Inflammatory Polyarthritis Differential & RA Mimics
Six weeks, a good history, and the right serology separate RA from its impostors
Approach to polyarthritis
Is it inflammatory? Morning stiffness longer than an hour, soft synovial swelling, warmth, stiffness that eases with use and a raised ESR or CRP point to inflammatory arthritis. Hard bony enlargement with brief stiffness points to osteoarthritis instead.
How long? Duration is the first sorting step. Polyarthritis present for less than 6 weeks is often viral or reactive and resolves on its own; synovitis persisting beyond 6 weeks is more likely a chronic disease such as rheumatoid arthritis, which should not be diagnosed in the first weeks.
What pattern? Symmetric small-joint disease (MCPs, PIPs, wrists) fits viral arthritis, RA, SLE and CPPD pseudo-RA. Migratory large-joint disease suggests rheumatic fever or gonococcal infection. Asymmetric large-joint oligoarthritis suggests reactive, psoriatic or Lyme arthritis, and DIP involvement suggests psoriatic arthritis or osteoarthritis.
Clues. Young children at home, travel, blood-borne infection risk, urticaria, a recent sore throat, nail changes, erythema nodosum, clubbing and girdle stiffness each redirect the differential.
Viral arthritis
Common features. Viral polyarthritis is typically acute, symmetric and small-joint, often arrives with rash, and resolves within weeks with NSAIDs and no joint damage.
| Virus | Setting and clues | Confirmatory test |
|---|---|---|
| Parvovirus B19 | Adults, especially women, exposed to young children; abrupt MCP, PIP, wrist and knee arthritis, rash often absent; RF may be transiently positive | Parvovirus B19 IgM |
| Hepatitis B | Preicteric prodrome of acute infection; arthritis with urticaria that fades as jaundice appears | HBsAg, aminotransferases |
| Hepatitis C | Chronic infection; nonerosive arthritis, RF positive but anti-CCP negative; mixed cryoglobulinemia | HCV antibody, then HCV RNA |
| Rubella | Infection or vaccination, usually young women; transient small-joint arthritis | History, rubella serology |
| HIV | Acute polyarthralgia; severe reactive or psoriatic arthritis | HIV antigen/antibody test |
| Chikungunya | Travel to the tropics or Caribbean (Aedes mosquitoes); high fever, rash, severe symmetric polyarthralgia that can persist for months | RT-PCR early, IgM later |
Parvovirus B19. Parents, teachers and health workers are the classic patients; the arthritis settles over weeks, occasionally months. The same virus causes aplastic crisis in chronic hemolytic anemia and hydrops fetalis in pregnancy.
Hepatitis B and C. The HBV prodrome is an immune-complex, serum sickness-like illness; HBV is also the classic viral association of polyarteritis nodosa (mononeuritis multiplex, renal and mesenteric microaneurysms, lungs spared). HCV-related mixed cryoglobulinemia adds palpable purpura, low C4, glomerulonephritis and neuropathy. New polyarthritis should be screened for HCV, especially when RF is positive and anti-CCP negative.
Reactive and granulomatous mimics
Acute rheumatic fever. Two to four weeks after group A streptococcal pharyngitis, a migratory polyarthritis of large joints (knees, ankles, elbows, wrists) appears, each joint inflamed for a few days, with a dramatic response to aspirin or NSAIDs. Carditis, Sydenham chorea, erythema marginatum and subcutaneous nodules complete the major Jones criteria, and ASO or anti-DNase B titers document the preceding infection.
Post-streptococcal reactive arthritis. Arthritis after streptococcal infection that is nonmigratory, persistent and less responsive to NSAIDs, without other Jones features, is classified separately.
Lyme arthritis. Months after an untreated tick bite in an endemic area, intermittent large effusions develop, typically in the knee. Two-tier serology is positive, and 28 days of oral doxycycline is standard.
Löfgren syndrome. This acute form of sarcoidosis in young adults combines bilateral ankle periarthritis (periarticular soft-tissue swelling more than true synovitis), erythema nodosum and bilateral hilar lymphadenopathy, often with fever. The triad is diagnostic without biopsy, most cases remit spontaneously, and NSAIDs control symptoms; glucocorticoids are kept for severe disease.
