Quantitative vs. Semi-quantitative Measurements of Bacterial Load in Wounds: Assessment of 1053 Data Points from 41 Bacterial Species in a 350-patient Trial

Abstract Body: Introduction: Quantitative tissue biopsy, which provides colony counts per gram (CFU/g) is the gold standard to accurately measure bacterial load in wounds. However, the complexity and cost of this procedure limit its use, thus semi-quantitative cultures are most commonly used world-wide to guide treatment decisions. Herein we compare microbiological findings of quantitative and semi-quantitative culture of 41 bacterial species from 350 chronic wounds to assess the correlation between the two methods. Methods: Biopsies (n=428, gathered across 14 clinical sites) were assessed via quantitative and semi-quantitative cultures. Total bacterial load (TBL) was quantified as colony-forming unit per gram (CFU/g). SQ results were obtained by serial dilution of biopsy onto a culture plate divided into four quadrants representing occasional, light, moderate, and heavy growth. Results: 1053 data points were analyzed. Linear regression analysis demonstrated a statistical correlation of r = 0.85 between semi-quantitative and quantitative measures where semi-quantitative of moderate to heavy growth corresponded to TBL >105 CFU/g. Distribution of the TBL corresponding to the semi-quantitative categories was statistically analyzed using Mann Whitney Wilcoxon tests. Distinct semi-quantitative categories for TBL were observed. However, occasional growth and light growth each spanned 3-logs (102-105 CFU/g and 103-106 CFU/g, respectively), while moderate and heavy growth spanned 4 logs (104-108 CFU/g and 105-109 CFU/g). Conclusions: These findings indicate that although quantitative culture analysis was strongly correlated with semi-quantitative growth categories, the overlap in range of CFU/g per SQ category can confound appropriate antimicrobial treatment decisions.