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INTRODUCTION:Bleeding disorder of unknown cause (BDUC) is a challenging diagnosis that predominantly affects women. Previous investigations into connective tissue disorders (CTD) and vitamin C have not been conducted. AIM:To examine the association between hypermobility-related disorders, vitamin C status and BDUC. METHODS:Patients were selected following laboratory and genetic screening that yielded negative results for known hemostasis disorders. Sixty patients with BDUC and an ISTH BAT score ≥ 10 underwent clinically examination for skin hyperextensibility and for hypermobility assessed by Beighton score. Vitamin C was analyzed by high-performance liquid chromatography. Genetic screening for causal variants in 42 CTD genes was performed. RESULTS:The majority of patients were female (56/60). Median ISTH BAT score was 13 (range 10-23). Beighton score was positive in 29/60 patients compared to 1/20 healthy controls (HC) (p < .001). Hyperextensive skin was observed in (18/60) patients, and none (0/20) of the HC (p = .0041). Ten patients met the clinical diagnostic criteria for hypermobile Ehlers-Danlos syndrome (hEDS), and one patient was diagnosed with Noonan syndrome. Genetic screening excluded various subtypes of EDS with known genetic backgrounds. Average vitamin C level was adequate, but lower than in HC (55.9 vs. 70.4 μmol/L; p = .001). Suboptimal, or low vitamin C were identified in 19/60 compared to 1/20 HC (p = .018). CONCLUSION:Our study demonstrates that BDUC is frequently associated with hypermobility disorders and low vitamin C status. Our results could pave the way for a randomized study of vitamin C supplementation in patients with BDUC.

Background Inherited platelet disorders (IPDs) are rare diseases characterized by reduced blood platelet counts and/or impaired platelet function. Recognizing IPDs is advisable but often challenging. The diagnostic tools include clinical evaluation, platelet function tests, and molecular analyses. Demonstration of a pathogenic genetic variant confirms IPDs. We established a method to assess the platelet phenotype on blood smears using immunofluorescence microscopy as a diagnostic tool for IPDs. Objectives The aim of the present study was to validate immunofluorescence microscopy as a screening tool for IPDs in comparison with genetic screening. Methods We performed a blinded comparison between the diagnosis made using immunofluorescence microscopy on blood smears and genetic findings in a cohort of 43 families affected with 20 different genetically confirmed IPDs. In total, 76% of the cases had inherited thrombocytopenia. Results Immunofluorescence correctly predicted the underlying IPD in the vast majority of patients with 1 of 9 IPDs for which the typical morphologic pattern is known. Thirty of the 43 enrolled families (70%) were affected by 1 of these 9 IPDs. For the other 11 forms of IPD, we describe alterations of platelet structure in 9 disorders and normal findings in 2 disorders. Conclusion Immunofluorescence microscopy on blood smears is an effective screening tool for 9 forms of IPD, which include the most frequent forms of inherited thrombocytopenia. Using this approach, typical changes in the phenotype may also be identified for other rare IPDs.

