Guide to Interpretation of Positive Blood Cultures



I arrange the potential Gram stain results that one can be called w/ as follows: Gram(+) cocci, Gram(+) rods, Gram(–) rods, Gram(–) cocci, yeast. Gram(+) cocci are grouped by ‘morphology’ since the lab usually tells you this: clusters, pairs, chains, etc. Fill in with orgs.



The orgs are deliberately ordered this way: Gram(+) orgs are often [skin] contaminants, Gram(–) orgs & yeast are not. Remember that clinician adjudication is the ‘gold standard’ for deciding what is a contaminant!



Now, w/ the bugs organized, when you get that call from the micro lab, here are 3 questions to ask yourself/the lab:

1) Number of positive bottles/cultures and time to positivity?

2) ‘Shape’ of the bacteria?

3) Aerobic or anaerobic bottle?



Fewer positive cultures & longer time to positivity suggests a contaminant. Apply Q1) to Gram(+) orgs. Time to positivity tough to interpret unless extreme (ex. 8h v 48h). Can use Number of positive cx fact to your advantage – before abx, obtain more cx & increase the denominator!



Apply Q2) to Gram(+) rods since their shapes are so distinct. There are some uniquely shaped Gram(–) rods too, but rare (think Fusobacterium). Here’s a comparative chart of GPRs to illustrate. A great reason to go to micro lab and review the Gram stain!



Apply Q3) to Gram(+) rods & perhaps Gram(–) rods too. For GPRs, preferential growth in the aerobic v anaerobic bottle helps organize the ‘shape’ chart. Ex: while you await speciation for that aerobic box car shaped GPR, these clues suggest Bacillus, usually a contaminant!



For Gram(–) rods, the pearl is that Pseudomonas is a ‘strict’ aerobe and ought to grow preferentially in the aerobic bottle – thus, a GNR that grows in the anaerobic bottle first is less likely Pseudomonas. Of course, always exceptions to these pearls, so await speciation!



Dr. Varun Phadke @ https://twitter.com/VarunPhadke2



#BloodCultures #BCx #Positive #Interpretation #Contaminant #Contamination #Interpretation #Laboratory #Microbiology #Diagnosis