Sterility testing of platelet concentrates prepared from deliberately infected blood donations

In general the bacterial count in freshly donated blood is low and even lower in the corresponding platelet concentrates (PCs). By use of flow cytometry (FACS) for sterility testing, the reliability of early versus later sampling times was evaluated.

Blood donations were spiked with various numbers of Staphylococcus epidermidis, Staphylococcus aureus, Bacillus cereus, and Klebsiella pneumoniae. The corresponding PCs were prepared by the buffy-coat method and stored at 22 degrees C. A 20-mL sample was collected from each PC directly after preparation and after 8 hours. Samples were stored at 35 degrees C. Sterility testing of both PCs and samples was by FACS analysis at different time points.

All stored PCs were found positive by FACS analysis, with detection times ranging between 8 and 24 hours (K. pneumoniae, B. cereus), 8 and 91 hours (S. aureus), and 144 hours (S. epidermidis). In the samples incubated at 35 degrees C, bacteria were detected after 8 to 19 hours (K. pneumoniae, B. cereus), 8 to 67 hours (S. aureus), and 19 to 43 hours (S. epidermidis). Some of the samples did not contain bacteria.

Detection times for slow-growing bacteria are significantly shortened when PC samples are incubated at 35 degrees C: the numbers of bacteria in freshly prepared PCs may, however, be so low that the samples drawn for sterility testing do not contain a single bacterium. Our results do not support a shortening of the 24-hour or greater sampling time recommended by the manufacturers of established test systems, because also for consistent detection by FACS, bacteria need to grow in the PCs to sufficient numbers.