Introduction Immune mediated thrombocytopenia (ITP) is a platelet disorder in which the immune system attacks and destroys the body's own platelets. Mechanisms leading to low platelet count in ITP are multifactorial, involving both increased peripheral platelet destruction and decreased platelet production. This is caused by autoantibodies targeting megakaryocytes and platelet-specific glycoproteins, as well as cytotoxic T cells directly acting on platelets. Although antibodies in some cases can be found in patients with ITP, there is no well-established diagnostic test and the diagnosis is thus, one of exclusion. While most cases of ITP resolve within 3 months with or without treatment, some patients do not respond to conventional treatment and develop a refractory ITP. It is likely that in the group of patients diagnosed with refractory ITP, some does not have an immune mediated thrombocytopenia, but other causes of low counts (PLC)s. With the development of high throughput sequencing and the increasing knowledge of inherited bleeding disorders, the possibility to identify the underlying cause of disease in these patients is steadily growing. In ITP, there is a high degree of interindividual differences in bleeding rates, where some patients have no bleeding symptoms despite of unmeasurable PLC, whereas other patients show severe bleeding symptoms at PLC > 30 x10 9 /L. The underlying etiology of these discrepancies is currently unknown and complicates the clinical care of these patients. We hypothesize that genetic defects influence the bleeding pattern in true ITP patients as well as being one of the underlying causes of thrombocytopenia in patients misdiagnosed with “refractory ITP”. The aim of this pilot study was to investigate whether upfront genetic screening of patients suspected of ITP could be a valuable diagnostic tool to determine presence of inherited thrombocytopenia, secondary thrombocytopenia and genetic variants that might affect bleeding pattern. Method Patients were included after informed consent following the declaration of Helsinki. We performed whole genome sequencing (WGS) in patients diagnosed with presumed ITP at the Skåne University Hospital, Sweden. Whole genome sequencing was performed at the Center for Genomic Medicine at Rigshospitalet, Copenhagen. An in-silico gene panel consisting of 113 genes known to be associated with bleeding disorders as well as 86 genes associated with ITP (Using the VarSeq gene prioritisation algorithm phorank) was analysed and pathogenicity was classified according to the ACMG/AMP guidelines. Patients' medical records were examined, laboratory work up was performed and clinical data regarding demographics and information regarding bleeding was collected. A final evaluation of susceptible variants took place at multiple disciplinary team conferences. Results Twenty-one patients were enrolled (9 women, 12 men, 17 were adults and 4 children). Mean age of adult patients was 50,5 years and mean age of paediatric patients was 11,5 years. Out of 21 patients, 7 were shown to have genetic variants with relevance to their disease. All variants were found in adult patients. Two patients had variants related to inherited thrombocytopenia (X-linked thrombocytopenia(WAS) and monoallelic Bernard-Soulier Syndrome(GP1BB)), 4 had variants in genes related to secondary ITP (3 with variants associated with common variable immunodeficiency (NFKB1, TNFRSF13B) and 1 with systemic lupus erythematosus (TREX1)) and 1 had a variant associated with platelet dysfunction (platelet delta storage pool deficiency (UNC13D)). Only the variant associated with X-linked thrombocytopenia was previously described (class 5) whereas the remaining 6 variants were initially classified as variants of unknown significance (VUS). Functional work-up and co-segregation studies indicated them as probably pathogenic (class 4) but confirming studies are still pending. Conclusion In this pilot study, 33% of patients with presumed primary ITP had variants with relevance to their clinical phenotype, and we were able to diagnose 1 patient with an inherited bleeding disorder (X-linked thrombocytopenia). Whole genome sequencing seems to be a useful diagnostic tool in the work up of patients with thrombocytopenia where ITP is suspected to ensure the diagnosis and could also add important information when assessing bleeding risk in individual patients.

The authors regret the affiliation of Clinical Pathology Department, Faculty of Medicine, Mansoura University, Egypt was omitted for Maha Othman. This has been corrected and added as below. The authors would like to apologise for any inconvenience caused. GoldVariants, a resource for sharing rare genetic variants detected in bleeding, thrombotic, and platelet disorders: Communication from the ISTH SSC Subcommittee on Genomics in Thrombosis and HemostasisJournal of Thrombosis and HaemostasisVol. 19Issue 10PreviewThe implementation of high‐throughput sequencing (HTS) technologies in research and diagnostic laboratories has linked many new genes to rare bleeding, thrombotic, and platelet disorders (BTPD), and revealed multiple genetic variants linked to those disorders, many of them being of uncertain pathogenicity when considering the accepted evidence (variant consequence, frequency in control datasets, number of reported patients, prediction models, and functional assays). The sequencing effort has also resulted in resources for gathering disease‐causing variants associated with specific genes, but for BTPD, such well‐curated databases exist only for a few genes. Full-Text PDF Open Access

We report a genome-wide association study of venous thromboembolism (VTE) incorporating 81,190 cases and 1,419,671 controls sampled from six cohorts. We identify 93 risk loci, of which 62 are previously unreported. Many of the identified risk loci are at genes encoding proteins with functions converging on the coagulation cascade or platelet function. A VTE polygenic risk score (PRS) enabled effective identification of both high- and low-risk individuals. Individuals within the top 0.1% of PRS distribution had a VTE risk similar to homozygous or compound heterozygous carriers of the variants G20210A (c.*97 G > A) in F2 and p.R534Q in F5. We also document that F2 and F5 mutation carriers in the bottom 10% of the PRS distribution had a risk similar to that of the general population. We further show that PRS improved individual risk prediction beyond that of genetic and clinical risk factors. We investigated the extent to which venous and arterial thrombosis share clinical risk factors using Mendelian randomization, finding that some risk factors for arterial thrombosis were directionally concordant with VTE risk (for example, body mass index and smoking) whereas others were discordant (for example, systolic blood pressure and triglyceride levels). Genome-wide association analyses identify 93 risk loci for venous thromboembolism (VTE). A polygenic score derived from these results identifies individuals at increased VTE risk equivalent to monogenic forms of the disease.

