Abstract: (1) Background: Polycythaemia is defined by an increase in haemoglobin (Hb) concentration, haematocrit (Hct) or red blood cell (RBC) count above the reference range adjusted to age, sex and living altitude. JAK2 unmutated polycythaemia is frequent but under-investigated in original publications. In this retrospective cohort study, we investigated the clinical and laboratory data, underlying causes, management and outcomes of JAK2 unmutated polycythaemia patients. (2) Methods: The hospital database was searched to identify JAK2 unmutated patients fulfilling WHO 2016 Hb/Hct criteria for PV (Hb 〉 16.5 g/dL in men and 〉 16 g/dL in women, or Hct 〉 49% in men and 〉 48% in women, or RBC mass 〉 25% above mean normal predicted value) between 2008 and 2019. Clinical and laboratory data were collected and analysed. (3) Results: From 727,731 screened patients, 294 (0.04%) were included, the median follow-up time was 47 months. Epo and P50 showed no clear pattern in differentiating causes of polycythaemia. In 30%, the cause remained idiopathic, despite extensive work-up. Sleep apnoea was the primary cause, also in patients under 30. Around 20% had received treatment at any time, half of whom had ongoing treatment at the end of follow-up. During follow-up, 17.2% developed a thromboembolic event, of which 8.5% were venous and 8.8% arterial. The mortality was around 3%. (4) Conclusions: Testing for Epo and P50 did not significantly facilitate identification of underlying causes. The frequency of sleep apnoea stresses the need to investigate this condition. Idiopathic forms are common. A diagnostic flowchart based on our data is proposed here. NGS testing should be considered in young patients with persisting polycythaemia, irrespective of Epo and P50 levels.

Abstract: Background: Real life data of underlying causes of JAK2-negative polycythemia is sparse. We aimed to analyze clinical and laboratory data of patients at our tertiary referral hospital, in order to identify the most frequent underlying causes of JAK2-negative polycythemia. Particularly, we intended to analyze the prevalence and causes of hereditary erythrocytosis. Methods: The hospital database was searched to find patients ≥15 years of age with polycythemia (inpatients and outpatients) between 1 st Oct 2008 and 31 st Jul 2019. Using a stepwise process, we focused on patients in whom a JAK2 result was available. For the diagnosis of polycythemia, including reactive and relative, the diagnostic criteria of the 2016 WHO classification for polycythemia vera were used: hemoglobin & gt;165 g/L in men and & gt;160 g/L in women, or hematocrit & gt;49% in men and & gt;48% in women. Results: Files of 727,731 patients were screened, and in 4,391 polycythemia was mentioned as a diagnosis on the electronic record. Of these, polycythemia was confirmed in 1,483 based on laboratory values. From them, 391 were tested for JAK2 mutation, and 294 were negative, thus representing our study cohort. The median age at polycythemia diagnosis was 46 years (r 15 - 89), and the majority of patients were males (N=242, 82%). The median Hb and Hct value were 172 g/L (r 157 - 224) and 51% (r 45 - 68) respectively. The patients were followed up for a median time of 4 years (r 12 days - 21 years). In 61% of patients (179/294) reactive polycythemia was diagnosed, while in 4.8% (14/294) relative polycythemia was present. In 30% of patients (89/294) the cause of polycythemia was undetermined, whereas in 70% (205/294) an underlying cause explaining polycythemia was found (Table 1). In 12 patients (4.1%), a congenital cause was identified. Among them, hemoglobinopathy or high-oxygen affinity Hb (N=8, 2.7%) were the most common (Table 1). Sleep apnea confirmed through a polysomnography (N=55, 18.7%), followed by smoking (N=51, 17.3%), increased HbCO & gt;5% (N=14, 4.8%), respiratory diseases (N=13, 4.4%), and non-cancer kidney disease, for instance renal cysts (N=12, 4.1%), were the most common associated causes. Polysomnographies were performed on 60 patients, of whom sleep apnea was confirmed in 92% (N=55), and a significant proportion of these patients were younger than 40 years old (N=18, 33%). From all patients with undetermined causes (N=89), we focused on those with persistent polycythemia for further investigations. Accordingly, 25 patients were identified, all male and 72% (N=18) younger than 40 years of age. In 12/25 patients a congenital erythrocytosis NGS panel was performed, and in 3/12 patients 4 different heterozygous mutations were found (two of which were in one patient). All mutations were characterized as variants of unknown significance (VUS). None of these mutations were previously described in the literature, and their finding in patients with polycythemia suggests a pathogenic likelihood (Table 2). Conclusion: In one third of JAK2-negative patients with polycythemia, despite extensive workup, no underlying cause was found. Sleep