OA coding

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Does anyone have any info on correctly coding OA? We aren't to code primary or secondary unless so stated by the physician but there aren't codes for unspecified OA for many sites. For example, OA of ankle directs you to code M19.07X but the tabular states primary osteoarthritis. There is a code for unspecified OA of the knee (M17.9) but if it's bilateral, the index directs you to M17.0 which is primary OA. The only other option I've found is M19.90 but while it is for unspecified OA, the site is unspecified as well. Any suggestions? :confused: