Secondary bacterial culture of platelets to mitigate transfusion-associated sepsis: A 3-year analysis at a large academic institution

Background: In 2019, the United States Food and Drug Administration published its final recommendations to mitigate bacterial contamination of platelets. We sought to evaluate our secondary bacterial culture (SBC) strategy in light of those recommendations. Study Design and Methods: A retrospective analysis was conducted of SBC data (October 2016-2019) at our institution. SBC was performed upon receipt (Day 3 after collection); 5 mL of platelet product was inoculated aseptically into an aerobic bottle and incubated at 35°C for 3 days. For 8 months, a 10-mL inoculum was trialed. No quarantine was applied. All positive cultures underwent Gram staining and repeat culture of the platelet product (if available). A probable true positive was defined as concordant positive culture between the initial and repeat culture. The incidence of probable true- and false-positive cultures were reported descriptively and differences evaluated by sampling volume. Results: Over 3 years, 55 896 platelet products underwent SBC, yielding 30 initial positive results (approx. 1/1863 platelets); 25 (83.3%) signaled within 24 hours of SBC. The rates of probable true positive, false positive, and indeterminate for 5 mL were 0.027% (1/3771), 0.002% (1/45 251) and 0.018% (1/5656), respectively. The respective rates for 10 mL were 0.018% (1/5323), 0.07% (1/1521), and 0%. Seven of eight (87.5%) false-positive SBCs occurred with a 10-mL inoculum. No septic transfusion reactions were reported. Conclusion: SBC continues to interdict bacterially contaminated units of platelets. Our findings suggest higher rates of false positivity using large-volume inocula.