The performance of the Alfred 60/AST device, an automated bacterial culture device which uses laser nephelometry to detect and quantify bacterial growth, was evaluated. The instrument is effective at screening negative samples and is more reliable at detecting bacteria than yeasts. Microscopy can be used to reduce the false-negative numbers.
Earlier targeted therapy for bacteraemia optimizes patient outcomes and reduces broad spectrum antibiotic use. Standardized susceptibility testing results are available at 36-48 h. Direct disc susceptibility testing from blood culture broth reduces time to results but the inoculum is not standardized. No studies have looked at the clinical utility of direct susceptibility results. This retrospective cohort study aimed to assess the correlation between direct and formal testing methods as well as the clinical utility of direct susceptibility results. 160 episodes of bacteraemia with paired direct and formal susceptibility testing were studied. Direct disc testing was performed on blood culture broth. Formal testing was performed on isolates, using automated broth microdilution or Etests. The rate of error was 9.0 % (95 % CI 7.0-11.6 %). In 10 cases (6.3 %, 95 % CI 3.0-11.2 %), inappropriate antibiotics were used due to direct susceptibility results, including two cases with ineffective (as opposed to too broad) antibiotics being used. Antibiotics were changed in 28.1 % of cases once direct susceptibility data was available. There was a decreased time to effective antibiotics in 9.3 % (95 % CI 5.3-15.0 %), and a decreased time to a targeted antibiotics in 14.3 % (95 % CI 9.3-20.8 %) of cases. Despite the error rate, the advantages of earlier times to effective and targeted antibiotics justifies continuing direct testing in bacteraemia episodes with Gram-negative rods. In the Gram-positive group, given the contamination rate, the availability of adjunctive PCR, and the fact that early identification of the isolate could equally influence antibiotic choices, direct susceptibility testing may no longer be warranted.
Spezzotti has identified several issues that may have resulted in an underestimation of the performance of the Alfred 60/AST device.First, Dr. Spezzotti questions the authenticity of one of the nine false-negative samples (MB12216254), based on the Alfred 60/ AST device flagging this sample as "invalid." Although this is correct, we followed the manual's recommendations (see page 66 in reference 2), which currently does not suggest excluding such a sample. Furthermore, additional samples which gave the correct result also had such issues. If the manual were to be updated, exclusion of these samples would result in a sensitivity of 88.0%.In the 3 indwelling catheter specimens or additional false negatives, Dr. Spezzotti suggests alternative Alfred 60/AST cutoffs to optimize detection. However, these recommendations are contrary to the current manual. We would argue that all indwelling catheter specimens would need to be reevaluated with the new protocol to know the true performance of the Alfred 60/AST for these samples. Nevertheless, the exclusion of all indwelling catheter samples and the use of our original cutoff results in a sensitivity of 98.0%.Standard broth, as provided by the distributor, was used in our evaluation. Although Dr. Spezzotti mentions another "specific dedicated product" to boost the detection of specific organisms, we are unaware of such a product for urine analysis, and it seems impractical to run duplicate broths for each sample to optimize detection.Although the above issues could potentially be rectified by better optimization of the instrument and culture medium inoculation, we disagree with the final point raised by Dr. Spezzotti concerning laboratory evaluations. We defined our "gold standard" prior to analysis and disagree that the results of the Alfred 60/AST should form part of the gold standard with respect to performance. Furthermore, all discordant results were investigated clinically with all patients found to have symptoms or signs consistent with urinary tract infections.
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