A guideline for the management of specific situations in polycythaemia vera and secondary erythrocytosis: A British Society for Haematology Guideline.

Abstract

This guideline was compiled according to the British Society for Haematology (BSH) process at b-s-h.org.uk. The Grading of Recommendation Assessment, Development and Evaluation (GRADE) nomenclature was used to evaluate levels of evidence and to assess the strength of the recommendations. The GRADE criteria can be found at https://www.gradeworkinggroup.org. The literature review was conducted on 2 March 2017. Databases searched include MEDLINE (OVID), Embase (OVID) and CENTRAL (The Cochrane library) using search terms (and relevant MESH terms) polycythaemia vera, erythrocytosis, familial, high oxygen affinity haemoglobin, defects of oxygen sensing pathway, diagnosis, investigation, molecular, mutation, JAK2, MPL, CALR, bone marrow, red cell mass, erythropoietin, risk, management, treatment, cytoreduction, venesection, hydroxyurea, interferon, busulfan, pipobroman, radioactive phosphorus, aspirin, anagrelide, ruxolitinib, thrombosis, haemorrhage, pregnancy, pruritus surgery and management. The search covered the period from 2005, the date of last version of the guideline (McMullin et al, 2005), to February week 3 2017. Exclusions included articles not in English, studies not in humans, single case reports and small case series. A total of 6062 articles were identified which, with exclusions and duplications, resulted in 1215 articles which were reviewed. Review of the manuscript was performed by the BSH Guidelines committee General Haematology Task Force, the BSH Guidelines Committee and the General Haematology sounding board of BSH. It was also placed on the members section of the BSH website for comment. A patient representative from MPN-Voice (www.mpnvoice.org.uk) participated in the guideline writing meeting. The guideline has been reviewed by MPN-Voice; this organisation does not necessarily approve or endorse the contents. The previous BSH guideline for the management of erythrocytosis was published in 2005 (McMullin et al, 2005) and amended in 2007 (McMullin et al, 2007). Here, we re-evaluate the literature formulate guidance on the management of specific situations encountered in polycythaemia vera (PV) and the management of the other types of secondary erythrocytosis. Recommendations for the diagnostic pathway of investigation of an erythrocytosis, risk stratification and management of PV are in the accompanying guideline (McMullin et al, 2018). We review evidence and outline guidance on management of acute thrombotic events and secondary prevention of thrombosis in PV. The unusual thrombotic events, splanchnic vein and cerebral vein thromboses are discussed and haemorrhage. The specific situations of surgery and pregnancy and guidance on management of pruritus are included. The evidence for the management of other causes of erythrocytosis, including idiopathic erythrocytosis, congenital erythrocytosis, hypoxic pulmonary disease and post-transplant erythrocytosis, is reviewed and recommendations made. Thrombotic events are the major cause of morbidity and mortality in PV, and their prevention is the main objective of treatment. About one-third of patients present with a thrombotic event (Elliott & Tefferi, 2005; Hultcrantz et al, 2015; Kaifie et al, 2016) and younger patients diagnosed with PV have an increased risk of early death from cardiovascular disease over the general population, accounting for 45% of all deaths in PV (Marchioli et al, 2005; Hultcrantz et al, 2015). Arterial thrombosis involving large arteries and peripheral vascular disease are more common than venous thromboembolism in PV. A prospective study (Marchioli et al, 2005) found that three quarters of PV patients who experience a thrombotic event had arterial and one quarter venous thrombosis. Concerning the management of thrombosis: cardiovascular risk factors should be assessed and controlled, particularly hyperlipidaemia and hypertension, smoking and diabetes mellitus. The long-term cardiovascular risk should be formally assessed at baseline and annually using a validated score, such as that determined by the computerised scoring tool, QRISK, although the impact of myeloproliferative neoplasms (MPN) on this score is not defined (National Institute for Health and Care Excellence [NICE], 2014). This assessment is usually conducted in primary health care, but the results should be used in primary and secondary care to optimise the management of vascular risk factors. Risk stratification for thrombosis identifies age ≥65 years and previous thrombosis as high-risk factors for further thrombosis. The role of thrombocytosis as a risk factor in PV is less clear than in essential thrombocythaemia (ET) (Di Nisio et al, 2007). The mechanism behind thrombosis is complex and factors include functional cellular changes, such as increased leucocyte-platelet aggregation and endothelial activation, and the