First results using automated epifluorescence microscopy to detect Escherichia coli and Staphylococcus epidermidis in WBC-reduced platelet concentrates

Use of the RQI test for bacterial screening of whole blood platelets

We compare our experience using a new rapid qualitative immunoassay (RQI) test (platelet Pan Genera Detection, Verax, Worcester, MA) for bacterial screening of whole blood platelet (WBP) pools with our previous WBP bacterial screen, pH testing. All WBP pools were RQI tested at the time of issue. All RQI+ pools were cultured in an automated culture system, with subsequent bacterial identification if the culture was positive. During approximately 5.5 months, 7,733 WBP pools were RQI tested. There were 14 positive RQI tests; 12 WBP pools were sterile when cultured and considered false-positive RQI tests. One pool was positive for coagulase-negative Staphylococcus, while another was positive for group B Streptococcus. The specificity and positive predictive value of the RQI test were 99.85% and 14.3%, respectively. The specificity and positive predictive value of the RQI test were higher than pH testing, leading to less waste of sterile WBP pools.

Microscopic detection of viable Staphylococcus epidermidis in peri-implant tissue in experimental biomaterial-associated infection, identified by bromodeoxyuridine incorporation

Infection of biomedical devices is characterized by biofilm formation and colonization of surrounding tissue by the causative pathogens. To investigate whether bacteria detected microscopically in tissue surrounding infected devices were viable, we used bromodeoxyuridine (BrdU), a nucleotide analogue that is incorporated into bacterial DNA and can be detected with antibodies. Infected human tissue was obtained postmortem from patients with intravascular devices, and mouse biopsy specimens were obtained from mice with experimental biomaterial infection. In vitro experiments showed that Staphylococcus epidermidis incorporated BrdU, as judged from staining of the bacteria with anti-BrdU antibodies. After incubation of bacteria with BrdU and subsequent staining of microscopic sections with anti-BrdU antibodies, bacteria could be clearly visualized in the tissue surrounding intravascular devices of deceased patients. With this staining technique, relapse of infection could be visualized in mice challenged with a low dose of S. epidermidis and treated with dexamethasone between 14 and 21 days after challenge to suppress immunity. This confirms and extends our previous findings that pericatheter tissue is a reservoir for bacteria in biomaterial-associated infection. The pathogenesis of the infection and temporo-spatial distribution of viable, dividing bacteria can now be studied at the microscopic level by immunolabeling with BrdU and BrdU antibodies.

Applications of real-time PCR in the screening of platelet concentrates for bacterial contamination

Although there have been major improvements over the past few decades in detection methods for blood-borne infectious agents, platelet concentrates are still responsible for most cases of transfusion-transmitted bacterial infections. To date, real-time PCR is an indispensable tool in diagnostic laboratories to detect pathogens in a variety of biological samples. In this article, the applications of this powerful technique in the screening of platelet concentrates for bacterial contamination are discussed. Next to pathogen-specific (real-time) PCR assays, particular attention is directed to the recently developed 16S rDNA real-time PCR. This assay has been proven as a convenient way to detect bacterial contamination of platelet concentrates. The assay is sensitive and enables rapid detection of low initial numbers of bacteria in platelet concentrates. The short turnaround time of this assay allows high-throughput screening and reduction of the risk of transfusion of bacterially contaminated units. As with every method, real-time PCR has its advantages and disadvantages. These and especially limitations inherent to generation of false-positive or -negative results are emphasized. The universal nature of detection of the assay may be suitable for generalized bacterial screening of other blood components, such as red blood cells and plasma. Therefore, it is necessary to adapt and optimize detection in red blood cells and plasma with real-time PCR. Further sophistication, miniaturization and standardization of extraction and amplification methods should improve the total performance and robustness of the assay. Hence, real-time PCR is an attractive method in development as a more rapid screening test than currently used culture methods to detect bacterial contamination in blood components.

Use of a pH meter for bacterial screening of whole blood platelets

BACKGROUND

Bacterial contamination of blood products is a leading cause of transfusion-related morbidity and mortality. Transfusion services are now compelled to employ methods of detecting bacteria in platelet (PLT) components. The use of pH screening of whole-blood PLTs (WBPs) was evaluated with a pH meter at the time of issue as a surrogate test for bacterial contamination.

STUDY DESIGN AND METHODS

All WBPs selected for transfusion in May through September 2004 were tested individually for pH at time of issue. Those with a pH value of less than 7.0 were cultured in an automated culture system for 5 days. The white blood cell (WBC) and PLT counts in 56 representative WBP units that failed pH screening were compared to WBP units with acceptable pH values.

RESULTS

Of the 37,060 WBP units that underwent pH screening, 405 had a pH value of less than 7.0 (1.1%). Four of those units were culture positive (1.0%) for Staphylococcus aureus, Bacillus subtilis, diphtheroids, and coagulase-negative Staphylococcus. Only one cocomponent red blood cell (RBC) unit was culture-positive and grew the same bacteria (S. aureus) as the WBP unit. The rate of pH failure increased with WBP storage length with the greatest rate of pH failures occurring in 5-day-old WBPs. The units that failed pH screening had significantly more WBCs and PLTs than units with acceptable pH values.

CONCLUSION

pH screening of WBPs at issue prevented transfusion of bacterially contaminated WBPs and RBCs. This method, however, results in significant PLT wastage. Higher WBC and PLT content likely explains pH failures not due to bacterial contamination.

