If patient has inflammatory arthritis affecting multiple joints

If patient has inflammatory arthritis affecting multiple joints, possibilities are many: clinical pattern and associated signs may help along with some supporting role of lab / radiological tests.
1: RHEUMATOID is usually symmetrical, classically involves small joints of hands, spears DIP and spares spine (except cervical spine which can be affected). RA per se also doesn’t involve glomeruli, brain and doesn’t explain alveolar haemoptysis. Also other than Vasculitic skin issues, no other skin rash is seen in RA. Synovitis is hallmark and Erosions are a feature of RA unless treated adequately & if there are no erosions after two years it’s unlikely to be RA. Positive RA factor or anti CCP may support the diagnosis if clinical picture is convincing for RA. Extra articular features such as subcutaneous RA nodules, felty syndrome etc may be very useful help to support diagnosis etc.
2: PSORIATIC Arthritis can also be poly arthritis and can be mimicking like RA. However associated nail changes, DIP involvement, psoriatic skin rash etc if present all favour psoriasis but are not always seen. Distribution of joint involvement is different from RA as RA will involve PIP &/ or MCPs of almost all fingers whereas Psoriasis classically doesn’t involve all fingers but those fingers which are involved will have almost all joints affected, so RA spans transversely across the fingers, whereas Psoriasis affects fingers longitudinally. Psoriasis can also present as seronegative pattern (large joints involved in an asymmetrical fashion and predominantly in lower limbs more than upper limb ) OR spondylitis or Arthritis Mutilans which are not seen in RA etc. Joint erosions and deformities maybe there but not an hallmark like that in RA. Plus psoriatic Arthritis can cause extra bone formations due to syndesmophytes etc which is never seen in RA (later almost always cause Peri articular bone loss but no extra bone formation).
3: SLE, Sjogren, polymyositis, cryoglobulinemia, Vasculitis, sarcoidosis, IBD etc can also present with arthralgias & poly arthritis etc but usually non deforming and erosions are not commonly seen. Associated clinical features and tests wills help make a diagnosis. Viral arthritis is common but usually self limited and non deforming except Parvovirus which can cause destructive mono or polyarthritis. Chronic viral infections such as HBV, HCV, HIV etc can cause recurrent episodes.
5: Rheumatic fever can also present with poly Arthritis but if someone never had it before 20, he / she will not have it after that. Associated skin rash, nodules, carditis, ASO titre, Anti DNAase etc help diagnose.
6: Still’s disease can present wth poly arthritis, skin rash, fever, neutrophilia and very high Ferritin in thousands more in young people.
7: Seronegative Poly arthritis can be seen in Ankylosing Spondylitis, Psoriasis, Behcet, Reiter’s syndrome, IBD related arthritis etc. Classically it’s large joints and asymmetrical with preferential involvement of lower limbs. Hands and feet small joints are not commonly involved. Spondylitis is common with sacroiliac involved in majority. Ascending spondylitis can also be there. Associated clinical features helps such as Uveitis, aortitis etc.
8: Poly articular Gout can also be confused with RA as it often causes deformities and punched out tophi related bony erosions etc. Tophi when found helps clear the ambiguity. Joint aspiration with Urate crystals will confirm the diagnosis.