Centrifugation of 50 ml of peritoneal fluid is sufficient for microbiological examination in continuous ambulatory peritoneal dialysis (CAPD) patients with peritonitis.

Concentration of peritoneal fluid of continuous ambulatory peritoneal dialysis (CAPD) patients has often been recommended for microbiological examination. This study presents data about the yield of microorganisms of different volumes through centrifugation. Twenty-eight samples of 50 ml and 1000 ml of the same dialysate bags were centrifuged, and assays were made for aerobic and anaerobic organisms on solid media and in enrichment broth. All pairs of 50 ml and 1000 ml samples except one (96%) showed identical results, suggesting that it is unnecessary to centrifuge more than 50 ml of fluid. Furthermore, it seems better to examine two or more consecutive bags per peritonitis episode, since microorganisms were found in seven of nine (77%) such episodes, compared to only five of eight (62%) episodes with only one bag. In addition, significantly more positive cultures were recorded (14 of 17 vs. 5 of 11; p less than 0.05) if samples were taken before administration of antibiotics. One patient's catheter was colonized with bacteria probably arranged in a biofilm. This fact seems to be of particular importance for CAPD patients and should be studied further.