Does it matter whether one uses MSI PCR testing versus IHC for MMR deficiency testing inorder to determine eligibility for immunotherapy?  

In colorectal cancers, PCR is likely more sensitive (+2-5%) for detecting MSI but more costly with a longer turnaround time. PCR may miss PMS2 and MSH6 alterations that are difficult to detect and are not always MSI-high (but they are germ-line alterations). IHC is easy to incorporate into universal...

Immunohistochemistry (IHC) assays detect dMMR, or deficiency in the mismatch repair proteins. Polymerase chain reaction, or PCR, detects microsatellite instability-high (MSI-high) status at the genomic level. Functionally, they mean the same thing - tumors that test positive for either assay should ...

Both IHC and PCR are sensitive and specific with a high concordance rate between these two methods (92–97%) (Sepulveda et al., PMID 28165299; Histopathology 2020). Importantly, there is a small percentage of dMMR CRCs with intact expression of MMR proteins by IHC, yet have a dysfunctional MMR ...

The answer in my practice is no. MMR IHC is an excellent screening tool and a quicker surrogate marker for MSI and is reflexively testing immediately on new CRC biopsies. I always check both for all of my patients, but in most cases, I do not wait for MSI testing on NGS prior to starting immunothera...