DAVIS ET AL. 586 TRANSFUSION Volume 39, June 1999 BACKGROUND: Corrected count increment (CCI) and percent platelet recovery (PPR) are measures of re- sponse to platelet transfusion that “correct” the count in- crement for blood volume and number of platelets trans- fused. Their potential for data distortion is described, and a regression analysis is suggested that is more in- formative and avoids the inherent problems associated with using ratios as outcome measures. STUDY DESIGN AND METHODS: Data from the first platelet transfusion for 585 patients from the Trial to Re- duce Alloimmunization to Platelets (TRAP) were used to model methods of analyzing posttransfusion platelet re- sponse. RESULTS: By linear regression analysis, unfiltered platelet components gave a greater posttransfusion in- crement on average (p = 0.001), but filtered platelets gave a greater increment per platelet transfused (p = 0.003). In contrast, CCI and PPR showed no difference between filtered and unfiltered platelets (p = 0.36 and p = 0.29, respectively) because they combined the effects of dose, filtration, and patient size. Slightly fewer patients are required for a study analyzed by regression analysis. CONCLUSION: Regression analysis of posttransfusion platelet increments should be used instead of CCI or PPR to compare the efficacy of platelet components. CCI and PPR should not be used to define platelet re- fractoriness as a study outcome, because these mea- sures are biased in favor of platelet preparation tech- niques that provide fewer platelets. T he posttransfusion platelet count is an important part of the evaluation of a patient’s response to a platelet transfusion. Posttransfusion platelet re- coveries in thrombocytopenic patients average 60 ± 15 percent of the transfused dose. 1 Approximately 30 to 35 percent of transfused platelets are normally pooled in the spleen. Recovery is further decreased in hypersplenic patients and may be above 90 percent in asplenic individu- als. Concurrent medical conditions such as bone marrow transplantation, hepatomegaly, and antibiotic therapy and a variety of other factors may also reduce the response to platelet transfusion. 2-5 The posttransfusion platelet count is affected by the viability of the platelets as well as the num- ber of platelets in the platelet component. It is also affected by the dilution of platelets in the patient’s blood volume. Measures that compensate for the number of platelets in the component and the patient’s blood volume have been considered a more precise comparison of the posttransfusion platelet response between platelet preparations or between patients. The corrected count increment (CCI) and the percent platelet recovery (PPR) are measures that have been used Corrected count increment and percent platelet recovery as measures of posttransfusion platelet response: problems and a solution K.B. Davis, S.J. Slichter, and L. Corash ABBREVIATIONS: BSA = body surface area; CCI(s) = corrected count increment(s); PC(s) = platelet concentrate(s); PPR = per- cent platelet recovery; TRAP = Trial to Reduce Alloimmunization to Platelets; UV = ultraviolet; WBC(s) = white cell(s). From Axio Research Corporation and the Puget Sound Blood Center, Seattle, Washington; and the Cerus Corporation, Con- cord, California. Address reprint requests to: Sherrill Slichter, MD, Puget Sound Blood Center, 921 Terry Avenue, Seattle, WA 98104-1256; e-mail: davis@biostat.washington.edu. Supported by grants U01 HL42799, U01 HL42802, U01 HL42805, U01 HL42810, U01 42811, U01 HL42815, U01 HL42824, and U01 HL42832 from the National Heart, Lung, and Blood Institute, National Institutes of Health. Received for publication March 31, 1998; revision received August 29, 1998, and accepted October 26, 1998. TRANSFUSION 1999;39:586-592. T R A N S F U S I O N P R A C T I C E