overexpression of activation and adhesion molecules by the neoplastic cells (Falanga & Marchetti, 2012). Some of these factors may explain the increased incidence of thrombosis at unusual sites, such as splanchnic vein thrombosis (SVT) and cerebral venous thrombosis (CVT). Factor V Leiden (F5 R506Q) and prothrombin gene mutation (F2 G20210A) have been associated with thrombosis in PV and ET (Trifa et al, 2014). However, routine testing for inherited or acquired thrombophilia is not recommended as it has is no impact on treatment and standard guidelines for thrombophilia testing should be applied for PV patients. Both primary and secondary prevention should include control of cardiovascular risk factors in accordance with current recommendations (NICE, 2014). Of interest, there is some evidence that angiotensin-converting enzyme inhibitors (ACEi), as anti-hypertensive agents, may have an additional role in controlling the haematocrit (Hct) (Barbui et al, 2017). In accordance with evidence for the management of PV, low-dose aspirin should be offered to all patients (Landolfi et al, 2004). Low-dose aspirin, which describes various doses (e.g. 75, 81 or 100 mg) used in different countries, is the only dose to have been assessed in recent clinical trials. The risk of major bleeding, particularly gastro-intestinal, increases with age, and proton pump inhibitors should be given to patients over 75 years of age as well as to those a with high bleeding risk. The role of the ADP-receptor antagonist, clopidogrel, in the long-term prevention of MPN-related thrombosis has not been investigated although it is widely used as an alternative in those who cannot tolerate aspirin. The Hct should be maintained below 0·45 in accordance with evidence from the Cytoreductive therapy in PV (CYTO-PV) study (Marchioli et al, 2013). Acute thrombotic events should be managed according to current guidelines. Cytoreductive therapy should be instituted to ensure control of blood counts to the therapeutic target. Venesection to reduce the Hct to <0·45 should be undertaken if it is not controlled at the time of acute thrombosis. For arterial thrombosis, low dose aspirin should be offered to all patients, and specialist advice should be sought. For venous thromboembolism (VTE), anticoagulation should be commenced. Low molecular weight heparin (LMWH) remains the first choice in the acute setting, followed by vitamin K antagonists (VKA) as per guidelines (NICE, 2012). Direct oral anticoagulants (DOACs) are increasingly used in the non-MPN population for prophylaxis and VTE therapy. However, there is limited data for their specific use in MPN, where they appear to be safe and efficacious (Ianotto et al, 2017). Indefinite anticoagulation for VTE is recommended because of the presence of continuing risk (NICE 2012; Watson et al, 2015; Kearon et al, 2016). In MPN, the role of continuing anticoagulation, particularly in VTE, has been assessed by several studies. Around a third of patients who stop anticoagulation after initial treatment suffer a recurrence (Barosi et al, 2013; Kaifie et al, 2016). Two retrospective studies (Hernández-Boluda et al, 2015; De Stefano et al, 2016a) showed that cessation of VKA after 3 months of treatment for a first VTE in MPN patients increased the risk of recurrent thrombosis compared with continued treatment. The later study also suggested a role for VKA in reducing the risk of arterial thrombotic recurrence and the possible benefit of adding aspirin to VKA to prevent recurrent arterial thrombosis. We are unable to make a specific recommendation about the addition of aspirin to VKA when a secondary thrombosis occurs in a patient already on VKA, as there is not yet evidence to support this. However, trials of this combination are in progress in other conditions and evidence may emerge. For unprovoked VTE, where there is no physical precipitating factor, we therefore recommend indefinite anticoagulation to reduce the incidence of recurrence. In a significant number of patients with recurrent thrombosis (De Stefano et al, 2008), cell counts were sub-optimally controlled. Cytoreduction should therefore be reviewed to ensure optimal counts are achieved. For recurrent arterial events, specialist advice should be sought from cardiovascular, stroke or vascular physicians. Unusual sites of venous thrombosis are more common in patients with MPN, especially in younger patients. The splanchnic circulation is particularly vulnerable in patients with JAK2 V617F mutation. SVT comprises thrombosis of hepatic, portal, superior mesenteric and splenic veins singly or in combination. The incidence of SVT during the course of PV ranges from 0·8 to 7·8% (Sekhar et al, 2013). Overall, portal-mesenteric axis venous thrombosis is commoner than hepatic vein thrombosis or Budd Chiari Syndrome (BCS), however BCS is more prevalent in PV (Smalberg et al, 2012). The mortality of MPN-SVT is high, at 20–40% at 10 years, due to the index occurrence or recurrent thrombosis, bleeding