From the Guidelines

The time to positivity (TTP) cutoff for differentiating contaminants from true pathogens in blood cultures is 2 hours.

This cutoff is based on studies that have shown that if a blood culture drawn from a catheter becomes positive more than 2 hours before a simultaneously drawn peripheral blood culture, there is a high probability of catheter-associated bloodstream infection (CA-BSI) 1.
A retrospective analysis of 64 cancer patients with suspected catheter-related bacteremia found that a cutoff of +120 min had 100% specificity and 96.4% sensitivity for diagnosing CA-BSI 1.
A prospective study of 93 cancer patients on an ICU also found that a cutoff of +120 min had 91% specificity and 94% sensitivity for diagnosing CA-BSI 1.
The Infectious Diseases Society of America (IDSA) and the American Society for Microbiology (ASM) recommend using a TTP cutoff of 2 hours to diagnose CA-BSI, although they note that this method is not performed routinely in most laboratories 1.

The TTP cutoff is used to differentiate between contaminants and true pathogens in blood cultures.
A cutoff of 2 hours is commonly used to diagnose CA-BSI.
The IDSA and ASM recommend using this cutoff, although it is not universally adopted.
The evidence supporting this cutoff comes from retrospective and prospective studies, as well as guidelines from professional organizations 1.

From the Research

The TTP cutoff for differentiating contaminants from true pathogens in blood cultures varies across studies, but most research suggests that a cutoff of 24-48 hours can be used to distinguish between the two 2, 3, 4, 5.
A study published in 2013 found that the mean detection time for clinically significant isolates was 17.87 hours, while contaminants had a mean detection time of 40.56 hours 2.
Another study published in 2017 found that 96.1% of pathogenic organisms were detected within 24 hours, and 100% were detected within 36 hours 3.
A study published in 2000 found that 87% of cultures containing pathogens were detected within the first 24 hours of incubation, and 95% of critical pediatric pathogens were detected in less than 24 hours 4.
A study published in 2014 found that 98% of aerobic Gram-positive and Gram-negative BSIs were detected by 48 hours, and culture results at 48 hours were 97% sensitive and had a negative predictive value of 99.8% 5.
A study published in 2019 found that the TTP cutoff value for CoNS, S. aureus, E. coli, and K. pneumoniae was 22.72 hours, 19.6 hours, 18.58 hours, and 16.43 hours, respectively, with most sensitive and specific predictor of BSIs 6.

The type of microorganism can affect the TTP, with Gram-negative strains tend to have a shorter TTP than Gram-positive strains 6.
The use of prior antibiotics can also affect the TTP, with a study finding that the median TTP was longer in patients who received prior antibiotics compared to those who did not 5.
The presence of drug-resistant organisms can also affect the TTP, with a study finding that methicillin-resistant S. aureus had a longer TTP than methicillin-susceptible S. aureus 2, 6.