Reflective molecular testing for myeloproliferative neoplasms in patients with elevated serum vitamin B 12

Vitamin B₁₂ deficiency is common and can result in megaloblastic anaemia and progressive neurological deterioration; estimation of total serum vitamin B₁₂ is normally the primary investigation. ¹ The association between myeloproliferative neoplasms (MPN) and elevated serum vitamin B₁₂ concentrations in patients has been recognized for many years; the elevated concentrations may reflect disease activity, e.g. increased proliferation. ² Elevated serum vitamin B₁₂ concentrations are often observed in patients with chronic myeloid leukaemia (CML), polycythaemia vera (PV), essential thrombocythaemia (ET) and primary myelofibrosis (PMF), and also in less common subtypes such as chronic eosinophilic leukaemia. ³ Paradoxically, analysis of serum methylmalonic acid, serum transcobalamin and plasma homocysteine reveals vitamin B₁₂ deficiency in a significant proportion of MPN patients at diagnosis. ⁴ The diagnosis of MPN is multidisciplinary, requiring clinical, laboratory, histomorphological, and increasingly, molecular evaluation. While the BCR-ABL1 fusion gene is the molecular hallmark of CML, the most common acquired driver mutation in PV, ET and PMF is the JAK2 V617F, with CALR exon insertion/deletion mutations also frequently observed in patients with ET and PMF. ⁵ Although elevated serum vitamin B₁₂ concentrations are not diagnostic of MPN, they have become a recurrent infrequent trigger for molecular testing for the JAK2 V617F, CALR mutations and/or BCR-ABL1 transcripts, even in the absence of other clinical or laboratory features of MPN.

In order to address the clinical value and laboratory impact of such requests, a retrospective audit was performed on all JAK2 V617F requests received at a molecular diagnostics centre for haematological malignancies. From January 2006 to December 2017 inclusive, 15,562 diagnostic requests for JAK2 V617F mutation analysis were received. In 78 (0.5%) of these, elevated serum vitamin B₁₂ (>900 pg/mL) was the sole indication provided. In this group, median age was 61 years (41 males, 37 females). Using standardized screening assays unchanged throughout the audit period, the JAK2 V617F mutation was not detected in any of these 78 patients, BCR-ABL1 transcripts were not detected in 36/78 and CALR mutations not detected in 13/78 where requested.

Selecting patients to screen for MPN-associated rearrangements and mutations requires careful consideration in order to optimize laboratory resources. The number of requests in patients with an elevated serum vitamin B₁₂ does not appreciably impact the overall laboratory workload. Nevertheless, reflective screening for JAK2 V617F, BCR-ABL1 fusion and CALR exon 9 mutations in patients with elevated vitamin B₁₂ concentration, but possessing no other clinical, haematological, or laboratory evidence of an MPN, is not efficient.