The use of the arterial line as a source for blood cultures

Abstract

Objective: To determine the reliability of blood cultures obtained through indwelling arterial lines as compared to that of blood cultures obtained by venipuncture.

Design: A prospective observational study.

Setting: Six-bed mixed medical surgical intensive care unit (ICU) of a 550-bed university-affiliated medical center.

Measurements: During a 3-month period blood culture sets, when clinically indicated, were drawn in parallel from indwelling arterial catheters and one-time venipuncture and the results compared. Each blood sample consisted of 15 ml and was distributed equally between three blood culture bottles: aerobic, anaerobic and one aerobic resin-containing bottle. Blood culture results from the two sources were compared according to preset definitions.

Main results: During the study period 90 parallel blood culture sets (540 bottles) were obtained from 36 patients. Forty-three (16%) venipuncture bottles were positive versus 88 (32%) arterial line culture bottles (p<0.001). Of the parallel sets, 83% yielded equivalent results – either both sterile or both growing the same organism. Amongst the discordant sets, the arterial line cultures grew 37 gram-positive and 18 gram-negative isolates not found in venipuncture sets (i.e. 50% of 109 arterial line isolates), while only two gram-positive isolates were solely grown in venipuncture cultures (4% of all 55 venipuncture isolates, p<0.001). On clinical correlation, all the gram-positive organisms in the discordant cultures were found not to reflect bacteremia, while five of the 18 gram-negative isolates (28%) grown only in arterial line cultures probably did reflect ongoing bacteremia.

Conclusion: The results of blood cultures taken from the arterial line are frequently equivalent to those taken from venipuncture. When discordant, the growth of gram-positive bacteria almost certainly reflects contamination or arterial line colonization, whereas the growth of gram-negative bacteria may have to be considered as reflecting bacteremia.