Comparison of contamination rates of catheter-drawn and peripheral blood cultures

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Summary

The aim of this study was to assess the sensitivity and specificity of catheter-drawn and peripheral blood cultures. Paired blood culture samples collected over a 44-month period from a 280 bed Brisbane metropolitan hospital were analysed, using standard clinical and microbiological criteria, to determine whether blood culture isolates represented true bacteraemias or contamination. Catheter-collected cultures had a specificity of 85% compared with 97% for peripheral cultures. In only two instances (0.2%) was the diagnosis of clinically significant bacteraemia made on the basis of catheter culture alone. This study concluded that catheter-collected samples are not a good test for true bacteraemia, and that peripheral cultures are more reliable when the results of the paired cultures are discordant.

Introduction

Contamination of blood culture specimens is a common and costly problem.1 Some studies have demonstrated that bacterial contamination is higher for catheter-collected specimens,2, 3, 4 while others suggest that there is no significant difference.5, 6, 7, 8 Further studies are required to resolve this. This study compared the sensitivity, specificity and positive and negative predictive values of peripheral and catheter-drawn cultures.

Section snippets

Methods

Paired peripheral and catheter-drawn blood cultures were assessed as being true or false positives using retrospective chart review and microbiology data. Analysis was made of the contamination rate, true bacteraemia rate, sensitivity, specificity and positive and negative predictive values.

The ‘Kestral’ pathology database of the Mater Hospital, Brisbane was searched for all blood cultures for which the site of collection was specified between January 1998 and August 2002. In total, 8444 blood

Results

Of the 962 pairs of blood cultures that were included in the study, the culture results were concordant in 797 pairs. In 659 pairs, both cultures were negative, and in 138 pairs, both cultures were positive for the same organism. Of the 165 discordant pairs, 132 had negative peripheral venous cultures and positive catheter cultures. Seven of these were determined as true bacteraemias while 125 were determined to be contaminants. The remaining 33 discordant pairs tested positive from the

Discussion

False-positive blood cultures are a major problem for clinicians and pathologists. The optimal strategy to minimize false positives remains controversial, as the results of studies have been conflicting. In this study, a large retrospective analysis of blood cultures was performed. The study found that catheter-drawn blood cultures have a low specificity and positive predictive value compared with peripheral cultures. The sensitivities were similar for the two groups.

These findings are in

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