of treatment. Although this study used a small percentage change in wound area as its cut off point (3%), it provides evidence of the relevance of percentage of area reduction as a useful measure in the first few weeks of treatment.

   Other wound types exhibit similar findings. van Rijswijk and Polansky²² studied patients with 56 pressure ulcers to compare time to healing with other variables over 2 weeks. A 50% reduction in pressure ulcer area after 15 days was seen, with half demonstrating an 80% reduction in area after 40 days. In a retrospective analysis of healing rates of 10 full-thickness pressure ulcers, Brown⁴¹ observed that plotting healing curves could be useful in clinical practice because they had the potential to indicate the progress of granulation, contraction, and epithelialization. This study supports previous findings that percentage of area reduction rather than absolute area reduction is an important parameter in predicting healing rates and that reduction in pressure ulcer area does not occur in a linear fashion.

   Healing rates are an indication of whether wounds are responding to treatment at a particular point in time, but they should not be used to predict anticipated healing rates. Care needs to be taken not to project healing rates to anticipate future outcomes as variables influencing healing - ie, infection - cannot be controlled. The benefits of plotting wound healing rates are described in Table 2. This information could help establish baseline healing rates for a variety of different wound types, which could facilitate meta-analysis of multiple studies, allow objective comparison of different treatments, and assist in reliable cost benefit analysis.

Clinical Appearance of the Wound Bed

   Clinicians broadly agree that changes in tissue type are indicative of the healing process - ie, the replacement of necrotic tissue with granulation or re-epithelialization. Supportive evidence is found in many clinical studies.^40,42-44 Johnson and Miller⁴⁴ demonstrate that care is required when using objective and subjective assessments to monitor wound progress, as sometimes these two approaches do not correlate and may confuse the inexperienced clinician. This study compared two methods of calculating wound area: 1) freehand tracing with digital planimetry and use of the Kundin Wound Gauge with two subjective assessments, and 2) characteristics of wound bed tissue type and nurses' perceptions of healing. All methods were compared to stereophotogrammetric measurements as a control. Although the measurements of surface area performed well, the subjective scales demonstrated poor reliability; some wounds markedly decreased in size but remained sloughy and others increased in size as a result of debridement and were interpreted as deteriorating.

   Some wound assessment charts use rating scales to quantify the percentage of tissue involved (eg, 25% to 50% granulation tissue). Although in clinical practice this approach has proved useful, no reliability or validity studies have been conducted to support use of this type of scale. In a small study by Mekkes and Westehof,⁴⁵ considerable intra- and inter-rater variations were reported for visual estimation of percentage area of wound bed tissue type, suggesting that visual approximations are too subjective to be clinically valuable. Interestingly, in a random, controlled trial, van Rijswijk⁴⁰ found that the presence or absence of granulation or necrotic tissue and pain did not influence treatment outcomes. However, she noted that leg ulcers with strong odor (P = 0.02) and high levels of exudate (P = 0.009) at initial assessment were less likely to heal, emphasizing the need for comprehensive wound assessment procedures.

Documentation of Wound Healing

   Precise wound measurement is only worthwhile if accurately documented and communicated to the interdisciplinary team. The primary purposes of a wound assessment chart should be to monitor the progress of healing and to detect non-healing wounds early. Wound assessment charts around the world are generally consistent in content, but many have not been rigorously evaluated. The validity or reliability of some pressure ulcer assessment tools such as the Pressure Ulcer Scale for Healing (PUSH)³¹ and the Pressure Sore Status Tool (PSST)³⁰ have been tested and demonstrate that if used carefully they can monitor healing effectively. The PUSH tool devised by the NPUAP has been modified several times as a result of widespread evaluation in a variety of clinical settings and provides a quick way to classify pressure ulcers in relation to surface area, exudate production, and tissue type. Currently, the recommended method of estimating surface area is by multiplying the greatest length and width of the wound, which has the advantage of being performed in all clinical settings without the use of additional resources. However, the precision of the PUSH tool could be increased by using inexpensive, portable digital planimeters that are becoming more widely available in clinical practice. This type of device has the potential to determine true wound area measurement, allowing more accurate classification of the PUSH tool subscore.

   Documentation of these parameters alone will not improve patient outcomes unless practitioners consider the clinical relevance of the information recorded and act upon it. A study by Briggs and Banks⁴⁶ demonstrated that a large proportion of documentation failed to accurately record specific details of either preventive care or treatment of patients with pressure ulcers. This is cause for concern, because legally, documentation constitutes written evidence of clinical practice (ie, if what clinicians do is not documented, it was not done).¹⁷ Other purposes of wound documentation include evaluation of care, protection against litigation, production of data to support applications to insurance payers, and maintenance of license to practice. Many examples of wound assessment charts³¹ demonstrate the characteristics of effective wound documentation.

Conclusion

   Calculation of wound surface area has been shown to be a reliable and valid method of monitoring wound healing. Yet the prediction of healing based on wound measurement is still not fully developed, especially for chronic wounds that are complicated by risk factors that make anticipation of final outcomes difficult. The literature indicates that percentage of area reduction is an important indicator within the first few weeks of treatment that helps differentiate between healing and non-healing wounds. It concludes that a percentage of area reduction of less than 20% to 40% over the initial 2 to 4 weeks is a reliable indicator that the wound is not responding well to treatment. Studies confirm that reassessment of treatment should take place between 2 and 4 weeks. Regular wound area measurements during this time would help avoid inappropriate therapeutic measures and may justify use of advanced treatment modalities in slow healing wounds.

   The level of precision required for wound measurement depends on whether an accurate estimation of size is required for a research study or whether broad indication of a wound's progress is sufficient. For any measurement to be of value, it must be capable of comparison, which can only happen if measurements are made under the same conditions using the exact same procedures. The use of simple wound measurement protocols could improve the accuracy of this important aspect of wound assessment (see Table 3). In order to improve the consistency and reliability of wound measurement, the wound ideally should be measured by the same clinician with the patient in the same position. The optimum time interval between wound measurements is approximately 7 days, as more frequent measurement is unlikely to demonstrate any noticeable or clinically relevant differences in wound dimension. Finally, accurate documentation of all of these parameters in the patient records and use of clinical photography will further improve the accuracy of wound measurement and facilitate communication within the interdisciplinary team.

   No method of wound measurement is perfect, even when used in conjunction with strict protocols. For these reasons, results will always need to be carefully interpreted in the light of clinical practice. Yet the importance of accurate wound measurement and assessment is emphasized by the rising number of clinical negligence cases involving wound mismanagement that have resulted in increased legal claims for compensation globally.⁴⁷

   Studies have demonstrated that conventional contact tracing and square counting are not capable of determining the true area of a wound. The increased availability of inexpensive, portable, digital planimeters capable of automatically converting and displaying measurements into cm² in clinical settings provides a quick, reliable, and precise method of calculating wound area. This type of device has the potential to make wound measurement easier and more accurate so that it could become a feature of routine clinical practice.

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