The implementation of high-throughput sequencing (HTS) technologies in research and diagnostic laboratories has linked many new genes to rare bleeding, thrombotic, and platelet disorders (BTPD), and revealed multiple genetic variants linked to those disorders, many of them being of uncertain pathogenicity when considering the accepted evidence (variant consequence, frequency in control datasets, number of reported patients, prediction models, and functional assays). The sequencing effort has also resulted in resources for gathering disease-causing variants associated with specific genes, but for BTPD, such well-curated databases exist only for a few genes. On the other hand, submissions by individuals or diagnostic laboratories to the variant database ClinVar are hampered by the lack of a submission process tailored to capture the specific features of hemostatic diseases. As we move toward the implementation of HTS in the diagnosis of BTPD, the Scientific and Standardization Committee for Genetics in Thrombosis and Haemostasis has developed and tested a REDCap-based interface, aimed at the community, to submit curated genetic variants for diagnostic-grade BTPD genes. Here, we describe the use of the interface and the initial submission of 821 variants from 30 different centers covering 14 countries. This open-access variant resource will be shared with the community to improve variant classification and regular bulk data transfer to ClinVar.

BACKGROUND:The transcription factor GATA1 is an essential regulator of erythroid cell gene expression and maturation and is also relevant for platelet biogenesis. GATA1-related thrombocytopenia (GATA1-RT) is a rare X-linked inherited platelet disorder (IPD) characterized by macrothrombocytopenia and dyserythropoiesis. Enlarged platelet size, reduced platelet granularity, and noticeable red blood cell anisopoikilocytosis are characteristic but unspecific morphological findings in GATA1-RT. OBJECTIVES:To expand the investigation of platelet phenotype of patients with GATA1-RT by light- and immunofluorescence microscopy on a blood smear. METHODS:We assessed blood smears by light- and immunofluorescence microscopy after May-Grünwald Giemsa staining using a set of 13 primary antibodies against markers belonging to different platelet structures. Antibody binding was visualized by fluorescently labeled secondary antibodies. RESULTS:We investigated 12 individuals with genetically confirmed GATA1-RT from 8 unrelated families. While confirming the already known characteristic of platelet morphology (platelet macrocytosis and reduced expression of markers for α-granules), we also found aggregates of nonmuscular myosin heavy chain II A (NMMIIA) in the erythrocytes in all individuals (1-3 aggregates/cell, 1-3 μm diameter). By systematically reanalyzing blood smears from a cohort of patients with 19 different forms of IPD, we found similar NMMIIA aggregates in the red blood cells only in subjects with GFI1B-related thrombocytopenia (GFI1B-RT), the other major IPD featured by dyserythropoiesis. CONCLUSION:Aggregates of NMMIIA in the erythrocytes associate with GATA1-RT and GFI1B-RT and can facilitate their diagnosis on blood smears. This previously unreported finding might represent a novel marker of dyserythropoiesis assessable in peripheral blood.

