Prevention and Management of Platelet Transfusion Refractoriness

Platelet transfusion refractoriness is a major complication of long-term platelet supportive care. Refractoriness may lead to fatal bleeding complications in thrombocytopenic patients. Major factors involved are factors related to the clinical condition of the patient as well as HLA alloimmunisation. Non-alloimmune factors may occur in up to 80% of the patients. However, platelet transfusion outcome is impaired in only 50% of the patients having these conditions. HLA alloimmunisation has been convincingly reduced by the use of leucocytedepleted transfusions. UV-B irradiation of platelet transfusions may be alternatively used to reduce HLA alloimmunisation. Despite these measures, patients with a history of pregnancy or non-leucocyte-depleted transfusions form HLA antibodies in a high proportion (up to 50%). HPA antibodies play a minor but relatively important role in patients with HLA antibodies. ABO antibodies may play a role in refractoriness, which can be abolished by transfusion of ABO-identical platelets. Screening for the presence of HLA and/or HPA antibodies is indicated in case of transfusion failure after ABO-identical or HLA-matched platelets. If no alloantibodies are detected, further analysis to define a role of drugrelated or autoantibodies is required. In case of HLA and/or HPA alloimmunisation associated with refractoriness, matched platelet transfusions are indicated. In case of non-alloimmune factors associated with increased platelet consumption, increasing the transfusion frequency can be considered. Additional investigations are still necessary to define risk factors for secondary HLA alloimmunisation and refractoriness due to non-immune factors to further decrease the incidence of refractoriness.     

Transfusion of platelet concentrates – clinical evaluation of two preparations

Abstract: The aim of this study was to compare the clinical effect of transfusion of platelet concentrates (PC) prepared from pooled buffy coats (BC) and PCs collected from a single donor (SD) by an apheresis technique. The influence of storage time and various clinical conditions was also studied. Thirty-two patients suffering from haematological malignancies were given a total of 326 platelet concentrates; 180 BC-PCs and 146 SD-PCs, median 7 transfusions per patient. BC-PCs contained 312±52×10⁹ and SD-PCs 383 ± 133 × 10⁹ platelets/unit (mean ± SD). The mean storage time of BC-PC was 3 d and that of SD-PC 1 d. The mean platelet count of the patients before transfusion was 11 ± 8 ×10⁹/L. Regression analysis showed a significant decrease of the post-transfusion platelet corrected count increment (CCI) during storage of PCs for 1–5 d (BC-PC: p <0.01; SD-PC: p <0.05). There was no difference in platelet increment between BC-PC and SD-PC. Human leukocyte antigen (HLA) alloimmunization was the major cause of clinical refractoriness to random donor platelet transfusions but splenomegaly also caused low CCI values.     

Efficacy of UVC-treated,pathogen-reduced platelets versus untreated platelets: a randomized controlled non-inferiority trial

Pathogen reduction (PR) technologies for blood components have been established to reduce the residual risk of known and emerging infectious agents. THERAFLEX UV-Platelets, a novel ultraviolet C (UVC) light-based PR technology for platelet concentrates, works without photoactive substances. This randomized, controlled, double-blind, multicenter, non-inferiority trial was designed to compare the efficacy and safety of UVC-treated platelets to that of untreated platelets in thrombocytopenic patients with hematologic-oncologic diseases. The primary objective was to determine non-inferiority of UVC-treated platelets, assessed by the 1-hour corrected count increment (CCI) in up to eight per-protocol platelet transfusion episodes. Analysis of the 171 eligible patients showed that the defined non-inferiority margin of 30% of UVC-treated platelets was narrowly missed as the mean differences in 1-hour CCI between standard platelets versus UVC-treated platelets for intention-to-treat and per-protocol analyses were 18.2% (95% Confidence Interval [CI]: 6.4-30.1) and 18.7% (95% CI: 6.3-31.1), respectively. In comparison to the control, the UVC group had a 19.2% lower mean 24-hour CCI and was treated with an about 25% higher number of platelet units, but the average number of days to the next platelet transfusion did not differ significantly between both treatment groups. The frequency of low-grade adverse events was slightly higher in the UVC group and the frequencies of refractoriness to platelet transfusion, platelet alloimmunization, severe bleeding events, and red blood cell transfusions were comparable between groups. Our study suggests that transfusion of pathogen-reduced platelets produced with the UVC technology is safe but non-inferiority was not demonstrated. (clinicaltrials gov. Identifier: DRKS00011156).     

