Volume Measures Using a Digital Image Analysis System are Reliable in Diabetic Foot Ulcers
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Depth was measured using the VeVMD “Line Tool,” which fits a straight line along the wound depth indicator. The “Zoom” feature in VeVMD was utilized, when necessary, to visualize the edges of the ulcer margin more clearly during measurement. VeVMD estimates volume using the mathematical formula for an elliptical spheroid, which is a function of wound depth and circumference. Wound volume measurements, as well as circumference, surface area, and depth, were recorded on case report forms.
Data collection procedures. Wound images were obtained, as described above, at baseline and every 2 weeks until: 1) the ulcer healed; 2) self or study withdrawal; or 3) 6 months of follow-up had been completed and stored on the study laptop. For this study, only the wound image from baseline was used. The “raters” for this study were 4 different research team members who were trained in: 1) wound assessment, including defining wound margins; 2) obtaining digital images; and 3) completing ulcer measurements using VeVMD software. Detailed study protocols for completing wound size measurements were available for consultation at all times.
Each rater independently completed measures of wound size from the baseline image (Time 1). The order in which each of the raters measured the wound images was generated using a random sequence table. Two weeks after the initial measurement, each rater repeated measurements (Time 2) of the wound images using a different randomly generated order than was used the first time. To minimize bias, each rater deleted his or her ulcer outline and depth line from the selected image on VeVMD before the next rater measured the ulcer.
Analyses of inter- and intra-rater reliability. The units of analysis for the inter- and intra-rater reliability analyses were volume measurements at Time 1 and Time 2. Inter-rater analyses were performed using Time 1 volume measurements. Inter-rater reliability was defined as the correlation between volume measurements from 2 different raters for the same ulcer, at the same time (Time 1). The analyzed data consisted of 4 measurements for each ulcer, corresponding to the 4 different raters. A summary estimate of the inter-rater reliability was computed by averaging the correlations corresponding to each possible pair of raters. An estimated standard error (SE) for the summary inter-rater reliability estimate was obtained using bootstrap methods9; bootstrap samples (B = 2000) were drawn, treating ulcers as the sampling units.
Intra-rater reliability was defined as the correlation between Time 1 and Time 2 volume measurements made by the same rater on the same ulcer, using the same wound image for both measurements, which had been obtained at the baseline study visit. A summary estimate of the intra-rater reliability was computed by averaging the 4 within-rater correlations, and standard errors were obtained using bootstrap methods, treating ulcers as the sampling units. Analyses were performed in SAS, Version 9.2 (SAS Institute Inc, Cary, NC).
A total of 34 subjects from the primary study had full-thickness DFU and were eligible for inclusion in this study. Compared to the others, 1 subject had an extremely large ulcer. This subject was excluded from further analyses because the ulcer could influence the results of the study much more than the other ulcers. Of the remaining 33 subjects, the mean age of subjects was 54.7 years (± 11.27). Subjects were predominantly white (90.9%), non-Hispanic (93.9%), and male (90.9%).