Quality control in homograft valve processing: when to screen for microbiological contamination?

Human donor heart valves remain essential for many reconstructive heart procedures. Heart valve donations are a scarce resource which must be used efficiently and safely. Infection transmission remains a potential risk with homograft valve use. Early experience with homograft valves identified high rates of microbial contamination at collection and initiated the practise of immersion in an antibiotic cocktail. Many centres rely on the microbiology screening after exposure to the antibiotic cocktail. We in our centre accept or reject valves on the basis of the microbiology screening at the time of collection prior to immersion in antibiotic solution. We wanted to compare our rate of valve discard and the rate of microbial contamination at implant with other centres. Valves are collected for the Irish Heart Valve Tissue Bank through partnership between the National Centre for Cardiothoracic Surgery and the Irish Blood Transfusion Service. Valves are collected in a surgical theatre setting and processed in dedicated section of the Irish Blood Transfusion Board. Tissues are screening for microbiology at collection and also at implantation. A total of 564 human heart valves and valve conduits were processed through the service during the study period. 167 (29.6%) were discarded during the processing and storage stages. The major reason for this in 117 cases was unsatisfactory microbiology on initial tissue screening. Repeat screening of accepted valves at the time of implantation identified positive cultures in only 0.9%. Optimal use of these limited resources is clearly important. However recipient safety remains paramount. One-fifth of collected valves are discarded at the processing stage due to positive microbiology screening. This is a higher rate of discard then other centres which reject 5.6-10% due to positive microbiology. However our rate of contamination at time of implant is lower then the 3% rate reported elsewhere. We are satisfied that our current discard rate, although significant, reflects rigorous quality control and the optimal balance between valve availability and patient safety.