Vancomycin-dependent antibodies associated with thrombocytopenia and refractoriness to platelet transfusion in patients with leukemia

Two patients with leukemia experienced profound thrombocytopenia and refractoriness to platelet transfusion during vancomycin treatment. In one patient, withdrawal of drug and administration of platelet transfusions restored platelet counts to near normal levels (approximately 100 x 10(9)/L), however, subsequent challenge with vancomycin due to recurring infection again precipitated severe thrombocytopenia (platelets less than 10 x 10(9)/L) and life-threatening hemorrhagic symptoms. Potent vancomycin-dependent antiplatelet antibodies were detected in the serum of both patients during the refractory period using staphylococcal protein A rosette formation. Employing a monoclonal antibody-antigen capture enzyme-linked immunosorbent assay (ELISA), the patients were found to have vancomycin-dependent IgG antibodies that bound specifically to platelet glycoproteins (GP) IIb and/or IIIa. One of these antibodies failed to react with platelets deficient in GPIIb/IIIa obtained from an individual with Glanzmann's thrombasthenia. These findings provide the first major evidence for drug-dependent antibodies in association with severe thrombocytopenia and refractoriness to platelet transfusion in alloimmunized leukemia patients and, further, provide the first demonstration of vancomycin-dependent antibodies reactive with platelets.     

Anti‐HLA alloantibodies in surgical patients refractory to platelet transfusion

Alloimmune platelet refractoriness (alloPR) among actively bleeding surgical patients with thrombocytopenia represents a life‐threatening problem. Here we present three cases in which surgical bleeding was complicated by life‐threatening thrombocytopenia and alloPR. We demonstrate that the human leukocyte antigens (HLA) antibodies associated with alloPR are broadly reactive and in high concentration, are not removed by hemodilution, and are not absorbed by transfusion of multiple doses of platelet concentrates. HLA alloPR may be under‐recognized among surgical patients. Research is needed to develop pre‐operative screening methods that will identify patients in need of specialized platelet support using HLA compatible donor products. Am. J. Hematol. 89:E133–E137, 2014. © 2014 Wiley Periodicals, Inc.     

The role of ABO matching in platelet transfusion

Abstract: A prospective controlled trial was performed to determine whether the use of ABO-identical platelets from the start of treatment might provide higher post-transfusion platelet increments, reduce the number of platelet transfusions and ultimately delay the onset of refractoriness. Forty newly diagnosed patients with haematological diseases were randomized to receive either pooled ABO-identical platelets or pooled platelets unmatched for ABO group throughout their course. The corrected platelet count increments (CCI) were calculated for the first 25 transfusions of each patient and non-immune factors present at the time of each platelet transfusion were documented. The mean CCI for the first 25 transfusions in the ABO-identical group was significantly higher (6600 ±: 7900 SD) than that achieved with ABO unmatched platelets (5200 ±: 7900; p<0.01). The effect was most marked for the first 10 transfusions for each patient where the CCI was 64% higher in the ABO-identical group (8200 ±: 7500 vs 5000 ±: 8100; p<0.0002). Patients given ABO-identical platelets required only about half as many transfusions in the first 30 days (10 versus 17, p<0.05) or during the first admission (11 versus 21 p<0.01) as patients in the ABO-unmatched group. A smaller percentage of patients in the ABO-identical group became refractory (36% vs 75% p<0.03). The data suggest that patients requiring long-term platelet support should be transfused with ABO-identical platelets.     

Laboratory testing for platelet antibodies

Laboratory testing for immune‐mediated thrombocytopenias involves identification and classification of antibodies present in patient sera or attached to patient platelets. This article summarizes the available types of platelet antibody testing and applications in disorders such as neonatal alloimmune thrombocytopenia, post‐transfusion purpura, multiple platelet transfusion refractoriness, immune thrombocytopenia, and drug‐induced thrombocytopenia. Am. J. Hematol. 88:818–821, 2013. © 2013 Wiley Periodicals, Inc.     

