Are Semi-Quantitative Clinical Cultures Inadequate? Comparison to Quantitative Analysis of 1053 Bacterial Isolates from 350 Wounds.

Thomas E Serena,Philip G Bowler,Gregory S Schultz,Monique Y Rennie,Anna D'Souza

doi:10.3390/diagnostics11071239

Thomas E Serena, Philip G Bowler + Show 3 more

Open Access

https://doi.org/10.3390/diagnostics11071239

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Abstract

Early awareness and management of bacterial burden and biofilm is essential to wound healing. Semi-quantitative analysis of swab or biopsy samples is a relatively simple method for measuring wound microbial load. The accuracy of semi-quantitative culture analysis was compared to ‘gold standard’ quantitative culture analysis using 428 tissue biopsies from 350 chronic wounds. Semi-quantitative results, obtained by serial dilution of biopsy homogenates streaked onto culture plates divided into 4 quadrants representing occasional, light, moderate, and heavy growth, were compared to total bacterial load quantified as colony-forming units per gram (CFU/g). Light growth, typically considered an insignificant finding, averaged a clinically significant 2.5 × 105 CFU/g (SE = 6.3 × 104 CFU/g). Occasional growth (range: 102–106 CFU/g) and light growth (103–107 CFU/g) corresponded to quantitative values that spanned a 5-log range; moderate and heavy growth corresponded to a range of 4-log and 6-log, respectively, with a high degree of overlap in range of CFU/g per category. Since tissue biopsy and quantitative culture cannot be widely practiced and semi-quantitative analysis is unreliable, other clinically relevant approaches are required to determine wound bioburden and guide best management practices. Fluorescence imaging is a point-of-care technology that offers great potential in this field.

Highlights

Elevated levels and diversity of bacteria characterize chronic wounds, prolong healing, and increase risk of infection
Treatment selection is reliant on clinical expertise and, if needed and available, diagnostic information obtained from wound sampling and/or other methods
Semi-quantitative microbiological analysis of wound swab samples is widely used and is relatively inexpensive compared to biopsy samples and fully quantitative microbiological analysis methods

Summary

Introduction

Elevated levels and diversity of bacteria characterize chronic wounds, prolong healing, and increase risk of infection. Wound care providers estimate bacterial load by a combination of clinical examination of clinical signs and symptoms and semi-quantitative cultures of wound samples. Studies have demonstrated poor sensitivity (~20%) of clinical signs and symptoms (CSS) in detecting elevated levels of bacteria [13,14,15,16,17], even for Pseudomonas aeruginosa [18], which is known for its hallmark symptoms. Quantitative analysis performed on tissue biopsies is considered to be the ‘gold standard’ for detecting high microbial load in chronic wounds; the majority of wound care practitioners are not able to take wound biopsies, and microbiological analysis is a more time consuming and costly procedure than semi-quantitative swab analysis [19,20,21]. Semi-quantitative microbiological analysis is lauded as being more cost effective and has gained widespread use world-wide [22]

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