MICROBIOLOGY
‘Relative fluorescence unit value is the unit of measurement that the analyser algorithm uses to determine blood culture positivity’
Overwritten algorithm The element of this study that pointed to the solution to the problem of high numbers of false-positive blood culture bottles being reported was the indication of when during the day the bottles were received (ie during core working hours or when only emergency on-call cover is available). It was found that 75% of cases of false-positive blood culture bottles were received during on-call hours, compared to only 25% during core working hours. This is confounded, however, by the fact that on-call hours equate to double the amount of time of core working hours in any 24-hour period (approximately 16 hours, compared to eight hours of core working). An increase in false-positive blood cultures originating during this period of time therefore may be expected. However, it was also noted anecdotally that when a biomedical scientist from medical microbiology was on-call, false-positive bottles were conspicuous by their absence the following day. This led to an investigation of the data
transfer between the BacT/ALERT 3D analyser and the Masterlab laboratory information management system (LIMS). This instigated closer analysis of the rules on
Instrument Manager, which finally led to the deduction that the rules were wrong. This caused Masterlab to overwrite the correctly assigned algorithm when the bottle was scanned into the analyser, thereby assigning the incorrect (non-PLUS) algorithm, which caused the bottle then to flag positive. It is unclear why it didn’t flag every bottle
as falsely positive. When blood cultures were entered on the analyser during core working hours, the relatively rapid post-entry booking in of the specimen demographics would reset the algorithm in line with the correct bottle type when the laboratory number was assigned to the respective bottle number. During the on-call period, however, when the bottle was entered on the analyser late at night or in the early hours of the morning, it could be several hours later that this procedure would occur, thereby giving the incorrect algorithm time to become a problem when the demographics were entered, and the laboratory number assigned to the bottle. When medical microbiology staff were on-call, however, bottles were treated in the same way as they would be during core working hours, hence the absence of false positives the following day as a result of this complication.
Rules amended, case closed In conclusion, it can be seen that the issue of high numbers of blood culture bottles flagging as falsely positive in the medical microbiology laboratory at Nobles Hospital on the Isle of Man was due to a fault in the rules processing on the intermediary Instrument Manager, which transfers data between the analyser and the host LIMS. This fault overwrote the bottle barcode and assigned the incorrect algorithm to the bottle, thus requiring a small but significant proportion of blood culture bottles to be processed as positive when they were in fact sterile. This report has looked into various other potential reasons for the increase in false-positive bottles, but has found no evidence to suggest secondary factors. Once the data processing rules were amended, bottles stopped being flagged falsely positive by the BacT/ALERT 3D analyser, and therefore this case may now be regarded as closed.
The author would like to acknowledge the support provided by colleagues in the microbiology laboratory at Nobles Hospital, especially Dr Rizwan Khan for initiating the study, Rebbeca Shields and Chris Helm for proof-reading and providing technical information, and Ruth Cameron at bioMérieux.
Craig Pownall is a specialist biomedical scientist in medical microbiology at Nobles Hospital, Isle of Man.
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