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J Lab Physicians. 2019 Apr-Jun;11(2):154-160. doi: 10.4103/JLP.JLP_137_18.

Evaluation of performance of direct disk diffusion test from positively flagged blood culture broth: A large scale study from South India.

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Department of Microbiology, Jawaharlal Institute of Postgraduate Medical Education and Research, Puducherry, India.



Rapid turnaround time of blood culture reports should be the main motive for a clinical microbiologist for optimal patient care. Categorical agreement (CA) between direct disk diffusion (dDD) and reference disk diffusion (rDD) may vary between laboratories.


This study was designed to determine the CA and understand various types of errors associated with antibiotic organism combination, so that caution can be derived while interpreting and reporting dDD results in the earliest meaningful time frame.


In the present study, dDD results were compared to the rDD results from the positive blood culture bottles. CA and various types of errors were evaluated.


A total of 965 pathogens and 7106 organism antibiotic combinations were evaluated in this study. Overall, there was a CA of 96% which was extremely satisfactory. The categorical disagreement was found only in 4% of organism antibiotic combinations; majority of which were major error (ME, 2.1%) followed by very ME (1%) and minor error (0.9%). The errors were marginally high for Enterobacteriaceae testing against β lactam- β lactamase inhibitor combinations, for Pseudomonas species against aminoglycosides and ciprofloxacin and Staphylococcus species against cefoxitin, one should be vigilant while reporting dDD result of these antibiotic organism combinations.


dDD is of paramount importance for early institution of targeted therapy and is considered as one of the key stewardship intervention. Our study gives an insight that every laboratory must perform dDD for positively flagged blood culture specimens; the result of which should be confirmed later by performing rDD. One should be vigilant while reporting dDD result of BL BLI for Enterobacteriaceae; aminoglycosides and CF for Pseudomonas species; cefoxitin for Staphylococcus species and HLG for Enterococcus species. Supplementary tests such as MRSA latex should be included when necessary.


Blood culture; direct susceptibility test; disk diffusion

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