Repeat JAK2 V617F testing in patients with suspected essential thrombocythaemia

Correspondence to Dr Stephen E Langabeer, Cancer Molecular Diagnostics, St. James's Hospital, Dublin D08 W9RT, Ireland; slangabeer@stjames.ie Molecular investigation for characteristic initiating mutations, in addition to clinical, haematological and histopathological evidence, has become an integral part of myeloproliferative neoplasm (MPN) diagnosis. Up to 15% of patients with MPN of essential thrombocythaemia (ET) have no evidence of the canonical mutations (termed ‘triple-negative’) and are associated with a distinct clinical course particularly in regard to thrombotic risk therefore influencing treatment.2 Low JAK2 V617F allele burdens are by themselves insufficient to result in a diagnosis of an MPN in the absence of other diagnostic criteria.3 Additionally, expansion of the JAK2 V617F-positive haematopoietic clone(s) is highly variable among MPN patients and may occur over months or years.4 5 It is therefore possible that a proportion of patients with a persistent thrombocytosis and suggestive clinical features may not be diagnosed as ET or alternatively, misdiagnosed as triple-negative ET if no canonical mutation is detected. In addition to the identification of MPL exon 10 in 5 patients (1.7%) and JAK2 exon 12 mutations in three patients (1.0%), the NGS assay identified the JAK2 V617F in repeat samples from four historical patients (1.4%) with a persistent thrombocytosis and suspected ET in whom this mutation was previously not detected by allele-specific PCR.