How long does it take to test sensitivity after a culture is positive?

Time to Antimicrobial Susceptibility Testing After Positive Blood Culture

With rapid automated systems, antimicrobial susceptibility testing (AST) results can be available in approximately 9.6-10 hours after a blood culture flags positive, compared to 25.9 hours with traditional overnight methods. 1

Timeframes for Susceptibility Testing

Rapid Automated Methods

• Rapid AST systems (e.g., MicroScan Walkaway-96) complete identification and susceptibility testing in 2-7 hours once initiated on positive blood culture broth 1
• When initiated immediately upon recognition of bacterial growth (morning workflow), mean time to susceptibility determination is 9.6 hours (range 4-68 hours) from culture positivity 1
• This rapid approach significantly reduced mortality (8.8% vs 15.3%, p=0.016) compared to traditional methods 1

Traditional Overnight Methods

• Conventional AST using overnight techniques requires 25.9 hours (range 16-78 hours) from positive culture to susceptibility results 1
• Tests are typically initiated between 9-10 PM on the day bacterial growth is recognized, with results available the following morning 1

Direct Susceptibility Testing from Positive Broth

• Direct sensitivity testing (DST) performed immediately from positive blood culture broth provides results 24 hours earlier than conventional methods 2
• DST shows high concordance with standard AST: 83.7% categorical agreement and 96.2% essential agreement 2
• Minor errors occur in 12.5% of cases, major errors in 3.33%, and very major errors in only 0.47% 2

Clinical Impact of Rapid Susceptibility Testing

Mortality and Morbidity Benefits

• Rapid molecular identification combined with AST reduces median time to optimal antibiotic therapy from 14.7 hours to 4.7 hours 3
• In high multidrug-resistant organism settings, rapid testing with antimicrobial stewardship reduces 30-day mortality from 19.2% to 8.1% 3
• Mean time from positive culture to therapy change decreases from 31.2 hours to 16.3 hours (p<0.0005) with rapid methods 1

Hospital Outcomes

• Hospital length of stay decreases by 2.48 days with rapid diagnostic testing 3
• Mean total hospitalization costs reduced from $19,256 to $15,062 (p=0.0118) 1

Important Caveats and Pitfalls

Workflow Considerations

• Laboratory workflow significantly impacts turnaround time - even rapid tests may be batched into 8-hour work shifts due to operational constraints 1
• Results must be communicated immediately to clinicians via telephone or electronic systems for maximum clinical benefit 1, 3
• Without active antimicrobial stewardship and real-time communication, mortality benefits are not realized 3

Technical Limitations

• Direct susceptibility testing from positive broth is not FDA-approved but shows acceptable error rates for clinical decision-making 2
• Conventional culture and susceptibility testing remains the definitive reference standard 3
• Some fastidious organisms may require longer incubation periods beyond standard timeframes 1

Optimal Implementation Strategy

• Initiate AST immediately upon recognition of bacterial growth rather than waiting for evening batch processing 1
• Integrate rapid molecular panels (1-hour identification) with rapid phenotypic AST systems 3
• Establish 24-hour/7-day laboratory and antimicrobial stewardship coverage to maximize clinical impact 3
• Prioritize rapid testing for critically ill patients, those with suspected multidrug-resistant organisms, or ESBL/carbapenem-resistant Enterobacterales 3