Do anaerobic bottles add value? A comparative study of blood culture bottle combinations in a tertiary care centre

Bloodstream infections (BSIs) require rapid identification and susceptibility testing to guide timely antimicrobial therapy. Although paired aerobic and anaerobic blood culture bottles are routinely used, the incremental benefit of anaerobic bottles in settings with low anaerobic bacteremia prevalence remains uncertain. This study compared paired aerobic bottles with paired aerobic anaerobic bottle combinations in terms of pathogen isolation rate, time-to-positivity (TTP), and contaminant recovery.

This 12-month descriptive study included 1009 adult inpatients with suspected sepsis. For each patient, 30 mL blood was collected: 20 mL from a single venipuncture site/central line inoculated equally into one aerobic and one anaerobic bottle, and 10 mL from another site inoculated into an additional aerobic bottle. Bottles were incubated in the bioMérieux BacT/Alert Virtuo system for up to five days. Positive bottles were processed, and organisms were identified using MALDI-TOF MS. Analysis compared aerobic-anaerobic pairs with aerobic pairs with one aerobic bottle common for each pair. Isolation rates were analysed using McNemar's chi-square test, and mean TTP using paired t-test.

A total of 3027 bottles were analysed. Pathogen isolation rates were similar between aerobic pair (11.49%) and aerobic-anaerobic pairs (10.8%) (P = 0.621). Mean TTP did not differ significantly (14.7 h vs. 14.84 h; P = 0.927). Recovery rates and TTP for Gram-positive bacteria, Gram-negative bacteria, Enterobacterales, non-fermenters, and Candida species were comparable across both combinations. Obligate anaerobes were rarely isolated (0.2%) and were detected only in anaerobic bottles. Contaminant isolation was significantly lower in aerobic-anaerobic pairs (5.15% vs. 7.73%; P = 0.0184), although contaminant TTP was similar.

Aerobic pair and aerobic-anaerobic pair demonstrated comparable diagnostic yield and TTP for clinically significant pathogens. In low-prevalence settings, selective rather than routine use of anaerobic bottles may represent a more resource-efficient strategy.