Calreticulin Mutation by Immunohistochemistry: Can It Replace PCR?

Before 2013, the diagnosis of about 30% to 45% cases of primary myelofibrosis (PMF) and essential thrombocythemia (ET) posed a diagnostic difficulty because of the missing reliable clonal marker. Calreticulin (CALR) mutation was identified as a recurrent mutation in about 60% to 88% of JAK2/MPL-negative PMF and ET. Molecular methods like Sanger sequencing and polymerase chain reaction (PCR) are considered gold standard, but they have limited availability, complex techniques, and labor intensive. In contrast to molecular methods, immunohistochemistry (IHC) is a widely available, rapid, simple, and cost-effective option. There are only few studies evaluating the utility of IHC for CALR mutation detection. Hence, we studied the role of IHC in CALR mutation detection and compared it with PCR. Thirty-one JAK2V617F-negative PMF and ET were evaluated for CALR mutation status. PCR was done and interpreted by comparing bands with the expected product size. The bone marrow biopsy was simultaneously put up for IHC using antimutated CALR monoclonal antibody (CAL2). CALR mutation was detected in 64.5% (20/31) cases. Prevalence of CALR mutation in JAK2-negative PMF and ET was 60.9% (14/23) and 75% (6/8), respectively. Sensitivity, specificity, positive predictive value, and negative predictive value of IHC analyzed were 89.4%, 100%, 100%, and 84.6%, respectively. A very good level of agreement (κ=0.86) was observed between PCR and IHC. We suggest that IHC is the best screening test to detect CALR mutation in resource limited countries with limited availability and affordability of molecular methods.