Prevention of Primary HLA Class I Allo-Immunization with Leucocyte-Poor Blood Components Produced without the Use of Platelet Filters

In a prospective study the incidence of allo-immunization and platelet refractoriness was investigated using a consequently leucocyte-poor blood product regime. Twenty-five previously non-transfused patients with acute leukaemia (11 men, 7 women) or autologous bone marrow transplantation for Hodgkin's or non-Hodgkin's lymphoma (2 men, 5 women) received at least 80 donor units of filtered red cells (filtration within 24 h after donation, leucocyte content 8.5 +/- 3.9 x 10(6)/U) and/or of platelet concentrate (produced by the buffy coat method, leucocyte content: 7.8 +/- 4.2 x 10(6)/U). A 1-hour recovery of 20% in three consecutive transfusions, in the absence of clinical factors known to impair increment, was defined as platelet refractoriness. HLA class I antibody screening with a panel of 60 cells was performed before the first transfusion and after 80 U of blood components. Of 25 patients who entered this study, 6 patients developed platelet refractoriness after a mean of 38 units of blood components (range 26-45 U); all 6 were female with a history of multiple pregnancies. In 19 patients regarded as non-refractory, no HLA antibodies were demonstrated (13 men, 6 women). This study, though limited in size, suggests that the use of blood products containing less than 1 x 10(7) leucocytes/donor unit prevents primary HLA class I immunization and platelet refractoriness.     

Human leukocyte antigen (HLA) class I antibodies and transfusion support in paediatric HLA-matched haematopoietic cell transplant for sickle cell disease

The relevance of donor-specific human leukocyte antigen (HLA) antibodies in HLA-mismatched haematopoietic cell transplant (HCT) is known, but the importance of HLA antibodies in HLA-matched HCT is unclear. We hypothesized that HLA antibodies detected before HCT would cause platelet transfusion refractoriness during HCT and investigated this in a multi-centre study. Pre-HCT samples from 45 paediatric patients with sickle cell disease (SCD) undergoing HLA-matched HCT were tested for HLA class I antibodies. The number of platelet transfusions received before day +45 was compared between those with and without antibodies. Thirteen of 45 (29%) patients had a positive HLA class I antibody screen, and these patients received significantly more platelet transfusions than patients without antibodies (median 19 vs. 7·5, P = 0·028). This platelet transfusion association remained significant when controlling for conditioning regimen. Among alloimmunized patients, there was no association between the panel-reactive antibody and the number of platelet transfusions. Patients with HLA class I antibodies also had a higher incidence of acute graft-versus-host disease (GVHD): 6/13 (46%) vs. 3/32 (9%), P = 0·011. Pre-HCT HLA class I alloimmunization is associated with increased platelet transfusion support and acute GVHD in paediatric HLA-matched HCT for SCD. Further studies are needed to investigate the pathobiology of this association.     

The mean fluorescence intensities of anti‐HLA antibodies detected using micro‐bead flow cytometry predict the risk of platelet transfusion refractoriness

There are no accepted methods to predict the development of platelet transfusion refractoriness (PTR) due to human leucocyte antigen (HLA)‐alloimmunization. Hence, matched platelets are usually given only to patients demonstrating PTR, necessarily resulting in some ineffective random donor platelets (RDPLT) transfusions. To assess its utility in predicting PTR, we retrospectively tested samples from 387 patients receiving chemotherapy for acute leukaemia or autologous transplantation using a micro‐bead flow cytometry assay. The average of the mean fluorescence intensities (avgMFI) of the class I beads in the screening assay was correlated with outcomes of RDPLT transfusions during a 2 week period. Antibodies were detected in 57 patients; 66 developed PTR, of whom 28 were alloimmunized. avgMFI usefully predicted the development of PTR (area under the receiver operating curve 0·87, 95% confidence interval: 0·77–0·96). A logistic regression model estimated the probability of PTR to be >90% when avgMFI >5440. These results indicate that micro‐bead flow cytometry assays could inform a risk‐adapted strategy for managing thrombocytopaenic HLA allo‐immunized patients.     

Audit of Practice in Platelet Refractoriness

Objectives: A significant number of patients become refractory to platelet transfusion and prompt investigation of the cause will encourage appropriate selection of platelet products. Methods: We surveyed haematologists to assess perceived practice concerning platelet refractoriness because of the high cost and limited availability of HLA-compatible platelets. Some 56 of 58 consultant haematologists participated. Results: Clinicians differed on their definition of platelet refractoriness, and non-immune factors were not considered as important as immune causes of platelet refractoriness. A working group, including an invited moderator, was established to produce guidelines on recommended practice for the management of platelet refractoriness. Re-audit after implementation of the guidelines showed that more patients receiving HLA-compatible platelets had been tested for HLA antibodies. There was a mean 50.9% reduction in the use of HLA-compatible platelets. Conclusions: Increased testing for leucocyte and platelet antibodies resulted in reduced demand for and more selective use of HLA-compatible platelets, with no apparent increase in haemorrhagic complications.     