Systemic and seronegative mimics
SLE. Lupus arthritis is symmetric and small-joint, with pain often exceeding visible swelling and no erosions. Jaccoud arthropathy produces ulnar deviation and swan-neck deformities that correct passively because they come from capsular and tendon laxity, not bone destruction. ANA with anti-dsDNA or anti-Sm, cytopenias, low complement, rash and serositis support the diagnosis.
Psoriatic arthritis. Patterns include asymmetric oligoarthritis, DIP-predominant disease, symmetric polyarthritis, axial disease and arthritis mutilans. Dactylitis, enthesitis and nail pitting or onycholysis are hallmarks, serology is usually negative, and radiographs show pencil-in-cup erosions (a tapered phalanx seated in a cupped neighbor) with new bone formation. Plaques may hide in the scalp, umbilicus or gluteal cleft, or appear after the arthritis.
Hemochromatosis and CPPD. Hemochromatosis produces firm, bony swelling of the second and third MCPs with hook-like osteophytes and chondrocalcinosis; transferrin saturation and ferritin make the diagnosis. CPPD can present as pseudo-RA, a chronic symmetric wrist and MCP synovitis in older adults with chondrocalcinosis and CPP crystals on aspiration. Chronic tophaceous gout can also be polyarticular, with tophi mistaken for rheumatoid nodules, which is why uric acid belongs in the workup.
| Condition | Joint pattern | Key discriminator |
|---|---|---|
| Rheumatoid arthritis | Symmetric MCP, PIP, wrist; DIPs, lumbar spine and SI joints spared | Anti-CCP, nodules, marginal erosions with periarticular osteopenia |
| SLE | Symmetric small joints | Nonerosive, reducible Jaccoud deformities; anti-dsDNA |
| Psoriatic arthritis | DIPs, asymmetric, dactylitis | Nail pits, pencil-in-cup erosions, seronegative |
| Hemochromatosis | 2nd–3rd MCPs | Hook-like osteophytes, high transferrin saturation |
| CPPD pseudo-RA | Wrists, MCPs in older adults | Chondrocalcinosis, CPP crystals |
Mimics in older adults
Polymyalgia rheumatica. Patients over 50 have bilateral shoulder and hip-girdle aching and prolonged morning stiffness with high ESR and CRP and a normal CK. The response to prednisone 12.5 to 25 mg/day is prompt and dramatic, and every patient is asked about headache, jaw claudication and visual symptoms of giant cell arteritis. Elderly-onset RA can begin with PMR-like shoulder involvement.
RS3PE. Remitting seronegative symmetrical synovitis with pitting edema causes abrupt wrist and hand synovitis with pitting edema of the hands in older adults. It is RF-negative, responds well to low-dose glucocorticoids and is sometimes paraneoplastic.
Hypertrophic osteoarthropathy. Digital clubbing, painful periostitis of the long bones (tibia, radius, ulna) with periosteal new bone on radiographs, and symmetric arthritis or effusions form the syndrome. In adults it most often signals lung cancer, and it also accompanies chronic lung infection and cyanotic heart disease, so image the chest.
Hypothyroidism. Arthralgias, viscous noninflammatory effusions, CPPD, carpal tunnel syndrome and a myopathy with high CK can all mimic inflammatory disease; check TSH.
Workup and referral
- History and examduration, joint count and pattern, skin, nails, eyes, travel and exposures
- Core blood testsCBC, ESR, CRP, RF, anti-CCP, ANA, HBsAg, HCV antibody, parvovirus B19 IgM if acute, uric acid
- Targeted testsTSH, iron studies, ASO, Lyme serology, chest radiograph as clues dictate
- Imaginghand and foot radiographs for baseline erosions
- Referralsynovitis beyond 6 weeks, especially with anti-CCP, RF, erosions or high CRP
RF versus anti-CCP. RF is sensitive but nonspecific: it is positive in HCV, Sjögren disease, endocarditis, other chronic infections and many healthy older adults. Anti-CCP is about 95% specific for RA, predicts erosive disease and can appear years before symptoms.
Early treatment. Persistent synovitis with anti-CCP or RF should prompt early rheumatology referral and a DMARD, usually methotrexate. Joint damage accumulates fastest early, so the aim is DMARD therapy within about 3 months of symptom onset. Glucocorticoid bridging is optional and should be brief and low-dose (the 2021 ACR guideline conditionally favors starting methotrexate without it); glucocorticoids never replace the DMARD.