apnea was the main cause of secondary forms, even in young patients, underlining the importance of performing a polysomnography in the workup of such patients. The NGS erythrocytosis panel offers easy and accessible characterization of some idiopathic forms, however there is still a majority of these patients unable to be characterized. In this cohort the investigation with NGS showed 4 mutations which have not been reported in the literature. The presence of these in patients with polycythemia suggests the probability of a relationship in its pathogenesis. Figure 1 Figure 1. Disclosures Jalowiec: Kite Pharma Gilead Sciences': Honoraria, Speakers Bureau. Rovo: AG Alexion: Research Funding; CSL Behring: Research Funding; Novartis: Research Funding; AG Alexion: Honoraria; BMS: Honoraria; Novartis: Honoraria; AstraZeneca: Honoraria; OrPhaSwiss GmbH: Honoraria; Swedish Orphan Biovitrum AG: Honoraria; Amgen: Other: Financial support for congresses and conference travel; AstraZeneca: Other; BMS: Other; Sanofi: Other; Roche: Other.

Abstract: Background: Natural killer (NK) cell-based immunotherapies are emerging as a new, cutting-edge, promising cancer treatment. NK cells have been used to generate autologous as well as allogenic chimeric antigen receptor NK (CAR-NK) cells, and are currently being tested in multiple clinical trials. The advantage of these cells over CAR-T cells is that they show a superior toxicity profile, less on-target/off-tumor effects, and their toxicity mechanism could be independent of CAR. Additionally, allogenic CAR-NK cells are less immunogenic and cause less graft versus host disease in comparison to CAR-T cells. NK-cell collection during apheresis is the first step in the production of CAR-NK-cells. Efficient collection of NK-cells is paramount to successful manufacture of a CAR-NK cell product. Here, we present our single-center experience on individualized high-flow autologous lymphocyte apheresis in heavily pre-treated patients who qualified for a CAR-T treatment. In this abstract, we demonstrate data on NK-cell collections that derived from our research on T-cell collections. Study design and methods: In this retrospective, descriptive study, we compared collection efficiencies (CE%), absolute efficiencies (AE; absolute yield) and relative efficiencies (RE; AE/total number of absolute circulating pre-apheresis cells) for NK- and T-cell collections. Recruitment of cells was assumed if RE & gt;1. The analysis was performed for 9 heavily pre-treated diffuse large B-cell lymphoma patients who underwent autologous T-cell collections for CAR-T treatment between October 2018 and November 2019. These 9 patients were chosen because the data on pre-apheresis NK- and T-cell concentrations, and harvest NK- and T-cell yield, were available for them. For calculation of CE% a standard formula was used (Figure 1, A). Results: A total of 9 high-flow lymphocyte collections achieved the cell yield of 1x10 9 cells for T- cells and 88% (n=8) achieved the cell yield of 1x10 9 cells for NK-cells. The median pre-apheresis cell concentration for NK-cells was 0.21 x10 9/L (range 0.05 - 0.29) and for T-cells was 0.69 x10 9/L (0.11 - 2.09). The median average blood flow was 77 ml/min (49 - 132), the median processed blood volume was 13.51L (7.35 - 31.17), while the median ratio of processed blood volume to total blood volume was 2.57 (1.46 - 6.67). The median AE for NK-cells with 1.58x10 9 (0.55 - 5.9) was markedly lower but not statistically significant (p=0.14) than that of T-cells with 5.5x10 9 (1.88 - 18.41). The CE of NK-collections was median 89% (29 - 125), while the CE of T-cell collections was median 68% (31 - 93) and they were not statistically different (p=0.605). The same was observed for RE which was 3.04 (0.63 - 4.31) for NK-cells and 1.91 (0.68 - 4.52) for T-cells with no statistically significant difference (Independent-Samples Mann-Whitney U Test, p=0.836). Figure 1 presents the differences in AE (B), CE (C) and RE (D) between NK- and T-cell collections. Conclusions: During high-flow lymphocyte apheresis, NK- as well as T-cells are recruited, as in the majority of collections the cell yield is greater than the absolute circulating pre-apheresis cell count (when RE was & gt; 1). Thus, the origin and immunophenotype of these NK- and T-cells should be subject of further investigation. CE for NK- and T-cells are similar and high enough to perform efficient cell collections of both cell types for immunotherapy. Figure 1 Figure 1. Disclosures Jalowiec: Kite Pharma Gilead Sciences': Honoraria, Speakers Bureau. Daskalakis: Amgen: Other: Travel grant; Roche: Other: Travel grant; Gilead: Membership on an entity's Board of Directors or advisory committees, Other: Travel grant; Celgene/BMS: Consultancy, Other: Travel grant; Novartis: Membership on an entity's Board of Directors or advisory committees, Other: Travel grant; NovoNordisk: Other: Travel grant.