Therapeutic efficacy and safety of platelets treated with a photochemical process for pathogen inactivation: the SPRINT Trial

We report a transfusion trial of platelets photochemically treated for pathogen inactivation using the synthetic psoralen amotosalen HCl. Patients with thrombocytopenia were randomly assigned to receive either photochemically treated (PCT) or conventional (control) platelets for up to 28 days. The primary end point was the proportion of patients with World Health Organization (WHO) grade 2 bleeding during the period of platelet support. A total of 645 patients (318 PCT and 327 control) were evaluated. The primary end point, the incidence of grade 2 bleeding (58.5% PCT versus 57.5% control), and the secondary end point, the incidence of grade 3 or 4 bleeding (4.1% PCT versus 6.1% control), were equivalent between the 2 groups (P =.001 by noninferiority). The mean 1-hour posttransfusion platelet corrected count increment (CCI) (11.1 x 10(3) PCT versus 16.0 x 10(3) control), average number of days to next platelet transfusion (1.9 PCT versus 2.4 control), and number of platelet transfusions (8.4 PCT versus 6.2 control) were different (P <.001). Transfusion reactions were fewer following PCT platelets (3.0% PCT versus 4.4% control; P =.02). The incidence of grade 2 bleeding was equivalent for PCT and conventional platelets, although posttransfusion platelet count increments and days to next transfusion were decreased for PCT compared with conventional platelets.     

Leukocyte depletion of random single-donor platelet transfusions does not prevent secondary human leukocyte antigen-alloimmunization and refractoriness: a randomized prospective study

We studied the value of leukocyte depletion of platelet transfusions for the prevention of secondary human leukocyte antigen (HLA)- alloimmunization in patients with a high-risk of prior immunization induced by pregnancies. Seventy-five female patients with hematologic malignancies (mostly acute leukemia) and a history of pregnancy were randomized to receive either standard random single-donor platelet transfusions (mean leukocytes, 430 x 10(6) per transfusion) or leukocyte-depleted random single-donor platelet transfusions. Leukocyte depletion to less than 5 x 10(6) leukocytes per platelet transfusion (mean leukocytes, 2 x 10(6) per transfusion) was achieved by filtration. Of the 62 evaluable patients, refractoriness to random donor platelets occurred in 41% (14 of 34) of the patients in the standard group and in 29% (8 of 28) of the patients in the filtered group (P = .52); anti-HLA antibodies developed in 43% (9 of 21) of individuals in the standard group and 44% (11 of 25) of cases in the filtered group. The time toward refractoriness and development of anti- HLA antibodies was similar for both groups. We conclude that leukocyte depletion of random single-donor platelet products to less than 5 x 10(6) per transfusion does not reduce the incidence of refractoriness to random donor platelet transfusion because of boostering of anti-HLA antibodies.     

Intravascular and total body platelet equilibrium in healthy volunteers and in thrombocytopenic patients transfused with single donor platelets

Instrument platelet counts used in corrected count increment (CCI) and percent platelet recovery (PPR) formulas presume the transfused platelets are in equilibrium during the first hour after platelet transfusion. The timing of the pre-transfusion count affects CCI results, and we postulate that timing of CCI post transfusion affects CCI results. Platelet equilibrium using indium-111 platelet transfusions has not been reported. Platelet redistribution was studied in 16 healthy volunteers and 12 thrombocytopenic patients by generally infusing less than 72-hr stored single-donor platelets along with an aliquot of indium-111-labeled platelets by intravenous push. Counts were measured at 10, 15, 20, 60, and 120 min, and 24, 48, 72 hr along with continuous body scanning for 2 hr in healthy volunteers, and static organ scanning in patients and volunteers. Results indicated transfused platelets do not reach intravascular equilibrium for 60 min post-infusion and that the 10-min count cannot detect platelet refractoriness. However, total body equilibrium varies considerably between normal volunteers and thrombocytopenic patients. It is recommended to continue with the 1-hr post transfusion count. Am. J. Hematol. 58:165–176, 1998. © 1998 Wiley-Liss, Inc.     