Introduction Vitamin C is essential for the synthesis of collagen. Ehler-Danlos syndrome (EDS) is a group of collagen disorders associated with generalized joint hypermobility (HM), bleeding and bruising. The genetic cause of the HM EDS subtype (hEDS) is unknown. Referral to haematology clinics may result in the diagnosis: bleeding of unknown cause (BUC), where no specific haemostatic treatment is available. However, it is a clinical experience, published in many textbooks, that bleeding, and bruising associated with EDS improve by oral intake of vitamin C. No previous studies examined vitamin C levels in patients with BUC. The aim of the study is to investigate the association of vitamin C levels with HM and vascular fragility in patients with BUC. Methods Patients with bleeding tendency are referred to our Haemophilia Center. Following ISTH-BAT score, normal coagulation test (INR, APTT, fibrinogen, F8, F9, F11, F13, VWFAg, VWFRist) and inconclusive results of flow cytometry analysis of platelet function, patients are diagnosed with BUC. Patients with a high ISTH-BAT score > 10 were included in this study. We performed clinical examinations by a tourniquet test for capillary fragility and a Beighton score for generalized HM. A skin extensibility test was performed by pulling on the skin off the forearm and the lateral side of the throat until resistance was met. The test was considered positive (abnormal) if the skin extended more than 2 cm in both sites when pulled. Germline DNA was analyzed by whole genome sequencing (WGS) analysis and an in silico panel of 170 genes associated with bleeding and generalized HM was examined. Additionally, homozygous polymorphism in vitamin C genes ( GULOP, SLC23A1, SLC23A2, SVCT1, HP, GST and SOD2) known to cause reduced vitamin C levels, were assessed Vitamin C analysis: The participants received oral and written guidelines regarding intake of vitamin C in relation to the blood sample. The guidelines included no vitamin C supplements for 3 days before the blood sampling and no intake of vitamin C rich food on the day of the appointment. Plasma samples were immediately acidified by addition of 10% meta-phosphoric acid to ensure the stability of ascorbic acid, and subsequently analyzed by high-performance liquid chromatography. Severe vitamin C deficiency was defined as <11µM, deficiency as < 23 µM, suboptimal status as 23-49 µM and adequate status as >49 µM. Results Sixty probands and 20 healthy age- and gender-matched controls (HC) were included. Most patients were women (56/60, 93%), median age 48 (range 21- 75) and median BAT score 13 (range 10-21). By WGS a 67-year woman with Beighton score 7 was diagnosed with Noonan syndrome due to a pathogenic variant in PTPN11 (c.922A>G). Also, a 62-year woman without HM was diagnosed with mild FV deficiency due to a heterozygous variant in F5 (c.449A>G). No other causal variants were identified. In a patient with a homozygous variant in SLC23A1, vitamin C level was suboptimal 42µM.Suboptimal vitamin C levels were found in 19/60 patients and 2/20 HC. One patient was severely vitamin C deficient (7µM). Vitamin C levels were significantly lower in patients compared to HC (median 56 µM vs 76 µM; p = 0.012). Beighton score was positive in 29/60 patients and 1/20 HC (p<0.001). In 18/60 patients versus 0/20 HC the skin extensibility test was abnormal (p = 0.01). In 12/29 patients with a positive Beighton score the test for skin extensibility was also positive indicating a diagnosis of hEDS. Tourniquet test was positive (> 10 petechiae) in 18/60 patients and 4/20 HC (p>0.5). Patients with positive Beighton scores had significantly higher BAT scores than patients without HM (P= 0.014). No significant associations were found between vitamin C levels, a positive Beighton score, a positive Tourniquet test or a high BAT score respectively. Conclusions We found that a high proportion of patients with BUC have generalized HM, abnormal skin extensibility and positive tourniquet test. Indeed, patients with HM had increased ISTH-BAT scores compared to patients with negative Beighton scores. Our data support that hEDS may be an underlying cause of BUC. Further studies are needed to determine the genetic background of hEDS. Levels of vitamin C were reduced in patients with BUC compared to age and sex matched control. Our results could pave the way for a randomized study of vitamin C supplements versus placebo for bleeding and bruising in patients with generalized HM and BUC.

INTRODUCTION:Platelet function tests are used to screen and diagnose patients with possible inherited platelet function defects (IPFD). Some acquired platelet dysfunction may be caused by certain drugs or comorbidities, which need to be excluded before testing. AIMS:To identify current practice among centres performing platelet function tests in Northern Europe. METHODS:A total of 14 clinical centres from Sweden (six), Finland (two), Denmark (two), Norway (one), Estonia (two) and Iceland (one) completed the survey questionnaire, the population capture area of about 29.5 million. RESULTS:Six of the 14 (42.8%) centres providing platelet function assessment represent comprehensive treatment centres (EUHANET status). A Bleeding score (BS) or ISTH bleeding assessment tool (ISTH BAT score) is evaluated in 11/14 (78.6%) centres and family history in all. Five/14 centres (35.7%) use structured preanalytical patient instructions, and 10/14 (71.4%) recorded questionnaire on the preassessment of avoidance of any drugs or natural products affecting platelet functions. Preliminary investigations of screening tests of coagulation are performed in 10/14 (71.4%), while in 4/14 (28.6%), the diagnostic work-up of IPFD and von Willebrand disease (VWD) is performed simultaneously. The work-up of IPFD includes peripheral blood smear in 10/14 (71.4%), platelet aggregometry in all, flow cytometry in 10/14 (71.4%) and Platelet Function Analysis (PFA) in 3/11 (28.6%). Molecular genetic diagnosis is available in 7/14 (50%) centres. CONCLUSIONS:The considerable variability in the current practice illustrates the need for harmonization between the Northern European centres according to the international registers (i.e. EUHASS) and IPFD guidelines (ISTH, EHA).

