Drawing a blood culture while a patient is receiving high-dose antibiotic therapy can result in a false negative result. This can be avoided by planning when and how to take the blood cultures.
Ways to prevent a false negative blood culture due to antibiotic effect:
(1) Try to draw the blood cultures prior to initiating the antibiotic therapy.
(2) If the antibiotic is given intermittently and in relatively low doses, then consider drawing any blood cultures before the next dose, when the antibiotic concentration in the blood is at its lowest level.
(3) If the blood culture must be drawn while the concentration of antibiotics is relatively high, then consider:
(3a) Diluting the blood by adding it to a large amount of culture media.
(3b) Making sure that the antibiotic media is isotonic (to avoid lysis of bacteria with membranes damaged by the antibiotic effect).
(3c) Using an additive to diminish the effect of the antibiotic (antibiotic absorbing resin, dematiaceous earth, etc).
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