Bleeding risk and platelet transfusion refractoriness in patients with acute myelogenous leukemia who undergo autologous stem cell transplantation

Therapy for acute myelogenous leukemia can be complicated by alloimmunization to histocompatibility antigens (HLA), with resultant refractoriness to platelet transfusions. Autologous peripheral blood or bone marrow stem cell transplantation (referred here collectively as 'autoBMT') is emerging as a standard consolidative strategy in acute myelogenous leukemia (AML). We had noted life-threatening bleeding associated with platelet transfusion refractoriness following autoBMT; we therefore retrospectively analyzed 39 AML patients for this complication following BMT. All patients received high-dose chemoradiotherapy, followed by infusion of allogeneic sibling donor (n = 12, alloBMT) or autologous (n = 27, autoBMT) stem cells. HLA alloimmunization was assessed if patients were suspected of immune refractoriness to random donor platelet transfusions. Within 100 days of stem cell infusion, one of three alloBMT and six of 12 autoBMT recipients tested were HLA alloimmunized (not statistically significant, NS). Five of six HLA alloimmunized autoBMT patients experienced delayed bleeding, which contributed to their demise while still in remission (P < 0.001). Increased platelet requirements in HLA alloimmunized autoBMT recipients were observed between days 61 and 100 post-BMT, at a median of 211 platelet transfusions vs 0 in non-alloimmunized autoBMT patients (P < 0.01) and 17 in alloBMT patients. Our data suggest that platelet transfusion refractoriness, when associated with HLA alloimmunization, is a risk factor for increased platelet transfusion requirements, delayed bleeding, and poor outcome following autoBMT for AML.     

Clinical guidelines for testing for heritable thrombophilia

Pathogen reduction (PR) of platelet products increases costs and available clinical studies are equivocal with respect to clinical and haemostatic effectiveness. We conducted a multicentre, open‐label, randomized, non‐inferiority trial comparing the clinical effectiveness of buffy‐coat derived leucoreduced platelet concentrates (PC) stored for up to 7 d in plasma with platelets stored in platelet additive solution III (PASIII) without and with treatment with amotosalen‐HCl/ultraviolet‐A (UVA) photochemical pathogen reduction (PR‐PASIII). Primary endpoint of the study was 1‐h corrected count increment (CCI). Secondary endpoints were 24‐h CCI, bleeding, transfusion requirement of red cells and PC, platelet transfusion interval and adverse transfusion reactions. Compared to plasma‐PC, in the intention to treat analysis of 278 evaluable patients the mean difference for the 1‐h CCI of PR‐PASIII‐PC and PASIII‐PC was −31% (P < 0·0001) and −9% (P = n.s.), respectively. Twenty‐seven patients (32%) had bleeding events in the PR‐PASIII arm, as compared to 19 (19%) in the plasma arm and 14 (15%) in the PASIII arm (P = 0·034). Despite the potential advantages of pathogen (and leucocyte) inactivation of amotosalen‐HCl/UVA‐treated platelet products, their clinical efficacy is inferior to platelets stored in plasma, warranting a critical reappraisal of employing this technique for clinical use.     

Platelet transfusion improves clot formation and platelet function in severely thrombocytopenic haematology patients

Prophylactic platelet (PLT) transfusion is a common practice in severely thrombocytopenic patients that reduces mortality, but responses to platelet transfusions are variable and difficult to predict in individual patients. In this prospective study, we evaluated the outcome of PLT transfusions in 40 patients with haematological malignancies, linking corrected count increment (CCI) to clot formation and agonist-induced platelet activation after transfusion. The CCI was highly variable between patients and 34% showed no response (1-h CCI < 7,5). Short time since the last PLT transfusion and extended storage time of the PLT product were linked to poor transfusion response, while patient sex, C-reactive protein or the number of chemotherapy cycles prior to transfusion did not influence transfusion outcome. High CCI and good PLT responsiveness to agonist stimulation predicted efficient clot formation in rotational thromboelastometry, but transfusion did not restore poor PLT function in patients to the level of healthy controls. Our study provides new insights into factors affecting PLT transfusion outcome in haematology patients with severe thrombocytopenia, and suggests that the thrombocytopenic environment, or disease-associated factors, may hamper platelet responsiveness.     

