Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1322 page)

   Evaluation of serious localized infections, such as pneumonia, UTIs, and meningitis. Classic signs and symptoms may be absent in infants, the elderly and patients with certain medical or surgical conditions.

   
Methods:

   Most commercially available blood culture systems recommend inoculation of blood into two broth media: one aerobic and one anaerobic. Lysis– centrifugation methods may be used for routine detection of BSIs due to bacteria or yeast but are more typically used for detection of mycobacteremia or fungemia.

   
Turnaround time:
Generally, incubation for 5–7 days. Most true-positive blood cultures become positive within 24–48 hours after inoculation.

   Special Collection and Transport Instructions

   Decontamination of the collection site is the most important factor in preventing false-positive (contaminated) blood cultures. Inoculate media according to the manufacturer’s instructions. Usually, 8–10 mL of blood is inoculated into each blood culture bottle. A smaller inoculum volume, based on weight or age, is recommended for small children. Submission of two or three independently drawn (different venipuncture sites) blood cultures is recommended for the initial evaluation of patients with suspected BSI. Transport blood cultures to the laboratory at room temperature.

   Interpretation

   
Expected results:
No growth

   
Positive results:
Bacteremia or fungemia present. Positive blood cultures must be carefully evaluated to assess the possibility of false-positive culture, usually due to specimen contamination at the time of collection. Organisms, like viridans
Streptococci
, that are most commonly isolated as contaminants may also cause true BSI, usually in patients with some compromise in immune function. Interpret all positive blood cultures in the context of number of positive blood cultures as well as clinical and laboratory signs and symptoms. In patients with clinically relevant BSIs (true positive), the pathogen is typically isolated from a majority of cultures/bottles. In patients with contaminated blood cultures (false positive), a common contaminant is typically isolated in a single culture or bottle, whereas other cultures are drawn during evaluation remain negative.

   
Negative results:
Bacteremia and fungemia at the time of specimen collection are unlikely. False-negative results may be seen in patients with prior antimicrobial therapy. False-negative results may be caused by inoculation of blood culture bottles with less than the recommended volume of blood. Because clinically significant bacteremia may be intermittent, collection of two or three blood cultures is recommended to rule out bacteremia.

   Limitations

   The significance of positive blood cultures must be evaluated in terms of several factors, including patient signs and symptoms, the intrinsic pathogenicity of the blood culture isolate, number of positive cultures, number of isolates in culture (mixed cultures typically represent contamination), and cultures positive at other infected sites for the blood culture isolate.