Sir.—Clarifying the meaning of a blood culture from which a microorganism has been recovered (a positive blood culture) is a frequent reason for requesting an infectious disease consultation. The essence of the question usually being asked is whether recovery of the organism represents a true bacteremia (or fungemia) or a contaminant. Since we cannot always clearly differentiate between these two possibilities, this guide is meant to be a first step in the analysis of unclear cases. The response to this dilemma is simplified by estimating probability based on the appropriate data. This approach was developed for the analysis of clinical research data but has proven to be equally useful in practice.
Most frequently, the probability that an organism recovered from a blood culture represents a true infection is close to 100%, or else it is clearly a contaminant. A consultation is not generally requested in such obvious cases, and