Abstract: Background Axicabtagene ciloleucel (axi-cel) and tisagenlecleucel (tisa-cel) CD19 CAR-T therapies are licensed in the UK for relapsed/refractory large B-cell lymphoma (LBCL). Corticosteroids for CAR-T toxicity were administered to 28% and 10% of patients in ZUMA-1 and JULIET respectively, but real-world steroid use is reported to be much higher (Nastoupil et al, JCO 2020), with concerns that this may adversely impact on OS and PFS following CAR-T. Analysis of real-world datasets may facilitate the identification of modifiable risk factors for toxicity. Here, we report the UK experience of CAR-T toxicity and its management in 341 LBCL patients with a focus on predictors for corticosteroid use and the impact of steroids on OS/PFS post CAR-T therapy. Methods Data were collected in 10 UK centres from January 2019 to April 2021. CRS/ICANS were graded prospectively (ASTCT). Low (LG) and high-grade (HG) CRS/ICANS were classified as grade 1-2 and 3-5 respectively. Hyper-acute CRS was defined as onset within 24 hours of cells. Steroid cumulative dosing for CAR-T toxicity was calculated as dexamethasone equivalent up to day 30. Product selection was at the discretion of the treating physician. Toxicity management in the UK follows EBMT and ASCT guidance. Results A total of 341 patients received axi-cel (n=261) and tisa-cel (n=80) with a median follow-up of 14.8 (3.4-30) and 13.9 (range 7.5-27) months. Median age was 59 (range 18-80), 61% were male (M=207, F=134) and 79% received bridging therapy (BT). HG CRS, hyperacute CRS and HG ICANS with axi-cel (8.8%; 38%; 21% respectively) and with tisa-cel (7.5%; 26%; 5% respectively) were observed, with incidence of HG ICANS post axi-cel lower than published reports. Risk factors for HG CRS were age & lt;65yrs (p=0.014), LDH pre-lymphodepletion (LD) (p=0.04) and ECOG & gt;0 at infusion (0.001); and hyper-acute CRS was associated with stable (SD) or progressive disease (PD) after BT (p=0.036). On multivariate analysis (MVA), ECOG & gt;0 (OR 3.4, 95% CI 1.3-9.1, p=0.015) and SD/PD post BT (OR 1.8, 95% CI 1.03-3.3, p=0.039) were predictive of HG CRS and hyper-acute CRS respectively. Risk factors for HG ICANS were age & lt;65yrs (0.03), ECOG & gt;0 (p=0.009), SD/PD post BT (p=0.011), extranodal disease (p=0.02) and LDH & gt;normal on Day 0 (p=0.02). By MVA, LDH & gt;normal on Day 0 (OR 3.8, 95% CI 1.4-10.5, p=0.009) was predictive of HG ICANS. Steroids were used in 44% (n=115) of axi-cel treated patients, administered at day 6 (median; range 1-22), at a median dose of 165mg (range 10-2182) over a median of 8 days (range 1-86). Anakinra was given as an adjunctive agent for high-grade ICANS (n=26, 10%), CRS (n=10, 4%) or HLH (n=3, 1%) at a median of day 8 (range 5-43) for a median of 6 days (range 1-16). 18% of tisa-cel patients (n=14) received steroids, administered at day 4 (median; 2-26) at a median dose of 95mg (range 10-220) over a median of 4 days (range 1-8). Tocilizumab was used in 74% and 48% of axi-cel and tisa-cel patients respectively. Clinical factors associated with steroid use included ECOG & gt;0 (OR 1.8, 95% CI 1.2-5.6, p=0.002), and the emergence of SD/PD post BT (OR 1.7, 95% CI 1.02-2.8, p=0.04). SD/PD post BT also predicted for higher total steroid dose ( & gt;median; OR 1.7, 95% CI 1.1-2.9, p=0.03), and prolonged ( & gt;1 week) administration (OR 1.8, 95% CI 1.1-2.9, p=0.03) (Table 1). The impact of clinical parameters including toxicity and steroids on PFS and OS post-CAR-T were assessed. Adjusted for age, sex, ECOG and LDH & gt;normal pre-LD, HG ICANS (but not HG CRS) was associated with worse OS (HR 2.4, 95% CI 1.3-4.4, p=0.004). 