Quantification of Mouse Total Body Surface Area: Implications for Preclinical Burn Research

Clinically, burn severity is reported as the size (and depth) of burn wounds relative to total body surface area (TBSA). This nomenclature is also often used in rodent models of burns. Accordingly, accurate determination and reporting of rodent TBSA is required to ensure the rigor and reproducibility of preclinical burn research. Rodent TBSA is typically estimated indirectly as a function of body mass. Further, empirical quantification of rodent TBSA through pelt dissection does not consider differences in rodent and human anatomy, making comparison of relative burn size in rodents and humans a challenge. Here, we compared commonly used approaches to directly determine or indirectly estimate rodent TBSA to demonstrate the impact different approaches can have on the calculation of relative burn size. A total of n=48 C57BL/6J background mice (55% male) ranging from 4 to 45 weeks of age and 17 to 40 grams were used. Mice were weighed prior to euthanasia. After euthanasia, mouse length was measured from the nose to anus. Mice were then placed into clear polypropylene sheet protectors (21.6 x 27.9 cm) to trace the areas of both the dorsal and ventral surfaces as well as all four limbs (dorsal-ventral (DV) tracing). Next, the pelt was carefully excised from the body through cutting a lateral line from the mouth to the genitalia, then again proximally to distally on all four limbs. The pelt was gently placed on a sheet protector and traced when both relaxed and stretched. The ears and tail were removed and traced separately. Photographs were taken of all tracings next to a ruler for scale and analyzed in ImageJ. Stretched pelt measurements of TBSA were 34% (79.4±7.6 vs. 57.5±7.5 cm ² , P<0.001) and 30% (70.6±10.9 vs. 52.7±8.1 cm ² , P<0.001) greater than relaxed pelt TBSA measurements in male and female mice respectively. TBSA estimated by DV tracing was 9% greater in males (62.5±10.9 vs. 57.5±7.5 cm ² ) and 15% in females (60.6±12.3 vs. 52.7±8.1 cm ² ) compared to TBSA measurements made on relaxed pelts. Accordingly, empirically derived Meeh constants ( k ) from DV tracing were greater than those derived from relaxed pelt measurements for both males (7.14±0.59 vs. 6.58±0.72) and females (7.72±0.58 vs. 6.78±0.80). In contrast k values derived from stretched pelt measures of TBSA were significantly greater than those determined in relaxed pelts for males (8.91±0.87 vs. 6.58±0.72, P<0.001) and females (8.85±1.25 vs. 6.78±0.80, P>0.001). The combined ears and tail represent approximately 7% and 8% of the TBSA measured by the relaxed pelt approach, respectively. Exclusion of the tail and ears from the calculated TBSA results in derived k values that are ∼16-17% lower. The approach used to determine TBSA in mice significantly influences measured areas and thus derived k values. We suggest that stretching the pelt prior to tracing inflates TBSA values, where measurements made from relaxed pelts or by DV tracing likely provide more accurate estimates of actual TBSA. Further, exclusion of the tail and ears (the latter of which is not typically considered in estimates of TBSA in humans) may be a useful approach relating relative burn sizes of mice to those of humans.