Inherited platelet disorders (IPD) cover a heterogenous group of disorders with large differences in severity, disease mechanisms and prevalence. Pathogenic variants in more than 60 different genes, associated with megakaryocyte or platelet number and/or function, are causal of IPD. Due to disease heterogeneity IPDs are often difficult to diagnose, problematic to manage and underestimated. In the past decade, genetic diagnostics using whole-genome sequencing has revolutionised the field by identifying numerous novel genes involved in IPD aetiology as described in this review.

Background: GNE encodes UDP-N-acetyl-glucosamine-2-epimerase/N-acetylmannosamine kinase, the rate limiting enzyme of sialic acid biosynthesis. Biallelic variants in GNE have recently been associated with severe isolated macrothrombocytopenia, attributed to an increased clearance of desialylated platelets. Interestingly, treatment with the sialidase inhibitor oseltamivir has been reported to increase platelet counts in conditions such as immune thrombocytopenia (ITP) and influenza. We present a case of a 17-year-old boy (the proband) with severe congenital macrothrombocytopenia (platelet counts A, p.Arg451Gln). The proband was otherwise healthy, with no signs of GNE myopathy. ​​​​ Aims: To investigate the consequences of the identified variants in GNE and evaluate the effect of oseltamivir in GNE-associated thrombocytopenia. Methods: Sialylation of platelets, granulocytes, lymphocytes and monocytes was determined by flow cytometry in the proband and healthy controls (n = 5), using Sambucus nigra lectin (SNA) and Maackia amurensis lectin II (MAL II). Platelet sialylation was reassessed in the proband following treatment with oseltamivir (75 mg twice daily, off-label use). Informed consent was obtained from all participants. The study was approved by the regional ethical committee. Results: Sialylation of platelets and leukocytes was markedly decreased in the proband compared with the healthy controls, consistent with a deleterious effect of the compound heterozygous variants in GNE (Figure 1). Platelet sialylation was persistently decreased after 18 days of treatment with oseltamivir, and no clinically significant elevation of the platelet counts could be observed (Figure 1, Figure 2). (Less)

Background:Hereditary thrombocytopenias constitute a genetically heterogeneous cause of increased bleeding. We report a case of a 17-year-old boy suffering from severe macrothrombocytopenia throughout his life. Whole genome sequencing revealed the presence of two compound heterozygous variants in GNE encoding the enzyme UDP-N-acetyl-glucosamine-2-epimerase/N-acetylmannosamine kinase, crucial for sialic acid biosynthesis. Sialic acid is required for normal platelet life span, and biallelic variants in GNE have previously been associated with isolated macrothrombocytopenia. Furthermore, sialic acid constitutes a key ligand for complement factor H (FH), an important inhibitor of the complement system, protecting host cells from indiscriminate attack.Methods:Sialic acid expression and FH binding to platelets and leukocytes was evaluated by flow cytometry. The binding of FH to erythrocytes was assessed indirectly by measuring the rate of complement mediated hemolysis. Complement activation was determined by measuring levels of C3bBbP (alternative pathway), C4d (classical/lectin pathway) and soluble terminal complement complex assays.Results:The proband exhibited markedly decreased expression of sialic acid on platelets and leukocytes. Consequently, the binding of FH was strongly reduced and moderate activation of the alternative and classical/lectin complement pathways was observed, together with an increased rate of erythrocyte lysis.Conclusion:We report two previously undescribed variants in GNE causing severe congenital macrothrombocytopenia in a compound heterozygous state, as a consequence of decreased platelet sialylation. The decreased sialylation of platelets, leukocytes and erythrocytes affects the binding of FH, leading to moderate complement activation and increased hemolysis.

Background Deferoxamine retinopathy is the informally designated term used to describe a characteristic pattern of outer retinal degeneration in iron-overloaded chronic anemia patients who are treated with deferoxamine. We hypothesize that insufficiently treated iron overloading and not only deferoxamine is the cause of the retinal degeneration. Our case report is based on exposure histories of two anemia patients and literature review. Case presentation Both anemia patients presented with bilateral visual loss secondary to photoreceptor and retinal pigment epithelium degeneration. Chart review showed that visual loss came after a year-long slow, and rather monotonous rise in plasma ferritin concentrations, with no obvious relation to iron chelator exposure. In one patient, the onset of symptomatic visual loss came after a bout of fever followed by two additional febrile episodes, all accompanied by plasma ferritin spikes. Adjustment of iron chelation therapy did not improve visual function. Experimental studies clearly show that both systemic and intraocular exposure to iron ions can induce retinal degeneration. Conclusion The available evidence indicates that retinal degeneration in chronic anemia patients treated by deferoxamine is cause by insufficient iron chelation, not by deferoxamine. The actual role of iron chelating agents may be to promote a long enough survival to allow the slow development of retinal siderosis.

