Blood cultures should be drawn prior to the institution of antibiotics whenever possible. If empiric treatment is an emergency, blood cultures should still be drawn as soon as possible after institution of antibiotics. There are no data to suggest that the timing of culture in relation to the appearance of fever or chills will maximize the yield.
GUIDELINES FOR BLOOD CULTURE COLLECTION
VOLUME OF BLOOD PER SET
There is a direct relationship between the volume of blood obtained and the yield of a blood culture set. Forty to 60 ml of blood should be obtained per episode (in other words, 2-3 sets with 20 ml per set, and 10 ml per bottle).
NUMBER OF SETS OF BLOOD CULTURES
Single sets should not be used to evaluate any patient with suspected bacteremia or candidemia. The optimal yield is obtained with three or four sets of blood cultures. No more than four blood cultures should be obtained for any given 24 hour period.
SITE OF BLOOD CULTURE
Blood should be obtained from peripheral venous or arterial sites. Obtaining blood cultures from central venous catheters, arterial lines and inguinal vessels increases the likelihood of obtaining a false positive blood culture.
The practice of drawing blood for culture from catheters or the groin should never be performed when a peripheral (i.e., non-catheterized) site is available.
Labeling the site of each set of blood cultures, particularly regarding whether a set was drawn from a catheter, the groin, or not, is of utmost importance in helping to distinguish pathogens from contaminants in those cases in which no peripheral access can be found.
PREPARATION OF THE SITE FOR CULTURE
- After the vessel site is selected, a 5 cm area of skin should be disinfected by swabbing concentrically with 70% alcohol, from the venipuncture site outward.
- The site should be cleansed once again, this time with 2% chlorhexidine gluconate again in a circular motion.
- Allow the chlorhexidine to dry completely before performing venipuncture. This should take 1 – 2 minutes.
- While waiting for the site to dry, the plastic cap covering each blood culture bottle should be removed, and the rubber stopper should be decontaminated with 70% alcohol.
- 20 ml of blood should be withdrawn from the puncture site.
- Do not change needles between venipuncture and inoculation of the bottles, or between bottles. The risk of needlestick is increased, while the chance of contamination is not significantly lessened.