Key Point:

• The time between when blood samples taken through a central venous catheter and from a peripheral vein test positive for the same organism can help diagnose catheter-related bacteremia.

HOUSTON—Semiquantitative and quantitative catheter culture, the diagnostic gold standard for central venous catheter–related bloodstream infection, is accurate. However, it is time-consuming and requires that the catheter be taken out. If the results are negative, the patient has been subjected to the discomfort of catheter removal and reinsertion for nothing.

To avoid this, investigators have sought a more convenient method for detecting catheter-related bacteremia. One may finally have been found. This method, termed the differential time to positivity, requires only that simultaneous blood samples be obtained, when the infection is suspected, through both the central venous catheter and a peripheral vein; the samples are then cultured for organisms, and the results compared. A catheter-related bacteremia is highly likely to be present if a blood sample from the catheter tests positive at least two hours before a peripheral sample confirms infection with the same organism.

AN OPPORTUNE TIME

Why at least two hours? “That is the time differential with the best combination of sensitivity and specificity,” explained Hend A. Hanna, MD, who co-authored a recent report describing the new diagnostic method.[1] According to Dr. Hanna, who is an Assistant Professor of Infectious Diseases, Infection Control, and Employee Health at the University of Texas MD Anderson Cancer Center in Houston, a differential time of two hours or more has a sensitivity of 81% and a specificity of 92% for infections resulting from short-term (less than 30 days) central venous catheter use.

This cutoff had previously been shown to be an effective means of identifying infections associated with long-term use (30 days or more) of a central venous catheter. The study by Dr. Hanna and her colleagues confirmed this finding; in their analysis, a differential time of two hours or more had a sensitivity of 93% and a specificity of 75% for such infections.

Thus, the differential time to positivity appears to identify patients with catheter-related bloodstream infections, regardless of how long the device has been in place. There seems to be a limitation with this method, though: Specificity fell dramatically—to 29%—among a small number of patients who were receiving antibiotics at the time the blood samples were drawn. Sensitivity remained high in these patients, however.

STAPH INFECTIONS COMMON

The study looked prospectively at 191 cases of bacteremia in which the patient had simultaneous blood samples drawn periodically through a central venous catheter and from a peripheral vein; to be included in the study, the patient had to have clinical signs of infection, and cultures of samples from the two sources had to reveal the same organism. For the infection to be considered catheter-related, either of the following conditions had to be met:
• A catheter-tip culture had to show 15 or more colony-forming units (CFUs) of the same organism isolated from the peripheral sample.
• The sample drawn through the catheter had to have five or more times as many CFUs as were found in the peripheral sample; the same organism had to be isolated from both samples.

Based on this definition, 108 cases were found to be catheter-related infections; the other 83 infections were not. Three quarters of the catheter-related infections and two thirds of the other infections were caused by gram-positive organisms, usually staphylococci; however, a higher proportion of the non–catheter-related infections resulted from gram-negative bacilli.

More than 90% of the catheter-related infections and 70% of the other infections were associated with silicone percutaneous catheters. On the other hand, 19% of the non–catheter-related infections occurred in patients with rigid-wall polyurethane catheters.

Dr. Hanna and her research team found that the best way to distinguish the catheter-related infections was with a differential time to positivity of two hours or more. “The differential time to positivity should be incorporated into the diagnosis of catheter-related bloodstream infection,” she said. “But clinicians ultimately have to rely on their clinical judgment when deciding whether to remove a central venous catheter due to suspicion of infection,” she stressed.

—Timothy Begany

Reference
1. Raad 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.

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