100 day NRM was 3.5% for the whole cohort. Median OS was not reached. In contrast to other published series, early steroid use (HR 1.0, 95% CI 0.6-1.6, p=0.99), higher total doses (HR 1.2, 95% CI 0.7-1.9 p=0.6) and prolonged ( & gt;1 week) steroid exposure (HR 1.3, 95% CI 0.8-2.2, p=0.25) were not associated with worse OS. Early steroid use (HR 0.7, 95% CI 0.3-1.6, p=0.4), higher doses (HR 0.5, 95% CI 0.2-1.2, p=0.13) and prolonged exposure (HR 0.9, 95% CI 0.4-2.0, p=0.8) also had no significant impact on PFS. Conclusion Patients who are refractory to BT or with other markers of high disease burden at infusion are more likely to require steroids for CAR-T toxicity, with axi-cel associated with more HG ICANS and cumulative steroid exposure than tisa-cel. Despite this, steroid use for CAR-T toxicity did not negatively impact on PFS or OS. Rates of HG ICANS with axi-cel were comparatively lower in this UK dataset. Figure 1 Figure 1. Disclosures Sanderson: Kite, a Gilead Company: Honoraria; Novartis: Honoraria. Kuhnl: Kite: Honoraria; Novartis: Research Funding. Menne: Kite/Gilead: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel grant, Research Funding, Speakers Bureau; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel grants; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel grants; Astra Zeneca: Research Funding; Jazz: Other: Travel grants; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: Travel grants; Bayer: Other: Travel grants; Kyowa Kirin: Other: Travel grants; Daiichi Sankyo: Honoraria, Membership on an entity's Board of Directors or advisory committees; Atara: Honoraria, Membership on an entity's Board of Directors or advisory committees; Takeda: Other: Honoraria for Lectures; Roche: Other: Honoraria for Lectures. Chaganti: Celgene-BMS: Consultancy, Honoraria, Other: Travel support; Takeda: Consultancy, Honoraria, Other: travel support, Research Funding, Speakers Bureau; Novartis: Consultancy, Honoraria, Other: Travel support, Research Funding, Speakers Bureau; Kite, a Gilead Company: Consultancy, Honoraria, Other: Travel support, Research Funding, Speakers Bureau; Atara Bio: Consultancy, Honoraria, Research Funding, Speakers Bureau; Janssen: Research Funding; Incyte: Honoraria, Speakers Bureau. Nicholson: Novartis: Consultancy, Other: Conference fees; Kite, a Gilead Company: Other: Conference fees, Speakers Bureau; BMS/Celgene: Consultancy; Pfizer: Consultancy. Latif: Takeda UK: Speakers Bureau; Astellas: Consultancy, Honoraria, Speakers Bureau; Abbvie: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Novartis,: Consultancy, Honoraria; Daiichi Sankyo: Consultancy, Honoraria; Jazz: Consultancy, Honoraria; Kite: Consultancy, Honoraria, Speakers Bureau. Jones: Janssen: Consultancy; Kite/Gilead: Honoraria; Novartis: Honoraria. Sharplin: Kite Gilead: Honoraria; Novartis: Other: Travel Support. Jalowiec: Kite Pharma Gilead Sciences': Honoraria, Speakers Bureau. Roddie: Novartis: Consultancy; Celgene: Consultancy, Speakers Bureau; Gilead: Consultancy, Speakers Bureau. OffLabel Disclosure: Anakinra for CAR-T toxicity management

