Glanzmann thrombasthenia platelets compete with transfused platelets, reducing the haemostatic impact of platelet transfusions

Glanzmann thrombasthenia (GT) is a severe, rare, predominantly mucocutaneous, autosomal recessive bleeding disorder. GT patients have normal platelet counts and morphology, but show absent/severely reduced platelet aggregation owing to defective function/absence of the fibrinogen receptor glycoprotein (GP) IIb-IIIa (integrin aIIbb3) (George et al, 1990; Nurden & Nurden, 2014). Management of GT involves preventing bleeds where possible plus the expeditious management of bleeds that do occur. For persistent or severe bleeding, despite use of local measures and non-specific haemostatic agents (e.g. anti-fibrinolytic agents) and/or recombinant factor VIIa (rFVIIa; NovoSeven, Novo Nordisk, Bagsværd, Denmark), or for patients undergoing major surgery, platelet transfusions are recommended (Poon et al, 2004). As GT patients have normal platelet counts, a post-transfusion platelet count is of limited predictive value to accurately assess the transfusion response, given normal fluctuations observed in platelet counts within all patients (Male et al, 2006). Thus, there is a need for an easily performed, readily available laboratory measure of the response to platelet transfusions in GT patients to ensure that the patient is not platelet refractory, particularly prior to surgery or when treating a lifethreatening bleed. Laboratory tests that can potentially be used to show improvement in primary haemostasis and/or platelet function include the bleeding time (Jennings et al, 1991), flow cytometry (Nurden et al, 2002; Cesar & Vecino, 2009), and global haemostatic assays, such as thromboelastography or thrombin generation assays (Male et al, 2006). However, these tests suffer from lack of availability and reproducibility, and they do not provide a quick turnaround result as would be required in acute situations. We evaluated the Platelet Function Analyser (PFA-100 ; Siemens, Malvern, PA, USA) as a possible laboratory measure of response to platelet transfusions in four patients with GT (Carcao et al, 1998; Favaloro, 2008). Institutional ethics approval was obtained to conduct the study. All patients (Table I) showed the typical aggregation response defects seen in GT, lacked CD41 (GPIIb) by flow cytometry and were compound heterozygotes for ITGA2B. These four patients underwent PFA-100 testing after platelet transfusions on seven separate occasions for bleeding that failed to respond to local measures along with repeated infusions of rFVIIa and tranexamic acid (five episodes) and prior to wisdom teeth extraction (two episodes) (Table I). For all seven platelet transfusions, no apparent reduction (i.e. improvement) in PFA-100 closure times (CTs) posttransfusion was observed. We assessed whether this lack of improvement in CTs in some of the episodes was due to: (i) low haemoglobin levels, secondary to acute blood loss; or (ii) to the presence of anti-platelet antibodies that caused the patients to be platelet refractory. With respect to these possibilities, it should be noted that in 3/7 episodes, patients demonstrated normal haemoglobin levels and that all four patients had undergone multiple screening tests for the presence of anti-platelet antibodies [both anti-human leucocyte antigen (HLA) and anti-GPIIb-IIIa] using a qualitative solid phase enzyme-linked immunosorbent assay (PAK 12 by GTI diagnostics, Waukesha, WI, USA) designed to detect antibodies to HLA class I antigens and to platelet GPIIb-IIIa, GPIaIIa and GPIb-IX. On all 13 occasions, patients tested negative for anti-HLA and anti-glycoprotein antibodies. Given these results, it is extremely unlikely that the lack of improvement in PFA-100 CTs could have been caused by anti-platelet antibodies. GT patients, when transfused with a standard amount of either pooled or aphaeresis platelets (10 ml/kg; approximately 10 9 10 platelets/kg) will have both circulating endogenous GT platelets and transfused ‘normal’ platelets. Using flow cytometry, Cesar and Vecino (2009) reported that after a single platelet transfusion in a 10-year-old girl with GT, only 17% of circulating platelets were the transfused normal platelets. Consequently, when GT patients are given a standard platelet transfusion in the setting of a bleed, most of the platelets adhering to the injury site will still be the endogenous GT platelets as these are in the majority and not the transfused normal platelets. In this situation, we hypothesize that the GT platelets, that are capable of adhering to sites of vascular injury but then are