Automated Blood Culture Pdf

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Automated Blood Culture Test
Automated Blood Culture System
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BackgroundTo evaluate the VersaTREK (TREK Diagnostic Systems, Cleveland, Ohio) blood culture system against the Bactec9240 (BD Microbiology, Cockeysville, MD), for the recovery of bloodstream pathogens. MethodsVenous blood from patients with suspected bacterial sepsis was evenly distributed into bottles of each system. Positive signals were recorded and bottles processed onto standard media for organism recovery. False positive signals were regarded if no organisms were seen on Gram stain and no growth was observed.

Results177 bottles were available for analysis; the Bactec9240 system yielded 43 positive, 134 negative results and no false positive signals. The VersaTREK system had 58 positive signals with 14 being false positives.

ConclusionsIn our setting with high background burden of immuno-compromised patients, the VersaTREK system compared favourably with the Bactec9240 in recovering blood stream aerobic and facultative anaerobic pathogens from patients with suspected bacterial sepsis. A concern is the high false positivity rate. Due to its versatility to accommodate small and large workloads as well as using smaller volumes of blood, this system may establish itself as a useful alternative for the recovery of bloodstream pathogens. A total of 177 comparative bottles were analysed in the laboratory.

The Bactec9240 system yielded 43(24%) positive signals and no false positive signals. The VersaTREK system had 58(33%) positive signals of which 44(76%) were true positive and 14 (24%) were false positive signals (Table ). There were 5 discrepant results. Of the 60 positive signals detected, 31(52%) were from aerobic bottles and 29(48%) from anaerobic bottles. The culture positivity rate for the VersaTREK and the Bactec9240 excluding false positive results was similar i.e.

25% (44 of 177) and 24% (43 of 177) respectively. The most productive bottle in both systems was a VersaTREK Redox 1 bottle that had a positive signal at 1 hour and 6 minutes and isolated Staphylococcus aureus with the corresponding Bactec bottle flagging at 3 hours and 3 minutes. For the two discrepant Streptococcus pneumoniae isolates, the VersaTREK system emitted positive signals but no organisms were seen on Gram stain nor was any growth observed after 48 hours incubation. The VersaTREK system also emitted an additional 12 positive signals which were Gram stain smear negative and yielded no growth and were therefore also considered to be false positives. The instrument growth curves of the pressure changes plotted against time for these bottles were evaluated and did not indicate logarithmic bacterial growth.

The average TTR for these bottles was 8 hours.The TTD for the two systems is shown in Table. Thirty-one bottles had matching TTD data available for comparative analysis. No statistically significant difference in TTD was observed between the two systems. The positive and negative analytical percent agreements for organism recovery were 95% and 98% respectively. The two systems were also compared for signal detection.

The positive and negative analytical percent agreements for signal detection were 95% and 87% respectively. This is the first study to report a comparison of the VersaTREK system to the Bactec9240 system using clinical samples from patients with suspected bloodstream infections. The study was carried out in South Africa where HIV infection rates are very high and hence a greater risk exists among immune compromised patients to develop bloodstream infections ,. The true culture positivity rate for both blood culture systems was high (25%). Although this is much higher than what is usually reported (range 10 - 15%) from other centres it does not come as a surprise considering the high HIV endemicity in the country, the intensive care setting and the struggle to adhere to good infection control practices.Reports of comparisons of blood culture systems have appeared in the literature but these have been mainly between the various Bactec systems and the BacT/Alert system ,. There has been only one published study which evaluated the VersaTREK blood culture system and this comparison was against the BacT/Alert system. Overall we detected a fairly good correlation between the two systems that we evaluated.

However, the false positive signals emitted from the VersaTREK system accounted for a false positivity rate of 7.9%. This is much higher than that reported in Mirrets' study.Various reasons for the false positives must be considered. A delay in processing the bottles post signal detection may result in negative growth of fastidious organisms as they may be susceptible to autolysins as was noted with the two Streptococcus pneumonia isolates. Post incubation processing was delayed in some cases as the laboratory was not continuously staffed and this may be a factor as the average TTR for the false positive signal bottles was 8 hours. Other organisms may require additional culture media e.g. Nutritionally variant streptococcus and Campylobacter spp.

