Serum samples were collected before transfusion for direct platelet antibody screening and samples were then stored at -20oC until processing. Platelet antibodies were screened by the FC-PIFT. commercially available methods. The sensitivity, specificity and predictive values of the immunofluorescence test were determined taking into account that the majority of antiplatelet antibodies presented human leukocyte antigen specificity. Results Seventy-six samples from 32 female and 38 male patients with a median age of 43.5 years (range: 5-84 years) were analyzed. The sensitivity of the test was 86.11% and specificity 75.00% with a positive predictive value of 75.61% and a negative predictive value of 85.71%. The accuracy of the method was 80.26%. Conclusion This study shows that the flow cytometry platelet immunofluorescence test has a high correlation with the anti-human leukocyte antigen antibodies. Despite a few limitations, the method seems to be efficient, fast and feasible as the initial screening for platelet antibody detection and a useful tool to crossmatch platelets for the transfusional support of patients with immune platelet refractoriness. strong class=”kwd-title” Keywords: Blood platelets, Antigens, human leukocyte, Flow cytometry, Histocompatibility, Antigens, (S)-3,4-Dihydroxybutyric acid human platelet Introduction Refractoriness to platelet transfusions – platelet refractoriness (PR) – is defined as inappropriately low (S)-3,4-Dihydroxybutyric acid platelet count increments following two or more, preferably consecutive, transfusions(1). PR must be determined by objective data which determine platelet transfusion outcomes, such as the corrected count increment (S)-3,4-Dihydroxybutyric acid (CCI) and the predicted percentage recovery (PPR)(2). This condition may be caused by immune and non-immune factors. Non-immune causes, represent the main etiology (more than 80% of cases) of platelet refractoriness and include splenomegaly, fever/sepsis, antibiotics or disseminated intravascular coagulation(3). Immune causes, occurring in less than 20% of the cases, involve alloimmunization against human leukocyte antigens (HLA) and, to a lesser extent, human platelet antigens (HPA) following exposure through transfusion, pregnancy or transplantation. Among immune causes, HLA antibodies are responsible for approximately 80-90% of PR (S)-3,4-Dihydroxybutyric acid cases and HPA antibodies for approximately 10-20% of cases, associated with HLA antibodies or not. It is important to note that the presence of antiplatelet antibodies does not mean PR, since in approximately 30% of cases, they occur in the absence of clinically detected PR(4). The immune causes of PR can be diagnosed by IL2RG laboratory tests and patients should receive compatible platelet transfusions(5). Testing for HPA antibodies is technically demanding and few laboratories offer the exam in their routine. The platelet antibody detection tests available include microcytotoxicity using Amos modification that detects both IgG and IgM antibodies, the platelet immunofluorescence test (PIFT) either by microscopy or flow cytometry (Capture-P? and monoclonal antibody immobilization of platelet antigens (MAIPA)(2,6-8). The latter is the gold standard technique that permits the identification and quantification of platelet specific antibodies however this (S)-3,4-Dihydroxybutyric acid technique is very laborious and time-consuming. Thus, a fast, effective and low cost antibody-screening method, which could detect both HLA and HPA platelet antibodies is essential for the recognition and for clinical support in immune PR. The main aim of the study was to evaluate the efficiency of the flow cytometry platelet immunofluorescence test (FC-PIFT) as a screening test to identify immune PR. Methods A group of hematologic patients with clinically suspected PR treated at the hospital complex of the Universidade Estadual de Campinas during the period July 2006 to July 2011 was prospectively enrolled in this study. Serum samples were collected before transfusion for direct platelet antibody screening and samples were then stored at -20oC until processing. Platelet antibodies were screened by the FC-PIFT. Sera from 24 regular male blood donors with no history of previous transfusions were analyzed and a negative fluorescence standard curve was defined. Pooled platelets from two O blood group male donors with no history of previous sensitization were buffer washed and re-suspended in 0.1% phosphate buffered saline (PBS)/ethylenediaminetetraacetic acid (EDTA).