FACS is on the FAST Track to Bacteria Detection in Donor Blood
Background of Study
Schmidt and his researchers most recent study, published in Transfusion Medicine, volume 16, recognizes that screening blood is still extremely important, given the number of transfusions that take place on a daily basis. Detecting bacteria in platelets is the specific problem that this study addresses. Basically, two methods exist to detect bacteria in donated blood. The first method is the culture method. A sample of blood is placed in a culture medium and allowed to incubate. If a bacterium grows, then the blood cannot be used. While these samples are accurate, the process occurs so slowly that, in many cases, the blood was already transfuse before the positive result showed. Thus, the process takes too long for it to be use in emergencies and is not deemed clinically efficient. The second method is rapid detection which allows the process to occur more quickly. The Fluorescence-Activated Cell Sorting (FACS) method provides an opportunity for blood to be tested for bacteria much more quickly than with culture methods.
Summary of Study
These researchers sought to use the FACS rapid detection method to isolate six bacteria found often in platelets. These strains are Klebsiella oxytoca, ATCC 13182; Staphylococcus aureus, ATCC 49476; Seratia marcesens, ATCC 43862; Staphylococcus epidermidis, PEI 06-05; Salmonella choleraesuis, ATCC 43971; and Escherichia coli, ATCC 25922. The platelet concentrates (PCs) were introduced to the inoculated bacteria and stained for easier detection. The PCs and the bacteria strains were treated according to standard protocol for this type of experiement established at the Red Cross Institute in Frankfurt, Germany.
During analysis, blood samples that had incubated for between five and sixty minutes were deemed to be negative if they had less that 50 fluorescent counts of the bacteria. Samples deemed to be positive had over 100 fluorescent counts of the bacteria. Those that had between 50 and 99 fluorescent counts were allowed to incubate longer and then be retested. The results show that after five minutes of staining, results are comparable to those published in similar studies of varying methods. However, after adding a pre-incubation step for the bacteria, the results are more accurate.
Justification of the Study
Testing blood for bacteria before using it for transfusions involves two key areas of focus – the analytical sensitivity of the test and the clinical efficiency of the test. The first refers to how accurate the test is, and the second refers to how quickly the results can be obtained. These researchers admit that the analytical sensitivity for FACS is lower than that of culture sample methods. However, the time it takes to get results from a culture sample may be prohibitive. For example, in a recent test, 113 of 184 platelet concentrates were already placed into the recipient before the samples were found to be positive for bacteria. The testing time was seven days in this case (Te Boekhorst, et al., 2005). That is a very long time, particularly if the supply of blood is very low.
Schmidt, Weis and Heck (2005) researched a method a year ago that held promise in reducing the testing time to 24 hours with 100% accuracy. The researchers are quick to note that the results of the study were confined to the study conditions and needed to be replicated with further research.
Indeed, more research is necessary to increase the analytical sensitivity for FACS.. It is vital to find ways to increase the accuracy of these tests without sacrificing time or patients will die. According to Blajchman (2004), “Bacterial contamination of platelet products is generally
acknowledged to be the most frequent infectious risk associated with transfusion. Approximately 1/2000 whole-blood- derived or apheresis-derived platelet units are contaminated with bacteria. Bacterial sepsis is considered the second most common cause of death overall from transfusion (after ABO incompatibility), with reported mortality rates ranging from 1/ 20 000–1/85 000 donor-unit exposures.” Clearly, the medical community could avoid patient mortality by continuing to seek methods to test for infected blood.
Blajchman, M.A. (2004). Bacterial contamination of cellular blood components: Risks, sources and control. Vox Sanguinis 87 (Supplement 1): S98-S103
Schmidt, M. et al. (2006). FACS technology used in a new rapid bacterial detection method. Transfusion Medicine 16:355-361
Schmidt, M., Weis, C., and Heck, J. (2005). Optimized Scansystem™ platelet kit for bacterial detection with enhanced sensitivity: detection within 24 h after spiking. Vox Sanguinis 89: 135-139
Te Boekhorst, P.A., et al. (2005). Clinical Significance of Bacteriologic Screening in Platelet Concentrates. Transfusion 45:514-519