False Positive Coombs Tests Due to Bacterial Contaminants in Stored Pilot Tube Blood

Unexplained positive Coombs tests during crossmatches have occasionally occurred in this laboratory. A brief investigation was carried out to determine if bacterial contamination of pilot tube blood was involved in this phenomenon. Of 220 pilot tubes cultured, seven contained viable bacteria. These organisms were inoculated into sterile bloods and incubated. Coombs tests were then performed. Blood containing Pseudomonas effusa gave positive direct and indirect antihuman globulin reactions, and blood containing Bacillus subtilis gave positive indirect antihuman globulin reactions. Blood containing other organisms isolated in this study gave negative results with Coombs tests.