Chronic myeloid leukaemia (CML) may present with an isolated thrombocytosis and thus mimic essential thrombocythaemia (ET). The diagnostic criteria for ET proposed within the British Committee for Standards in Haematology guidelines (Harrison et al, 2010) are listed in Table 1. Here, A3 requires exclusion of CML, usually by demonstrating the absence of a BCR-ABL1 fusion gene or transcript in samples from bone marrow or peripheral blood.

However the text of the guideline indicated that BCR-ABL1 testing could be restricted to patients with atypical features, such as basophilia, left shift of neutrophils or the presence of small hypolobated megakaryocytes. Such features are easily overlooked and missing a diagnosis of CML has major consequences for the patient. We would therefore like to clarify that we recommend testing for the presence of aBCR-ABL1 fusion in all patients lacking JAK2 V617F or MPL mutations, as well as in patients with any atypical features in order to ensure that category A3 is fulfilled. We have therefore modified the diagnostic flow diagram from the guidelines (Fig 1).

Declaration of Interests

The BSH paid the expenses incurred during the writing of this guidance. None of the authors had conflicts of interest to declare. All authors have made a declaration of interests to the BSH and Task Force Chairs which may be viewed on request.