MICROBIOLOGY
False-positive blood cultures: an algorithmic incident report
Craig Pownall reports on a false-positive blood culture issue highlighted on the Isle of Man, and explains the process by which the cause of the problem was identified and a successful solution implemented.
A false-positive blood culture may be defined as a blood culture bottle that triggers the blood culture analyser algorithm to flag the bottle as positive, when organisms are neither seen nor grown when performing subsequent Gram stain and culture. Between 15 July and 31 December last
year, a record was kept of all false-positive blood cultures that were removed from the BacT/ALERT 3D blood culture analyser (bioMérieux) in the medical microbiology laboratory at Nobles Hospital on the Isle of Man, due to a recent high prevalence of this phenomenon occurring.
Data transfer Around mid-November, the problem was ascertained to be a data transfer issue between the Instrument Manager and the analyser, causing the analyser to default to the FA (aerobic) and FN (anaerobic) bottle
Table 1. Number of false-positive blood cultures reported in each month of the study during the second half of 2015.
Month
July (15th onwards) August
September October
November December
256
Number of false positives
5
10 9
30 6 0
types when scanning the respective bottles prior to loading on the analyser, when the bottle types should have been noted as FA- PLUS (aerobic) and FN-PLUS (anaerobic), respectively. The algorithm differences between these bottle types were responsible for the analyser flagging a number of negative bottles as positive. During the period of this study, 60 bottles were recorded as being flagged falsely as positive, with monthly numbers shown in Table 1. There was an average of 0.39 false-
positive blood cultures per day in the 152 days prior to the problem being solved (around 14 November 2015), with 0.36 false-positive blood cultures per day during this whole period of time (169 days). The numbers for October 2015 are high, in part due to a particularly busy day with regards to false-positive blood cultures, whereby seven bottles were flagged as positive in a single day (25 October). During the period of this study, 54 (90%)
bottles were anaerobic FN-PLUS, indicating that the anaerobic algorithm was much more
A selection of blood culture bottles used on the BacT/ALERT 3D.
susceptible to being affected by the switch to the default bottle type. Bottle lot numbers were recorded during this study to establish if there was a link with a certain batch that was causing the problem. The study was run for a sufficient amount of time that saw several batches in use, therefore showing that a particular ‘faulty batch’ of blood culture bottles was not responsible. Table 2 shows the relative numbers of each lot number for different bottle types. Not all batch numbers were able to be ascertained here, as in some cases demographics stickers were placed over the areas of the
Table 2. Blood culture lot numbers that were reported as false positives. Anaerobic bottles
Aerobic bottles
Lot Number 3042784 3042688 3041929 3042510 3043382
% 50 38 8 2 2
Lot Number 3042777 3043902
% 50 50
MAY 2016 THE BIOMEDICAL SCIENTIST
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