Positive news: Culture results a matter of time

Timing culture results to prevent BSIs

Radd I, Hanna HA, Alakech B, et al. Differential time to positivity: A useful method for diagnosing catheter-related bloodstream infections. Ann Intern Med 2004: 140:18-25

Diagnosing potentially deadly central venous catheter–related bloodstream infections may be difficult, but the authors found that a lot of it may be a matter of timing.

Differential time to positivity of at least 120 minutes between centrally and peripherally drawn blood cultures helps diagnose catheter-related bloodstream infection, especially in critically ill patients, the authors reported. Accurate diagnosis of such infections results in proper management of patients and in reducing unnecessary removal of catheters.

This prospective study from a tertiary care cancer center examined 191 infections with the same organism detected from simultaneously drawn central and peripheral blood cultures. Catheter-tip colonization or quantitative blood cultures defined catheter-related bloodstream infection. When the culture drawn from the catheter became positive at least 120 minutes earlier than the peripherally drawn culture, the odds of catheter-related bloodstream infection increased almost sixfold.

The study included all patients over a one-year period that had the same organism isolated from blood cultures drawn simultaneously through the central venous catheter and the peripheral vein. The key measurement was the time necessary for the blood cultures from the central venous catheter and the peripheral vein to become positive, as well as other relevant patient information.

As a diagnostic tool for catheter-related bacteremia (using a composite definition reference standard according to the Infectious Diseases Society of America guidelines), differential time to positivity of 120 minutes or more was associated with 81% sensitivity and 92% specificity for short-term catheters and 93% sensitivity and 75% specificity for long-term catheters. "Differential time to positivity of 120 minutes or more is highly sensitive and specific for catheter-related bacteremia in patients who have short- and long-term catheters," the authors concluded.