or leukaemic transformation (Hoekstra et al, 2011; Ageno et al, 2015). The presence of hypersplenism, SVT-related haemodilution and iron deficiency due to blood loss and/or masked polycythaemia can artificially lower the blood counts and a normal Hct may be found despite a high red cell mass (Kiladjian et al, 2008). Therefore, it is important to investigate all patients presenting with SVT without attributable local pathology, such as malignancy, sepsis or pancreatitis, for the presence of JAK2 V617F mutation, which is present in 80–90% of patients with MPN-SVT (Kiladjian et al, 2008), and if this is negative, testing should be done for a CALR mutation, which is present in about 2·5% (Sekhar et al, 2016). Although PV is commoner in males, PV-related SVT is commoner in young females. The reason for the predilection of splanchnic venous circulation to thrombosis in MPN is not clear. Many series report co-existing inherited or acquired thrombophilia in 25–30% patients, especially protein S deficiency which has been associated with unusual site thrombosis in MPN patients (Gisslinger et al, 2005). In patients with SVT or cerebral vein thrombosis (CVT), testing for inherited and acquired thrombophilia should be considered if it would influence management decisions, such as intensity and duration of anticoagulation in the face of higher risks of bleeding (Tait et al, 2012). Anticoagulation treatment in the acute phase should be undertaken with LMWH unless there are compelling contra-indications (Tait et al, 2012). In addition, the treatment of acute phase SVT includes interventions to recanalize the blocked veins, such as thrombolysis and insertion of stents. Optimal management results in long-term survival of 85% at 10 years in BCS. Patients requiring liver transplantation have a high mortality (Potthoff et al, 2015). The management of PV-related SVT therefore needs close coordination between hepatologists, interventional radiologists and haematologists to achieve an accurate diagnosis, anti-coagulate effectively, undertake interventional procedures and optimise cytoreduction in the acute phase. Recurrence risk is high and prevention is important because of significant morbidity and mortality associated with recurrence. Overall 25–30% of MPN-SVT patients suffer a recurrence by 10 years (Hoekstra et al, 2011; Ageno et al, 2015). Although recurrence is more likely in the splanchnic circulation (Amitrano et al, 2007), it can occur in other venous or arterial locations. Anticoagulant practice in MPN-SVT is heterogeneous (Ellis et al, 2014). Current VTE guidelines recommend continuing anticoagulation in the presence of continuing active risk (Kearon et al, 2012; Tait et al, 2012). On this basis, PV-related SVT is an indication for long-term anticoagulation. Several prospective and retrospective registry studies conclude that long-term anticoagulation reduces recurrence risk at splanchnic and other sites with an acceptable level of bleeding complications (Amitrano et al, 2007; Ageno et al, 2015; De Stefano et al, 2016b). Recurrence in the arterial circulation has been observed in patients not on anti-platelet agents (Hoekstra et al, 2011; De Stefano et al, 2016b). The strategy of using anti-platelet agents in addition to VKA is followed in selected patients with BCS with stents in-situ but registry data do not suggest any benefit from this combination for prevention of recurrence of thrombosis outside of this group. Concerning anticoagulation with DOACs, 20% of patients with non-cirrhotic SVT suffered thrombosis or haemorrhage at 2 years (De Gottardi et al, 2017). These agents have not been studied in MPN-related SVT where LMWH followed by warfarin is the standard of care. The impact of cytoreduction in MPN with SVT has not been systematically studied. Cytoreduction was used in 40–70% patients across registries and did not uniformly influence recurrence risk. However, abnormally high blood counts due to inadequate cytoreduction were present in over half the patients with recurrent thrombosis (De Stefano et al, 2016b) indicating the need for rigorous clinical monitoring of patients on cytoreduction. Venesection alone is not appropriate to treat PV and cytoreduction should be undertaken in the presence of SVT. Patients with PV-related SVT are younger and interferon may be suitable as a first line agent, however data on interferon in this group of patients are not yet available. There are no data to guide therapeutic targets of cytoreductive treatment. In particular, whether more aggressive cytoreduction offsets the residual risk of recurrent thrombosis is not known. One strategy is to use standard targets for treatment of PV with an aim to achieving normal Hct, white blood cell (WBC), neutrophil and platelet counts (Barbui et al, 2011). An alternative strategy is to use lower targets, as defined by experts in the ongoing trial of pegylated interferon 2 alpha in MPN including MPN-SVT aiming for a Hct ≤0·42, WBC count between 2 and 8 × 109/l, and