I don’t have a statistic on misdiagnosis ready, I’m just a clinical lab scientist and have worked with several docs at two outpatient clinics in the last five years. Once at each clinic, I had docs misdiagnose gonococcal arthritis as rheumatic arthritis, and were surprised when the rheumatology specialists sent the patients back with antibiotics and orders for STI workups. They didn’t seem aware that N. gonorrhea could cause arthritis.

https://www.ncbi.nlm.nih.gov/books/NBK470439/#NBK470439_pubdet

Review paper says that in the 80s, data suggested that 14% of diagnosed arthritis cases were gonococcal in nature, but those statistics may not hold up in the context of today’s diagnostic rates. Regardless, arthritic symptoms in relatively young patients should definitely indicate for an STI workup in addition to normal RA testing and imaging.