Fatima Noman, Azmat Jehan, Altaf Ahmed.
Reliability of direct sensitivity determination of blood cultures.
J Coll Physicians Surg Pak Jan ;18(10):660-1.

The aim of this study was to evaluate the error in interpreting antimicrobial sensitivity by direct method when compared to standard method and find out if specific antibiotic-organism combination had more discrepancies. All blood culture samples received at Microbiology Laboratory from 1st July 2006 to 31st August 2006 were ncluded in the study. All samples were inoculated in automated blood culture system “BACTEC 9240” which contained enriched Soybean-Casein Digest broth with CO2. Once positive, bottles were removed from system; gram staining of the positive broths was done. Susceptibility test was performed from positive broth, on MHA (Mueller-Hinton Agar), with antibiotics panel according to gram stain result. All positive broths were also sub-cultured on blood agar, chocolate agar and McConkey`s agar for only gram-negative rods. Next day, the zone sizes of all antibiotics were recorded using measuring scale; and at the same time susceptibility test was repeated from isolated colonies from subcultures, with inoculums prepared of McFarland 0.5 standard.2 Staphylococcus aureus (ATCC 29213); E.coli (ATCC 25922) and Pseudomonas aeruginosa (ATCC 27853) were included as quality control strain. Zone sizes were interpreted as `sensitive`(S), `resistant`(R) and `intermediate`(I) according to CLSI recommendation.6 Two results were compared and recorded. Out of a total 1083 combinations, zone diameters by standard method were either equal or greater than direct zone diameter (never smaller). Most of the discrepancies were in b-lactam/b-lactamase combinations, and aminoglycosides. While reporting these groups of antibiotics with direct sensitivity test, one should be cautious. These are the major antibiotic used for life-threatening infections. In case of being heavy/lighter standard inoculums or marginal zones, repeating with standard method should be preferred to minimize the chances of error.

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