The interface detector (ID) is an optical density sensor that affects the quality of single donor platelet collection using the CS 3000 Plus blood separator. The purpose of this study was to evaluate the effect of altering ID position on platelet yields and the contamination of leukocytes (WBC) in platelet concentrates (PCs). Dual-needle apheresis procedures (n = 93) were performed using an A35 collection chamber. Plateletpheresis products were separated according to interface detector offset (IDO) positioning into four groups: A: IDO = 6 (n = 33), B: IDO = 10 (n - 28), C: IDO = 12 (n = 18), D: IDO = 18 (n = 14). For 32% of the collections, the closed system apheresis kit with integral Sepacell filter (Baxter) was used and 33% of them were leukodepleted using the LRP-6 (PALL) filter. Our results showed that: (1) Although the mean blood volume and the time of apheresis were significantly higher, the mean platelet (PLT) yields were significantly lower in PCs of group A as compared to all other groups (P < 0.0001). (2) The mean WBC content was significantly higher in PCs of group D as compared to all other groups (P < 0.0001). (3) With the LRP-6 filter, a significantly higher WBC reduction as well as PLT loss in PCs was observed as compared to Sepacell leukapheresis filter. A higher PLT loss was observed with both filters when leukoreduction was performed within the first 6 hours as compared to 24 hours after the procedure. Conclusively, an IDO setting of 10 or 12 results in better platelet yields in PCs without increasing the WBC contamination. An IDO positioning of 18 or higher must be avoided or should be always combined with PCs leukodepletion. Finally, the best timing for leukoreduction is 24 hours after the plateletpheresis. © 2002 Wiley-Liss, Inc.