Special Articles: Review of Literature: General Infectious Diseases
Differential Time to Positivity: A Useful Method for Diagnosing Catheter-Related Bloodstream Infections
Differential Time to Positivity: A Useful Method for Diagnosing Catheter-Related Bloodstream Infections Raad I et al. Ann Intern Med. 2004;140:18
The study was conducted at the MD Anderson Cancer Center in Houston to determine if the difference in time to positive blood culture results between blood from peripheral vein versus central catheter could be used to accurately distinguish catheter-related bloodstream infections (CRBIS). Quantitative cultures were done as well. Care was taken to inoculate the media simultaneously. Catheters were removed aseptically if infection was suspected or if they were not needed. The 5-cm segment was cultured by semiquantitative roll-plate method. The study included 6138 simultaneous blood cultures, and the analysis was restricted to 191 who had positive results for the same organism in both the peripheral and catheter cultures. The breakdown of the results of cultures is shown in the following table:
The definition of catheter-related bacteremia was adopted from IDSA guidelines (Clin Infect Dis. 2001;32:1249) based on a significant catheter-tip colonization (at least 15 CFUs) for the same organism in both cultures or quantitative blood cultures that yielded at least 5-fold greater bacteria in blood drawn through the central venous catheter. The results are shown in the following table based on a time differential for positive growth that was greater or less than 120 minutes.
The authors conclude that cultures become positive more rapidly from catheter-drawn specimens with line-associated sepsis, and 120 minutes is the threshold for this difference which shows a high degree of sensitivity and specificity.
Comment: There are several important observations in this study including some that were pointed out in the editorial by Barry Farr from Virginia (Ann Intern Med. 2004;140:62):
© 2004 Lippincott Williams & Wilkins, Inc.
- The issue has important and practical application in patients with long-term catheters due to the difficulty and expense for replacement. Thus, the ability to avoid unnecessary catheter removal is highly desired.
- It is important to note that among the 6000 cases with suspected catheter-related bacteremia, only 3% had this diagnosis verified.
- The differential time to positive results applies only to patients with positive results in both cultures with the assumption that media were inoculated simultaneously with specimens from both sources.
- Blood cultures that yield Candida or S. aureus are likely to be true positives, but one or even both cultures that grow coagulase-negative staphylococcus are often contaminates. This applies when 2 cultures grow this organism based on molecular typing (Am J Med. 2000;109:697; Infect Control Hosp Epidemiol. 2000;21:213).
- The results in this study showed that specificity of time differential was lower in a subset of patients who had received antibiotics. This result is unexplained.
- Multiple studies have shown that simultaneous quantitative blood cultures yield 5 to 10 times the number of colonies in specimens from the catheter versus peripheral vein in catheter-related infections (Eur J Clin Microbiol Infect Dis. 1989;8:142). The method described here represents an alternative to this tedious and expensive quantitative method which is infrequently offered by clinical laboratories.