Successful treatment of platelet transfusion refractoriness: the use of platelet transfusions matched for both human leucocyte antigens (HLA) and human platelet alloantigens (HPA) in alloimmunized patients with leukaemia

Six patients, 4 with acute myeloid leukaemia and 2 with a myelodysplasia syndrome who were refractory to random donor platelet transfusions and alloimmunized to human leucocyte antigens (HLA) and human platelet alloantigens (HPA), were treated with HLA-and HPA-matched platelet transfusions. In all the patients refractoriness and alloantibodies to HLA as well as HPA-1b or HPA-5b were detected simultaneously. Sixty-seven transfusions (445 units) of HLA-and HPA-matched platelets were given and responses to them were, in general, satisfactory in all the patients. No major spontaneous bleeding occurred. Four patients underwent bone marrow transplantation despite alloimmunization. The percentages of platelet transfusion days with a platelet nadir below 20×10⁹/l were 88% for the last 3 random donor platelet transfusions and 39% for the first 3 HLA-and HPA-matched platelet transfusions, respectively (p=0.009, Fisher's exact test). Four patients received also HLA-matched platelets, but responses to them were poor. The small number of transfusions with HLA-matched platelets precluded comparisons to either the random donor or HLA-and HPA-matched platelet transfusions. It seems that HLA-and HPA-alloimmunized patients can be successfully supported with HLA-and HPA-matched platelet concentrates.     

Lymphocytotoxic antibody is a predictor of response to random donor platelet transfusion

The transfusion records of 189 patients with acute leukemia were analyzed to correlate lymphocytotoxic antibody (LCTAb) levels with response to a series of random-donor platelet transfusions (Tx). Twenty-one patients were studied twice at times of different LCTAb levels. All transfusions were given when patients were clinically stable without disseminated intravascular coagulation, bleeding, temperature > 101°F, or splenomegaly. The mean 1-hr and 24-hr corrected count increments (CCI) for all patients with negative LCTAb were 16,100 and 12,000, and values for patients with positive LCTAb were 5,600 and 2,600 (P < 0.0005). Thirteen patients had intermediate LCTAb (10–20%) and a variable response to Tx. Of the 137 patients with negative LCTAb levels 106 (77%) had good mean CCI of > 10,000 at 1 hr and > 7,500 at 24 hr following transfusion. In contrast of 60 patients with positive LCTAb (> 20% cytotoxicity), 53 (88%) had poor CCI of < 10,000 at 1 hr and < 7,500 at 24 hr after transfusion. Only 4 patients with positive LCTAb had a good response to random donor platelets at both 1 and 24 hrs. Eighteen patients had negative LCTAb with a high 1-hr and low 24-hr CCI. Thirteen of these had a history of positive LCTAb and in 9 there was an anamnestic rise following transfusion. Nine of 137 patients had negative LCTAb with low 1-hr and 24-hr CCI. LCTAb is highly predictive of response to random donor platelets. Cytotoxicity to > 20% of tested lymphocytes virtually precludes a good CCI at 1 and 24 hr.     

Use of leukocyte-depleted platelet concentrates for the prevention of refractoriness and primary HLA alloimmunization: a prospective, randomized trial

Compared with conventional transfusion regimes a strong reduction in HLA alloimmunization and refractoriness to platelet transfusions is obtained when both red blood cell concentrates (RBCs) and platelet concentrates (PCs) are depleted of leukocytes by filtration. Because most of the leukocyte contamination is introduced by transfusion of RBCs, filtration of RBCs appears rational, but uncertainty exists regarding the degree of leukocyte-depletion of PCs needed for the prevention of HLA alloimmunization and refractoriness. We conducted a prospective trial and randomized patients with acute leukemia to receive leukocyte-depleted PCs prepared either by centrifugation (mean leukocyte count 35 x 10(6)/PC of 6 U) or by filtration (mean leukocyte count less than 5 x 10(6)/PC of 6 U). Both groups received RBCs that were filtered after prior removal of the buffy coat. Clinical refractoriness occurred in 46% (12 of 26) of the evaluable patients that were transfused with centrifuged PCs and only in 11% (3 of 27) in the filtered group (P less than .005). De novo anti-HLA antibodies were detected in 42% (11 of 26) patients in the centrifuged group and only in 7% (2 of 27) of the patients receiving filtered PCs (P less than .004). In 8 of 11 alloimmunized patients in the centrifuged group antibodies were detected in the first 4 weeks of transfusion therapy while none of the patients in the filtered group became immunized against HLA antigens during that period. We conclude that for the prevention of HLA alloimmunization and refractoriness to platelet transfusions from random donors, both RBCs and PCs have to be leukocyte-depleted by filtration.