Objective Inherited platelet disorders (IPD) are rare diseases featured by reduced platelet count and/or impaired platelet function. Diagnostic tools include clinical evaluation, platelet function tests and genetic analysis. We have established a method to assess platelet phenotype on the blood smear by immunofluorescence microscopy as a diagnostic tool for IPDs. The aim of the study was to compare the outcomes of the immune-morphological analysis with those of genetic testing in a cohort of consecutive patients suspected for IPD.

The classic Bernard‐Soulier syndrome (BSS) is a rare inherited thrombocytopenia (IT) associated with severe thrombocytopenia, giant platelets, and bleeding tendency caused by homozygous or compound heterozygous variants in GP1BA, GP1BB, or GP9. Monoallelic BSS (mBSS) associated with mild asymptomatic macrothrombocytopenia caused by heterozygous variants in GP1BA or GP1BB may be a frequent cause of mild IT.

Recently, reports of severe thromboses, thrombocytopenia, and hemorrhage in persons vaccinated with the chimpanzee adenovirus-vectored vaccine (ChAdOx1 nCoV-19, AZD1222, Vaxzevria; Oxford/AstraZeneca) against severe acute respiratory syndrome coronavirus 2 have emerged. We describe an otherwise healthy 30-year-old woman who developed thrombocytopenia, ecchymosis, portal vein thrombosis, and cerebral venous sinus thrombosis the second week after she received the ChAdOx1 nCoV-19 vaccine. Extensive diagnostic workup for thrombosis predispositions showed heterozygosity for the prothrombin mutation, but no evidence of myeloproliferative neoplasia or infectious or autoimmune diseases. Her only temporary risk factor was long-term use of oral contraceptive pills (OCPs). Although both the prothrombin mutation and use of OCPs predispose to portal and cerebral vein thrombosis, the occurrence of multiple thromboses within a short time and the associated pattern of thrombocytopenia and consumption coagulopathy are highly unusual. A maximum 4T heparin-induced thrombocytopenia (HIT) score and a positive immunoassay for anti-platelet factor 4/heparin antibodies identified autoimmune HIT as a potential pathogenic mechanism. Although causality has not been established, our case emphasizes the importance of clinical awareness. Further studies of this potentially new clinical entity have suggested that it should be regarded as a vaccine-induced immune thrombotic thrombocytopenia.

Inherited thrombocytopenia is a heterogeneous group of hereditary disorders with varying bleeding tendencies, not simply related to platelet count. Platelets transform into different subpopulations upon stimulation, including procoagulant platelets and platelet microparticles (PMPs), which are considered critical for haemostasis. We aimed to investigate whether abnormalities in PMP and procoagulant platelet function were associated with the bleeding phenotype of inherited thrombocytopenia patients. We enrolled 53 inherited thrombocytopenia patients. High-throughput sequencing of 36 inherited thrombocytopenia related genes was performed in all patients and enabled a molecular diagnosis in 57%. Bleeding phenotype was evaluated using the ISTH bleeding assessment tool, dividing patients into bleeding (n = 27) vs. nonbleeding (n = 26). Unstimulated and ADP, TRAP or collagen-stimulated PMP and procoagulant platelet functions were analysed by flow cytometry using antibodies against granulophysin (CD63), P-selectin (CD62P), activated GPIIb/IIIa (PAC-1) and a marker for phosphatidylserine expression (lactadherin). Procoagulant platelets were measured in response to collagen stimulation. An in-house healthy reference level was available. Overall, higher levels of activated platelets, PMPs and procoagulant platelets were found in nonbleeding patients compared with the reference level. Nonbleeding patients had higher proportions of phosphatidylserine and PMPs compared with bleeding patients and the reference level, in response to different stimulations. Interestingly, this finding of high proportions of phosphatidylserine and PMPs was limited to PMPs, and not present in procoagulant platelets or platelets. Our findings indicate that nonbleeding inherited thrombocytopenia patients have compensatory mechanisms for improved platelet subpopulation activation and function, and that generation of phosphatidylserine expressing PMPs could be a factor determining bleeding phenotype in inherited thrombocytopenia.