platelet count between 100 and 200 × 109/l (Hoffman, 2012). As this study is not yet complete the evidence for the use of these targets is weak. Nevertheless, the fact that these patients do develop thrombosis with normal counts suggests functional thrombogenicity, which may be altered by using disease-modifying agents to achieve these lower targets. Small doses of cytoreductive agents are often sufficient to achieve therapeutic targets and care must be taken to avoid significant thrombocytopenia in the face of continuing anticoagulation. Treatment with ruxolitinib was found to reduce the volume of spleen in a third of MPN-SVT patients after 2 years (Pieri et al, 2017). Complications of treatment include bleeding, especially gastrointestinal haemorrhage and, in particular, variceal haemorrhage (Ageno et al, 2015; De Stefano et al, 2016b). A quarter of major bleeds are intracranial. Other than thrombocytopenia, cytoreduction does not have any impact on bleeding risk. Bleeding risk can be minimized by careful management of oesophageal varices and concurrent use of proton pump inhibitors. Cerebral vein thrombosis occurs in about 1% of MPN patients, primarily in those with ET bearing the JAK2 V617F mutation (Martinelli et al, 2014). In selected patients with unexplained CVT, testing for JAK2 V617F can identify the aetiology and assist management. Recurrence rates are higher in MPN-CVT, especially in spontaneous CVT, and a third of patients suffer recurrence despite anticoagulation and cytoreduction (Miranda et al, 2010; Martinelli et al, 2014). Recurrence is systemic, affecting venous and arterial circulation and often involves splanchnic veins. Patients with PV-related CVT should be treated with long-term anticoagulation and cytoreduction. The role of aspirin and JAK inhibitors have not been studied in this group of patients. Haemorrhage is both a less frequent and generally less severe clinical complication of PV than thrombosis. The reported incidence varies greatly between studies (Elliott & Tefferi, 2005; Marchioli et al, 2005; Kander et al, 2015; Kaifie et al, 2016). The principal sites affected are skin, mucous membranes and gastrointestinal tract. Patients with SVT are at a particularly high risk of bleeding from gastro-oesophageal varices. Factors contributing to bleeding in patients with PV include extreme thrombocytosis (platelet counts >1500 × 109/l) (Finazzi et al, 1996) and associated acquired von Willebrand syndrome (aVWS). This has been reported to affect 12% of PV patients, usually those with high platelet counts, and does not always correlate with bleeding (Kander et al, 2015; Mital et al, 2015). Other platelet function defects are reported in PV but they are not predictive of bleeding. However, the risk of bleeding is likely to be higher with combined use of anti-platelet therapies, such as dual anti-platelet therapy or anti-platelet therapy and VKA (Hallas et al, 2006; Kaifie et al, 2016). Other studies have reported splenomegaly (Kaifie et al, 2016) and a high leucocyte count (Chou et al, 2013; Lim et al, 2015) as predictors of haemorrhage. Blood counts should therefore be optimised. Other measures to consider include adjustment of any concomitant anti-platelet and/or anticoagulant therapy. Clinically significant bleeding may, paradoxicall,y require platelet transfusion (Terasako & Sasai, 1998) and a role for tranexamic acid has been suggested (Spivak, 2002). The utility of recombinant activated factor VII (rFVIIa) is unknown in MPN patients with uncontrolled life-threatening bleeding and perhaps worthy of further study. Polycythaemia vera patients undergoing surgery are, paradoxically, at risk of both haemorrhage and thrombosis. Even in treated PV patients with well controlled counts, there is an increased risk of haemorrhage, demonstrated by their increased transfusion requirements (Weingarten et al, 2015) compared to non-PV patients (Ruggeri et al, 2008). VTE incidence was increased 5-fold despite prophylaxis. It is therefore important that PV patients are assessed pre-operatively and abnormal counts optimised, balancing the acute need for surgical procedure against the risk of bleeding and thrombosis. Polycythaemia vera patients who bleed may have significant qualitative platelet abnormalities. Patients with a prior history of bleeding should be tested for platelet function and for aVWS preoperatively. Anti-thrombotic prophylaxis should be administered according to standard postoperative guidelines. There is a little evidence that use of cytotoxic agents impairs wound healing: their use in the 4–6 weeks after surgery needs to be considered based on thrombotic and haemorrhagic risk and could be avoided if venesection alone is sufficient. Polycythaemia vera is uncommon in females of reproductive age, occurring in less than 0·3 per 100 000 (Srour et al, 2016). There are only small case series describing the management of pregnancy in patients with PV (Harrison, 2005; Robinson et al, 2005; Griesshammer et al, 2008). Larger