Non-cultivable organisms may only be identified by molecular techniques. Prior antibiotic use could be a factor even though the media does contain antibiotic binding resin. The issue of high white cell counts could not be addressed in this study, although this system is stated not to be limited by this factor. Another possibility for the false positive signals may be that the threshold setting of the system for detection is too low. Furthermore, the system also vortexes the aerobic bottles compared to the slow rocking method employed by the Bactec9240 system and this may also contribute to additional false signals by creating pressure changes. However, there is no evidence to support our hypotheses.A change in the spectrum of organisms being isolated from episodes of sepsis has been noted over the last few years ,. The increased use of more invasive therapeutic devices as well as the administration of immunosuppressant agents has increased the incidence of septicaemia.

Studies have also shown a decline in the recovery of Streptococcus pneumoniae and other opportunistic pathogens with the introduction of the pneumococcal conjugated vaccine and highly active antiretroviral therapy - HAART ,.The organisms recovered in our study represent a good variety of potential pathogens that can cause a wide spectrum of clinical symptoms and disease. The Bactec9240 detected two additional isolates of Streptococcus pneumoniae. It must be noted that these isolates did emit a positive signal on the VersaTREK system, at 8 and 10 hours respectively, however these bottles were removed and processed after 24 hours owing to the closure of the blood culture laboratory overnight. Owing to the sensitive nature of this organism it might explain the poor recovery rate in this specific instance.

Automated Blood Culture Test

The VersaTREK detected additional organisms such as viridans streptococcus and Bacillus species which are often contaminants. In this study all these patients had normal inflammatory markers suggesting probable contamination (data not shown). Overall if one considers clinically significant blood culture isolates, this study demonstrated a fairly good analytical percent agreement for organism recovery.Our analysis regarding the time to detection revealed a faster overall TTD for all isolates with the Bactec9240 system although this was not statistically significant. Variations in the propriety culture media in each blood culture bottle including their additional nutrients and anti-coagulants may be contributory factors. Furthermore, the Bactec9240 systems' detection of CO 2 release may be faster than the VersaTREK system that measures headspace pressure.

Although there was no statistical significance in the TTD the difference may have clinical bearing when treating gram-negative sepsis.We cannot comment on the efficiency of this system for the recovery of fungi and anaerobes as these were not recovered during the study period. However, in another study the VersaTREK system was comparable to the BacT/Alert for the recovery of yeasts from blood cultures.Our study was limited by the fact that we could not assess the false positive signals by a lack of knowledge of prior antimicrobial usage or the means to detect bacterial 16S rRNA or fungal genomic sequences. Another limitation was the relatively small number of positive isolates for the calculation of the TTD for the two systems, however performance data in a real world setting is often difficult to obtain and always useful. Although this study was mainly laboratory based, additional clinical data and information regarding outcomes could have added additional weight to our conclusions. The issue of false negative cultures could not be addressed.

Another approach could have been to perform terminal cultures from the negative bottles in those isolates that were missed by the Bactec9240 system as well as the false positive signals, however this was not part of the study design. The ability of the VersaTREK system to work with much lower volumes was not assessed in this study as we used equal volumes to compare both systems, this however may be a major advantage of the system especially in the paediatric population. We made use of one clinician to oversee the blood culture collection process, however this was not evaluated and the likely limited variation might have bearing on the false positive rate.

In our setting with high background burden of immuno-compromised patients due to HIV as well as a higher prevalence of blood stream infections, the VersaTREK system compared favourably with the Bactec9240 in recovering blood stream pathogens from patients with suspected bacterial sepsis. However, a concern is the high false positivity rate observed in our study. Due to its versatility to accommodate small and large workloads as well as the added benefit of using much smaller volumes of blood, this system may establish itself as a useful alternative for the recovery of bloodstream pathogens.

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Automated Blood Culture System

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