Abstract: Background: Erythrocytosis corresponds to an increase in the red blood count, hemoglobin (Hb) concentration and hematocrit (Htc) above the reference range adjusted to age, sex and living altitude. Clinical and molecular data on patients suffering from primary erythrocytosis are sparse. Due to its rare incidence, primary erythrocytosis frequently represents a challenge for the diagnosis. The management of such patients is demanding as no clear standards of treatment have been defined. Objective: We aimed to analyze the clinical and molecular features of patients with idiopathic erythrocytosis at our clinic. Methods: We evaluate in our internal registry patients encoded as erythrocytosis (primary or secondary) which were currently in follow-up. All data was obtained from the clinical records of the patients. For patient selection, we focused on those in which a JAK2 determination was available. We identified 102 patients from them 78 patients were JAK2 positive and were excluded in this analysis. 24 patients were initially selected as possible idiopathic erythrocytosis, of which 8 patients were excluded due to possible secondary causes. Patients in whom the diagnosis of primary erythrocytosis was suspected were studied with a 13 genes NGS panel. The analyzed genes included EPOR (exons 7,8), VHL (orf including exon 1'), EGLN1, EPAS1, EPO (including several regulatory regions), JAK2 (exons 12-16), BPGM, HBB, HBA1, HBA2, HIF3A, OS9, SH2B3 (somatic). Only gene variants of the categories pathogenic, likely pathogenic and variant of unknown significance (VUS) according to ACMG guidelines 2015 are mentioned. Results: We identified 16 patients (15%) that did not fulfill the criteria for polycythemia vera nor had a secondary underlying cause of polycythemia and were diagnosed as idiopathic erythrocytosis. The median age was 32 years (r 20-76) and 81% (13/16) were males. The median Hb-value was 173 g/l (r 142-192), Htc 50% (r 43-65%). The median erythrocyte concentration was 5.87 G/l (r 5.18-6.57 G/l). No thromboembolic complications occurred in any of these patients either before or after diagnosis. Eleven patients (69%) were initially treated with venesection, which was interrupted during the follow-up except in two cases. The median follow-up in our clinic was 43 months (r 1 - 132). The median follow up since the interruption of venesection was 23 months (r 12-106). There was no evolution to leukemia, a myeloproliferative disorder or any kind of tumor. In the results of the molecular analysis we found that the EPO: c.-1306C 〉 A, rs1617640 reference allele was never homozygous. Indeed, all 16 patients showed an alternative allele: nine patients were heterozygous and 8 homozygous. EPO c.*772G 〉 T, rs551238 was similarly distributed: no patients homozygous for the reference allele, 8 heterozygous and 8 patients homozygous for the alternative allele. In 5 patients other gene variants were also detected: in two patients EGLN1: c.1088T 〉 G, p.(Leu363Arg) (the nucleotide is highly conserved, VUS, likely pathogenic), the other three were VHL: c.340+648T 〉 C (reported as quite frequent, so more likely VUS, but it is within the alternate exon 1 region described by Lenglet et al. thus further studies are recommended), EPO: c.-1206C 〉 A (not rare, VUS), EPO: c.*656G 〉 A (downstream variant, very rare, unknown significance) respectively. No other mutations were identified. Conclusion: In our clinic idiopathic erythrocytosis showed a population of patients predominantly constituted of young men, had an indolent evolution, neither presented with thrombotic complications before or after the diagnosis, nor with a hematological transformation. The high prevalence of EPO gene variant underlines the role of this association. The five patients with combined mutations did not show a different clinical pattern. The use of NGS to evaluate patients with unclear erythrocytosis might allow an improvement in clinical management avoiding unnecessary interventions. Disclosures Jalowiec: Novartis: Other: Travel grant; Amgen: Other: Travel grant; Pfizer: Other: Travel grant.