False-positive alarms for bacterial screening of platelet concentrates with BacT/ALERT new-generation plastic bottles: a multicenter pilot study

BACKGROUND

The microbial detection system BacT/ALERT (bioMérieux) is widely used to monitor bacterial contamination of platelet concentrates (PCs). Recently, the manufacturer introduced polycarbonate culture bottles and a modified pH-sensitive liquid emulsion sensor as microbial growth indicator. This reconfigured assay was investigated in a routine setting.

STUDY DESIGN AND METHODS

In each of eight transfusion centers, samples from 500 consecutive PCs were monitored for 1 week. For all PCs with a positive BacT/ALERT signal, retained samples and, if available, original PC containers and concomitant red blood cell concentrates were analyzed independently. Initially BacT/ALERT-positive PCs without bacterial identification in any sample were defined as false-positive. BacT/ALERT-positive PCs with bacteria in the first sample only were called potentially positive. PCs with bacteria in the first sample and the same strain in at least one additional sample were accepted as positive.

RESULTS

Five PCs (0.13%) were positive, 9 PCs (0.23%) were potentially positive, and 35 PCs (0.9%) were false-positive. The rate of false-positive BacT/ALERT results varied substantially between centers (<0.2%-3.2%). Tracings from false-positive cultures lacked an exponential increase of the signal during incubation. Most of these false-positives were due to malfunctioning cells in various BacT/ALERT incubation units.

CONCLUSION

Careful assessment of individual tracings of samples with positive signals helps to identify malfunctioning incubation units. Their early shutdown or replacement minimizes the high rate of unrectifiable product rejects attributed to false-positive alarms and avoids unnecessary concern of doctors and patients after conversion to a reconfigured BacT/ALERT assay.

Protecting the blood supply from emerging pathogens: the role of pathogen inactivation

Although the risk of infection by blood transfusion is relatively low, breakthrough infections still occur, Transfusion-related fatalities caused by infections continue to be reported, and blood is not tested for many potentially dangerous pathogens. The current paradigm for increasing the safety of the blood supply is the development and implementation of laboratory screening methods and restrictive donor criteria. When considering the large number of known pathogens and the fact that pathogens continue to emerge, it is clear that the utility of new tests and donor restrictions will continue to be a challenge when considering the cost of developing and implementing new screening assays, the loss of potential donors, and the risk of testing errors. Despite improving the safety of blood components, testing remains a reactive approach to blood safety. The contaminating organisms must be identified before sensitive tests can be developed. In contrast, pathogen inactivation is a proactive strategy designed to inactivate a pathogen before it enters the blood supply. Almost all pathogen inactivation technologies target nucleic acids, allowing for the inactivation of a variety of nucleic acid-containing pathogens within plasma, platelets, or red blood cells thus providing the potential to reduce transfusion-transmitted diseases. However, widespread use of a pathogen inactivation technology can only be realized when proven safe and efficacious and not cost-prohibitive.

Transfusion-transmitted bacterial infection: risks, sources and interventions

Records of the transmission of bacterial infections by transfusion date back to the beginning of organized blood banking. Despite tremendous strides in preventing viral infection through careful donor screening and viral testing, there has been little improvement in reducing the risk of bacterial sepsis since the introduction of closed collection systems. Based on the French Haemovigilance study, the British Serious Hazards of Transmission (SHOT) study and fatality reports to the United States Food and Drug Administration, the risk of clinically apparent sepsis exceeds the risk of HIV, HBV, and HCV transmission. Sources of contamination include the skin, blood, disposables, and the environment. Potential interventions to reduce transfusion-associated bacterial sepsis include improvements to donor arm preparation, diversion of the first aliquot of whole blood, introduction of bacterial testing and/or implementation of pathogen reduction methods.

Risk and prevention of transfusion-related sepsis

PURPOSE OF REVIEW

Transfusion-related sepsis is both the oldest recognized and most common transfusion-associated infectious risk. Despite an increased awareness and recognition of this problem, particularly with room-temperature stored platelets, strategies to prevent or reduce the occurrence of this problem have proved daunting.

RECENT FINDINGS

With the recent FDA approval of culture methods for platelet bacterial testing and the promulgation of accreditation standards by the College of American Pathologists and American Association of Blood Banks to limit and detect platelet bacterial contamination, it is anticipated that the frequency of this problem will now begin to diminish.

SUMMARY

As methods to reduce and/or inactivate pathogens emerge, it is hoped that transfusion-related sepsis will essentially disappear.

[Detection of bacterial contamination in platelet concentrates: perspectives]

Bacterial contamination of blood components represents today the highest infectious risk of blood transfusion, the risk is particularly high when it affects platelet concentrates. In France the prevention methods developed over the past six years (donor selection, phlebotomy site preparation, first 30 ml diversion, systematic leuko-reduction...) aimed at limiting the introduction of bacteria in blood and bacterial proliferation. Several methods have been tested for the detection of bacterial contamination in platelet concentrates but none have been generalised. Difficulties were met, due to the necessity of 1) detecting only the platelet concentrates presenting a real infectious risk, when the presence of bacteria is observed in 2.2% (2-4%) of donated blood and 2) guaranteeing the availability of platelet concentrates. New methods have been developed which seem able to bring responses to these difficulties. Several processes are being (or will be) assessed, including automated blood culture, bacterial genomic detection with or without amplification, flow cytometric methods. In parallel, an indirect method able to detect the presence of bacteria, based on oxygen consumption, will also be evaluated. One (or several) of these processes should allow, in the short-term, to detect platelet concentrates presenting an infectious risk. In the future, the interest of bio-chips for bacterial detection in biological fluids must be investigated.