Diagnostic Validity of Semiquantitative Swab Cultures

Swab cultures of wounds are noninvasive and most laboratories are capable of semiquantitatively processing these specimens. This study examined the diagnostic validity of semiquantitative swab cultures obtained using Levine's technique and compared semiquantitative and quantitative swab cultures. Two swab specimens were obtained from a sample of 44 chronic wounds using Levine's technique. One was processed using quantitative laboratory procedures and the other using semiquantitative laboratory procedures. The diagnostic validity of the findings from each swab culture process was determined by associating the culture findings of each with quantitative tissue cultures (reference standard) using receiver operating characteristic (ROC) curves and by evaluating concordance. The area under the curve (AUC) of the semiquantitative swab culture was 0.639, which was not significantly higher than the .50 diagonal chance (P = 0.0501), suggesting a non-informative test. The AUC for quantitative swab cultures was 0.821. The AUC of the quantitative swab culture was significantly higher than the diagonal chance line of 0.5 and was significantly higher than the AUC of the semiquantitative swab culture (P = 0.0128). The mean concordance of semiquantitative cultures in recovering all organisms was 57%. The mean concordance of quantitative swab cultures in recovering all organisms was 72%. The findings of this study suggest that swab specimens processed using semiquantitative processes do not provide culture findings that correlate well with culture findings from tissue specimens. More meaningful information can be obtained from swab specimens if they are quantitatively processed in the laboratory.