Abstract: 〈 b 〉 〈 i 〉 Introduction: 〈 /i 〉 〈 /b 〉 In acute inflammatory optic neuritis (ON) as a typical onset of multiple sclerosis (MS), only few studies have investigated plasma exchange (PLEX) as a sequential treatment after insufficient response to high-dose intravenous glucocorticosteroids. Therefore, we aimed to investigate treatment outcome on visual acuity (VA) with PLEX in patients with steroid-refractory ON. 〈 b 〉 〈 i 〉 Methods: 〈 /i 〉 〈 /b 〉 In our retrospective monocentric study, medical records were screened for patients with acute ON as their first relapse with sequential MS diagnosis or with an established MS diagnosis from the Bern University Hospital (Switzerland) that were treated with PLEX between 2016 and 2018 due to lacking steroid response. VA prior to steroid administration, and before and after PLEX were assessed and compared using the Friedman multiple comparison test. 〈 b 〉 〈 i 〉 Results: 〈 /i 〉 〈 /b 〉 In total, 18 patients were included in the analysis. Interval from symptom onset to PLEX was 20.3 days (mean, 95% CI 14.8–25.9). Relevant functional improvement (VA of ≥0.5, after a mean of 15.9 (13.3–18.5) days after start of PLEX) was detected in 16/18 (88.9%) with a significant amelioration as compared to VA before glucocorticosteroids and before PLEX ( 〈 i 〉 p 〈 /i 〉 & #x3c; 0.0001). VA improvement at a later time point (38.1 weeks, 25.2–51.0) was present in 15/16 (93.8%) patients. No serious adverse events were detected. PLEX could be performed via peripheral access in 13/18 patients (72.2%). 〈 b 〉 〈 i 〉 Conclusion: 〈 /i 〉 〈 /b 〉 Our study demonstrates significant improvements of VA with PLEX in a cohort of MS patients with steroid-refractory ON. High response rates may be due to the timely treatment initiation. Despite the small sample size, our data support the early use of PLEX in steroid-refractory ON with a favorable safety profile.

Abstract: (1) Background: Thrombophilia testing utility has remained controversial since its clinical introduction, because data on its influence on treatment decisions are limited. (2) Methods: We conducted a single-center retrospective cohort study of 3550 unselected patients referred for thrombophilia consultation at the Bern University Hospital in Switzerland from January 2010 to October 2020. We studied the influence of thrombophilia testing results on treatment decisions and evaluated the association between thrombophilia and thromboembolic and pregnancy-related morbidity events after testing up to 03/2021. (3) Results: In 1192/3550 patients (34%), at least one case of thrombophilia was found and 366 (10%) had high-risk thrombophilia. A total of 211/3550 (6%) work-ups (111/826 (13%) with low-risk thrombophilia and 100/366 (27%) with high-risk thrombophilia) led to an appropriate decision to extend or initiate anticoagulation, and 189 (5%) negative results led to the withholding of anticoagulation therapy inappropriately. A total of 2492 patients (69%) were followed up for 〉 30 days, with a median follow-up of 49 months (range, 1–183 months). Patients with high-risk thrombophilia had a higher risk of subsequent venous thromboembolic events and pregnancy-related morbidity compared to those without thrombophilia. (4) Conclusions: Our study demonstrated the limited usefulness of thrombophilia work-up in clinical decision-making. High-risk thrombophilia was associated with subsequent venous thromboembolism and pregnancy-related morbidity.

Abstract: (1) Background: Clinical and molecular data on patients with unexplained erythrocyto-sis is sparse. We aimed to analyze the clinical and molecular features of patients with congenital erythrocytosis in our tertiary reference center. (2) Methods: In 34 patients with unexplained erythrocytosis, a 13-gene Next-Generation Sequencing erythrocytosis panel developed at our center was conducted. (3) Results: In 6/34 (18%) patients, eight different heterozygous gene variants were found. These patients were, therefore, diagnosed with congenital erythrocytosis. Two patients had two different gene variants each. All variants were characterized as variants of unknown significance as they had not previously been described in the literature. The rest of the patients (28/34, 82%) had no detected gene variants. (4) Conclusions: Our experience shows that the NGS panel can be helpful in determining the reasons for persistent, unexplained erythrocytosis. In our cohort of patients with erythrocytosis, we identified some, thus far unknown, gene variants which may explain the clinical picture. However, further investigations are needed to determine the relationship between the molecular findings and the phenotype.