Dressings and Topical Agents for Chronic Wounds - NIHR Health ...

Methods8Assessment of outcome measuresA single standard outcome measure for woundhealing does not exist. Both objective and subjectivemeasures are widely used by researchers,but little effort has been made to determine thevalidity of many of these measurements.Comfort, ease of application, ease of removal,exudate, handling and cosmesis are frequentlyused measures of dressing performance, butthey are not validated outcomes on which tobase decisions of effectiveness. In this reviewthe most commonly validated outcome measuresencountered were based on wound healing. Theunambiguity of complete healing and its importanceto clinicians and patients alike (because of itspotential impact on quality of life and burden ofcare), make it the preferred outcome measure withwhich to compare studies of clinical effectiveness.In accordance with the peer-reviewed protocol,subjective outcome measures such as visual assessmentsof oedema, erythema, granulation and pusand debris were not included unless the authorsassessed their validity. We were unable to find anystudy that validated the subjective outcomemeasures reported.Objective measures of healing are usually based onwound area. Planimetry, often aided by computeranalysis, is the most frequently used method ofcalculating wound area, though other methods,such as the measurement of wound diameter orweight of a tracing drawn around the area of thewound, are also used.Measurements of wound volume are infrequentlyreported in the literature; these methods areoften cumbersome and their accuracy has not beenproven. 32 Computerised image analysis may in thefuture, as the equipment becomes more affordableand portable, prove to be a useful technique forthe assessment of wound volume.Even though objective measures reduce oreliminate subjective biases and reduce randommeasurement errors, they have certain inherentbiases if the patients being compared havewounds with different baseline size.A change in wound area is often expressed as thepercentage change, which unlike the absolutechange in area, takes into account the initial sizeof the wound. For two wounds healing at the samelinear rate (as measured by diameter reduction)percentage area calculations will show a largerchange for a small wound than for a big wound. Theconverse is true when the absolute change in areais measured, as for any unit reduction in woundradius, a bigger area reduction will occur for a largewound. This has important consequences for thevalidity of trial results where there is poor comparabilityin initial wound size at baseline between thetreatment groups. This is illustrated in Table 1.In large trials, randomised allocation shouldensure that the mean wound size and variancein each group is similar. In a small trial randomallocation is unlikely to result in an even distributionof wound sizes. This problem will persistin small trials, even when the average wound sizeappears to be comparable between groups, becausethe distribution of wound sizes about the mean islikely to differ. This is illustrated in Table 2.In a trial where there is poor comparabilitybetween groups for wound size at baseline, andTABLE 1 Percentage and absolute measures of wound healingcan give different results for relative effectivenessGroup AGroup BBaseline mean area (cm 2 ) 50 60Follow-up mean area (cm 2 ) 35 43Mean of % reduction in area 30 28Mean absolute reduction inarea (cm 2 ) 15 17Using % change, wounds in group A appear to have healedmore rapidly than those in group B. The converse is true whenthe outcome is expressed as absolute change in wound areaTABLE 2 Groups with similar means may have differentdistributionsGroup CGroup DWound size at baseline (cm 2 ) 10,10,30,30 4,4,4,70Mean area (cm 2 ) 20 20.5Standard deviation (SD) 11.5 33Groups C and D have approximately the same meanarea. If both groups of wounds heal at the same rate (thetreatments are equally effective) it could be expected that thethree small wounds in group D will heal before those in groupC.Therefore measuring outcome based on the number ofwounds healed within a certain time period will be biased.Similarly, the percentage change in area will appear greaterin group D, while the absolute change in area will appeargreater in group C