A Day Saved is a Life Saved: Direct Antimicrobial Susceptibility Testing
from Positively Flagged Blood Culture Bottles and their Concordance with
the Routine Method
Authors: Aggarwal Alisha , Abhishek Kumar S. , Tak Vibhor , Mehrotra Sukanya , Nag Venkat Goutham , Jain Vidhi 
Published: April 2024

Background:
Sepsis is a major health problem worldwide and is associated with high
morbidity and mortality with every hour delay in initiation of therapy. A conventional method of
blood culture and Antimicrobial Susceptibility Testing (AST) takes around 48-72 hours. Empirical
antibiotics need to be administered until the sensitivity report is made available. It has been estimated that 20-50% of the empirical antibiotics are inappropriate, resulting in prolonged hospital
stays, adverse effects, and emergence of drug resistance. Additionally, this also puts an extra financial burden on both the patients and healthcare settings. Performing direct Antimicrobial Sensitivity Testing (dAST) is an important tool to reduce turn-around time (TAT) by at least 18-24
hours, thus reducing morbidity and mortality among critically ill patients.

Methods:
Direct AST (dAST) was performed from the positively flagged blood culture bottles
received between December, 2021 to May, 2022 from Intensive Care Units (ICUs) on MuellerHinton Agar (MHA) using four drops of withdrawn blood. dAST was performed for six drugs:
Ceftriaxone-30 µg (CTR), Piperacillin/Tazobactam-100/10 µg (PIT), Meropenem-10 µg (MRP),
Ciprofloxacin-5 µg (CIP), Aztreonam-30 µg (AT), and Colistin (CL). The zone of inhibition was
interpreted as per CLSI M100 ed32, 2022 guidelines. A parallel conventional method was also
performed to examine for categorical agreement and disagreement. Identification was carried out
using MALDI-TOF MS from the colonies that appeared on the dAST plate on the subsequent day.

Results:
A total of 162 positively flagged blood culture bottles were included in the study. The majority of the Gram-negative organisms were from Enterobacterales (n=109), followed by Acinetobacter spp. (n=28) and Pseudomonas aeruginosa (n=25). Out of the 972 isolate-antimicrobial combinations, overall Categorical Agreement (CA) was seen in 936 (96.3%), whereas disagreement was
observed in 36 with minor error (mE) in 21 (2.2%), major error (ME) in 7 (0.7%), and very major
error (VME) in 8 (0.8%) when compared to the routine method. Categorical agreement (CA) of >
99% was seen in ceftriaxone (CTR) and ciprofloxacin (CIP). In comparison, the lowest CA was observed with meropenem (MRP) at 92%. Colistin dAST was performed using the E-strip method, and
the result obtained was highly convincing, with an overall disagreement of only 1.2%.

Conclusion:
Rapid dAST from positively flagged blood culture bottles proved to significantly
reduce the TAT from the time of sample collection to the first availability of antimicrobial susceptibility report with excellent categorical agreement of > 95% using the conventional disc diffusion
method. Results obtained were within the acceptance criteria set by U. S. Food and Drug Administration (FDA) guidelines of > 90% categorical agreement for a new method. We were able to
obtain excellent concordance for colistin using the E-strip method. Performing dAST not only
saves a “day”, but its proper implementation would save a “life”.