Underbilling

I have never run across this particular problem before, a physician I am consulting for has underbilled 5 CPT® codes, and lost a significant amount of revenue. This occurred when they implemented their EMR in September of last year. I have filed an initial appeal with BCBS, and received a denial. Not surprising. They say we can ask for an external review, at a cost of course. Any opinions on whether this will be worthwhile? Any other thoughts? Thank you in advance for any insight you can give.