case series are published for pregnancy management in patients with ET or cohorts of patients with all MPNs (Alimam et al, 2016; Skeith et al, 2017). The guidance detailed here refers to these case series, practices extrapolated from the management of pregnant patients with ET and personal practice. Pregnancy is a prothrombotic state with increased risk of thromboembolism in patients with PV. Consequently, there is a significant risk of obstetric complications, such as fetal loss throughout all trimesters, intra-uterine growth retardation, prematurity, maternal thromboembolism and haemorrhage. Previously significant fetal loss and maternal morbidity was seen, but a recent prospective study of pregnancy outcomes in MPNs showed better outcomes. There were no maternal deaths or thrombotic events (Alimam et al, 2016). This improvement in pregnancy outcomes is probably partly due to a more protocol-based management and a multidisciplinary approach. Pregnancies in patients with PV should be managed under the joint care of an obstetrician experienced in the care of high-risk patients and a haematologist experienced in MPNs. Management should start with preconceptual planning for both male and female patients. The Hct should be kept within a gestational-appropriate range and potentially teratogenic medications (such as hydroxycarbamide) should be stopped a minimum of 3 months prior to planned conception in males and females. There is insufficient data regarding the safety of anagrelide during pregnancy and it should therefore be withdrawn at least 3 months prior to conception in females. Interferon should be considered for those patients who require cytoreductive therapy, though patients should be counselled regarding the potential for interferon to reduce fertility. All patients should receive low dose aspirin (unless there are patient-specific contraindications) throughout pregnancy and the postpartum period. Patients with high risk pregnancies should be identified (Table 1). These are patients with previous arterial or venous thrombosis or haemorrhage attributed to PV, previous pregnancy complications (>3 first trimester losses, >1 s or third trimester loss, birth weight <5th centile for gestation, intrauterine death or stillbirth, pre-eclampsia), extreme thrombocytosis before or during pregnancy, diabetes mellitus or hypertension requiring pharmacological treatment. Those patients with a prior history of thrombosis and/or fetal morbidity should commence cytoreduction therapy with interferon and prophylactic LMWH in addition to aspirin. The dose and frequency of LMWH should be adjusted according to renal function, body weight and previous history. For those patients of normal body weight, no renal impairment and with a history of previous venous thrombosis or fetal morbidity, standard dose thromboprophylaxis (e.g. enoxaparin 40 mg/day) should be commenced as soon as pregnancy is confirmed. This should be increased to intermediate dose thromboprophylaxis (e.g. twice daily enoxaparin 40 mg) at 16–20 weeks. Patients with a previous history of arterial thrombosis should be commenced on intermediate dose prophylaxis (e.g. twice daily enoxaparin 40 mg) throughout pregnancy (Harrison, 2005). Interferon should be initiated for those patients with a history of PV-associated haemorrhage, extreme thrombocytosis (>1500 × 109/l) before or during pregnancy and/or diabetes mellitus or hypertension requiring pharmacological treatment. All other patients (standard risk pregnancy) should receive low dose aspirin throughout pregnancy with the addition of once daily prophylactic dose LMWH for 6 weeks postpartum. During pregnancy, the Hct should be maintained within the normal range for gestation with venesection (Table 2). Interferon can be considered if venesection fails to adequately control the Hct or is not tolerated. Patients should be reminded to avoid iron supplementation in the absence of proven iron depletion. Where supplementation is offered, this should be at a low dose with regular monitoring. The full blood count, blood pressure and urinalysis should be checked every 4 weeks until 24 weeks and then every 2 weeks. Fetal growth should be assessed by regular ultrasound. Uterine Dopplers should be performed at 20 weeks’ gestation to assess placental function by measuring the mean pulsatile index. If the index is >1·4, the frequency of growth scans should be increased and consideration given to escalating treatment to include LMWH and interferon. Dehydration should be avoided in all patients during labour and the third stage of labour should be actively managed. Thromboembolic deterrent (TED) stockings are advisable during labour and if immobile during the postpartum period. LMWH and aspirin should be stopped prior to elective caesarean section or once spontaneous labour has begun, as per local guidelines. Low molecular weight heparin should be restarted at the earliest opportunity post-partum provided there is no significant bleeding. The dose