What is the impact of time to positivity in blood cultures on the timing of AST results?

• A. Longer time to positivity accelerates AST results.
• B. Time to positivity has no effect on AST reporting.
• C. Shorter time to positivity delays results.
• D. Longer times delay reporting of AST results.

Answer: (D)

Time to positivity measures how long a blood culture bottle takes to show detectable microbial growth after incubation. Antimicrobial susceptibility testing (AST) can only be performed once the culture is positive and there is enough organism present to yield reliable results. If the culture takes longer to reach positivity, the laboratory’s downstream steps—identification, setup for susceptibility testing, and the actual AST—start later, pushing the overall time to reporting AST results further out. In other words, a longer time to positivity delays the earliest point at which AST can be completed, even before any rapid methods after positivity are considered. Conversely, quicker positivity allows the lab to begin testing sooner, shortening the time to AST results.