Are Blood Tests for Infection Accurate If They are Taken from Patients' Intravenous Lines?

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The summary below is from the full report titled "Clinical Utility of Blood Cultures Drawn from Indwelling Central Venous Catheters in Hospitalized Patients with Cancer." It is in the 2 November 1999 issue of Annals of Internal Medicine (volume 131, pages 641-647). The authors are J.A. DesJardin, M.E. Falagas, R. Ruthazer, J. Griffith, D. Wawrose, D. Schenkein, K. Miller, and D.R. Snydman.

What is the problem and what is known about it so far?

Many sick patients have central intravenous lines (central IVs) so that they can receive medicines or fluids. Central IVs are long, thin tubes that are put into large veins, usually in the neck or upper chest area. They can stay in place longer than the regular types of IVs, which are usually placed in the smaller veins of the arms. When patients need blood tests, it is possible to draw blood samples from central IVs. Doing this can help avoid multiple painful needle sticks. However, experts have discouraged the use of central IVs to take samples for blood cultures, which are done to look for infection in the bloodstream. The concern is that blood cultures from central IVs may be "contaminated." This means that they may suggest infection when there really is no infection.

Why did the researchers do this particular study?

The researchers wanted to see whether blood cultures taken from central IVs tell accurately whether patients have a blood infection.

Who was studied?

185 patients who had central IVs and who had blood cultures at some point during their hospitalization on a cancer ward of a large hospital in Boston, Massachusetts.

How was the study done?

The researchers studied 551 pairs of blood cultures in these 185 patients. One sample in each pair was taken from a central IV and, within 4 hours, the other one was taken directly from a vein. The researchers then determined how often one of the paired cultures suggested infection and the other suggested no infection; how often both suggested infection; and how often both suggested no infection. Two infectious disease experts then looked at all of the information in each patient's records and decided whether the patient really had a bloodstream infection. Although these experts had the results of the blood cultures, they did not know which results came from central IVs and which came directly from veins.

What did the researchers find?

Negative tests for infection (no bacteria), whether taken from a central IV or directly from a vein, were very accurate indicators that the patient had no bloodstream infection. However, a test from a catheter that showed bacteria was not quite as accurate as a positive test from a vein, since in some cases a test from a catheter that suggested infection was not correct.

What were the limitations of the study?

This study only included cancer patients at a single hospital. These results may not apply to other types of patients at other hospitals.

What are the implications of the study?

Blood tests looking for infection taken from central IVs in cancer patients may give accurate results, particularly when they show no infection, and thus can help to avoid painful needle sticks. However, if tests taken from central IVs suggest that there might be an infection, additional tests may be necessary to confirm that the patient really does have an infection in the blood.