Processing Positive Blood Cultures

Following an instrument-flagged positive event, the bottle is removed from the system and a Gram stain and subculture is performed.

• If the sample is Gram stain positive, the morphology of the organism should be reported immediately to the physician. Subcultures and, if available, rapid techniques (e.g. molecular diagnostics) should be initiated immediately in order to provide further organism identification and antibiotic susceptibility testing should be performed as soon as possible.
• If a sample is Gram stain negative, additional stains, such as auraminerhodamine stain, acid-fast bacillus (AFB) stain or modified AFB stain, should be performed to screen for the organism or to search for specific organisms, e.g. Mycobacterium or Nocardia. If all the stains are negative, the blood bottle should be put back into the instrument to complete the full 5 days incubation period.

A positive blood culture is a critical result and must be reported as soon as available, due to the immediate impact on patient care decisions. When reports are delivered rapidly, studies have shown broadly improved outcomes and efficiencies in patient management.^{47, 48}

A study by Barenfanger, et al. validated that Gram stains of positive bloodcultures are a very important factor influencing appropriate therapy and patient outcomes.⁴⁹ The study documented a statistically significant increase in the mortality rate for patients who had blood cultures processed after a delay (i.e. Gram stain performed ≥1 hour after being detected as positive; P = 0.0389). The timely removal and reporting of Gram stain results have a positive impact on patient care and this study supports the need for 24/7 coverage of blood culture instruments.⁴⁹

Recent technological advances such as MALDI-TOF MS (Matrix-Assisted Laser Desorption Ionization Time of Flight Mass Spectrometry) provide the ability to rapidly deliver definitive organism identification. Molecular diagnostics can identify the most common pathogens in positive blood cultures as well as specific antibiotic resistance genes associated with common bloodstream pathogens. Rapid identification allows physicians to prescribe more targeted and effective antimicrobial therapy earlier to positively influence outcomes.^50-52

Additionally, antimicrobial susceptibility testing (AST) should be performed on positive blood cultures to provide the clinician with a complete result. Appropriate use of antibiotics is crucial in cases of bloodstream infections and sepsis. Accurately determining the antimicrobial resistance profile of the causative pathogen in order to select the most effective antibiotic therapy can have a significant impact on patient outcomes.

It is now possible for laboratories to deliver phenotypic AST results for gram-negative bacteria directly from positive blood cultures. CLSI has published a rapid, direct-from-blood-culture disk diffusion (DD) protocol, with specific direct DD breakpoints for Enterobacterales and P. aeruginosa.⁵³ EUCAST has also developed a DD method delivering AST results within 4-8 hours of blood culture positivity.⁵⁴ Another new rapid AST system provides actionable results direct from Gram-negative blood cultures in an average of 5 hours.⁵⁵ These diagnostic solutions help clinicians address the challenge of bloodstream infections, and sepsis in particular, by allowing either timely de-escalation to a focused, more appropriate, and lower-cost therapy, or life-saving rapid escalation to more effective therapy when a multidrug-resistant (MDR) infection is present.