of LMWH should be once daily and should be continued for 6 weeks post-partum. Aspirin should be continued throughout the post-partum period. Breastfeeding is safe with both aspirin and LMWH, and permissible with interferon. The Hct should be monitored in the post-partum period and maintained at less than 0·45. Attention should be paid to the platelet count, as a rebound thrombocytosis may develop, necessitating the introduction of cytoreductive therapy. There is no evidence regarding the management of PV during fertility treatment. Ovarian hyperstimulation is associated with an increased risk of thrombosis and therefore cytoreductive therapy (with interferon) and thromboprophylaxis with LMWH should be considered. Pruritus is common in PV, occurring in up to 85% of patients (Mesa et al, 2017). Pruritus can predate or accompany the diagnosis of PV (Le Gall-Ianotto et al, 2017). It can occur spontaneously or be precipitated by water or changes in temperature and can have a significant negative impact on quality of life, affecting sleep, participation in social activities and bathing (Siegel et al, 2013). The intensity of pruritus varies but can be severe causing emotional depression, anxiety and even suicide ideation. The pathogenesis of pruritus in PV is not clear. Mechanisms involving basophils (Pieri et al, 2009) and mast cells (Ishii et al, 2009) have been postulated. It can also be associated with the iron deficiency that commonly results from frequent venesection therapy. Pruritus has been found to be correlated with granulocyte JAK2 V617F homozygosity and allele burden (Vannucchi et al, 2007; Gangat et al, 2008). The mechanism for this association is unclear though it may relate to cytokine levels. The management of pruritus is challenging. Symptoms can improve or resolve with control of blood counts, using venesection alone or with cytoreductive therapy (Le Gall-Ianotto et al, 2017). Improvement in pruritus has been reported with hydroxycarbamide (Sharon et al, 1986), interferon (although this drug can also worsen pruritus) (Lengfelder et al, 2000), busulfan and 32P (Jackson et al, 1987). Trials of ruxolitinib in PV have shown this agent to be effective in controlling pruritus in patients who have failed to respond to hydroxycarbamide (Vannucchi, 2015; Mesa et al, 2017; Passamonti et al, 2017). In those who are severely iron deficient as a result of venesection, pruritus may improve with iron replacement, if this is possible, but iron replacement must be undertaken with extreme caution and close supervision of blood counts. However, pruritus can persist in patients with PV once their blood counts have normalised (Gangat et al, 2008). Antihistamines (both H1 and H2 receptor antagonists) are commonly prescribed though demonstrate mixed results (Weick et al, 1982; Steinman & Greaves, 1985) and combination therapy at high doses may be required to be effective. Other adjunctive therapy that have been shown to be of benefit include selective serotonin reuptake inhibitors (SSRIs) (Tefferi & Fonseca, 2002) and anticonvulsants (such as gabapentin and pregabalin). Tricyclic antidepressants (Weisshaar et al, 2012), thalidomide and naltrexone have also been found to be beneficial (Phan et al, 2010). Sometimes creams containing menthol can also provide some relief (Patel et al, 2007). From a practical point of view, it is advisable to cut nails to reduce skin damage. Joint management with a dermatologist should be considered for refractory symptoms and for those patients requiring narrow-band-ultraviolet (UVB) or ultraviolet A (UVA) (Rivard & Lim, 2005). The addition of oral psoralen to UV light (PUVA) is only occasionally used. In patients who do not require cytoreductive therapy, the above options should be tried prior to initiating cytoreductive therapy just to control pruritus. Idiopathic erythrocytosis (IE) is a diagnosis of exclusion. It is an absolute erythrocytosis of no identifiable cause that is more frequent in males (Randi et al, 2016). Every effort should be made to exclude identifiable primary and secondary causes of erythrocytosis (including PV and congenital erythrocytosis) before a diagnosis of IE is made. Erythropoietin (EPO) levels are unhelpful as they are below normal in a third of patients, suggesting a primary erythrocytosis, and elevated in two-thirds, suggesting a secondary erythrocytosis. Recent studies using sequencing analysis have identified a number of patients who were previously diagnosed with IE as having mutations in either erythrocytosis-associated genes or novel genes (Bento et al, 2014; Camps et al, 2016), suggesting that at least some of these cases have a genetic basis. Although previous data suggested that the risk of thrombosis and/or haemorrhage was considerably elevated in IE, these observations were based on old evidence that predated the identification of the JAK2 V617F mutation and therefore some patients with PV are likely to have been included in the study cohorts. More contemporary data sugge