HSR&D Home » Research » NRI 01-005 – HSR&D Study
The Diagnostic Validity of Three Quantitative Swab Techniques
Sue E. Gardner, PhD RN
Iowa City VA Health Care System, Iowa City, IA
Iowa City, IA
Funding Period: July 2001 - June 2004
Swab cultures are often used to examine the bacteriology of suspicious wounds. Nonetheless, the accuracy of swab specimens has been questioned and many clinicians believe they are not useful for guiding treatment decisions. Furthermore, little standardization exists to guide specimen collection and it is unclear which technique represents "best practice." Theoretically, the technique that most accurately reflects wound "tissue" bioburden rather than surface contamination would provide the most clinically meaningful information. Undiagnosed and untreated localized infection of chronic wounds acts to prolong healing, increases the likelihood of complications, and extends human suffering. Similarly, the indiscriminate treatment of chronic wounds based on faulty culture specimens can also adversely affect patient outcomes, consume unnecessary resources, and lead to inappropriate use of antimicrobial agents. Examination of the diagnostic validity of three commonly used swab culture techniques would clarify their role in monitoring wounds for infection and, if valid, would provide data from which to direct swabbing technique practices. In this way, the most economical and effective means of identifying infection can be delineated, swab techniques standardized, and the timely diagnosis of infection improved.
The purpose of this study is to examine the diagnostic validity of three different quantitative swab techniques in identifying chronic wound infection. The primary research questions to be addressed are: 1. What are the sensitivity, specificity, predictive value, and accuracy of quantitative cultures obtained using each of three different swab techniques? 2. Are there differences in accuracy among the three swab techniques? 3. What is the concordance between qualitative cultures of each of the three swab techniques and qualitative cultures of tissue specimens with regard to recovery and identification of isolates?
Swab specimens were obtained from the chronic wounds using 1) wound exudate, 2) broad Z-technique, and 3) Levine technique (Levine, Lindberg, Mason, & Pruitt, 1976). Acquistion of swab specimens was followed with a wound tissue biopsy. The diagnostic validity of each technique will be determined by associating the quantitative culture findings of each swab specimen with the culture findings of concurrent tissue specimen (gold standard). The culture findings are categorized as negative or positive. Positive cultures are defined as those with 1,000,000 or greater organisms per gram of viable wound tissue or swab,, or containing b hemolytic Streptococcus at any level. The "true" infection status of the wound is defined as the culture findings of wound tissue. Sensitivity, specificity, predictive value, and accuracy will be calculated for each swab technique. Differences in accuracy among the three swab techniques will be examining the width and overlap of 95% confidence intervals constructed around these measures of accuracy. As an alternative to the specified criteria for positive culture (i.e., 1,000,000 CFU per gram of tissue or swab), sensitivity and specificity will also be evaluated for various cut-values. The receiver operating characteristic (ROC) curve for each swab technique will be plotted and the area under the curve will be calculated and compared. In addition, the concordance between the tissue culture and each swab culture will be examined using only qualitative culture findings (i.e., organisms recovered).
Preliminary analyses of 78 study wounds found that Levine’s performed best of the three swab techniques in terms of all four validity parameters. Sensitivity was .70, specificity .90, positive predictive value .79, and accuracy .81. Preliminary data analysis also reveals that Staphylococcus aureus is present in over 50% of the wounds of these subjects. Furthermore, the presence of Staphylococcus aureus is significantly related to number of organisms per gram of tissue (Fisher’s exact; p = .000). Finally, preliminary analyses have shown that persons with diabetes and infected wounds express erythema less frequently than persons without diabetes.
The findings from this study would be widely applicable throughout the VA given the large numbers of older, diabetic veterans with chronic wounds, who are at risk for developing wound infection. Similar to the health care system in general, there is wide variation within the VA in the methods used to diagnose wound infection.
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DRE: Diagnosis, Treatment - Observational
Keywords: Nursing, Quality assurance, improvement
MeSH Terms: Diagnosis and Prognosis, Chronic Disease, Treatment, Quality of Health Care