Distinguishing Rheumatoid Arthritis, Inflammatory Arthritis, and Osteoarthritis
The critical distinction lies in recognizing that rheumatoid arthritis (RA) is a specific type of inflammatory arthritis characterized by symmetric small joint involvement with morning stiffness >30 minutes, positive serology (RF/anti-CCP), and systemic inflammation, while osteoarthritis (OA) presents with mechanical pain in weight-bearing joints, minimal morning stiffness (<30 minutes), bony enlargement, and normal inflammatory markers.
Key Diagnostic Features
Rheumatoid Arthritis
Joint pattern: Symmetric involvement of small joints—specifically metacarpophalangeal (MCP), proximal interphalangeal (PIP), wrists, and metatarsophalangeal (MTP) joints—while characteristically sparing distal interphalangeal (DIP) joints, first carpometacarpal joints, and first MTP joints 1.
Morning stiffness: Duration exceeding 30 minutes is characteristic, with difficulty making a fist being a hallmark symptom 1.
Serology: Anti-CCP antibodies demonstrate 90% specificity with 60% sensitivity, while RF shows 70% specificity with similar sensitivity; however, 20-30% of RA cases are seronegative 1.
Inflammatory markers: CRP is preferred over ESR as it is more reliable and not age-dependent, though both can be normal even in active disease 1.
Radiographic findings: Non-proliferative marginal erosions with periarticular osteopenia are characteristic, with erosions predicting disease persistence 2.
Osteoarthritis
Joint pattern: Primarily affects DIP joints, PIP joints, thumb base (first carpometacarpal), and weight-bearing joints (knees, hips), with characteristic sparing of MCP joints 3.
Clinical hallmarks: Heberden nodes (DIP) and Bouchard nodes (PIP) represent bony enlargement, often with lateral deviation of interphalangeal joints 3.
Pain characteristics: Pain worsens with usage and weight-bearing, with only mild morning stiffness lasting less than 30 minutes; symptoms are often intermittent affecting one or few joints at a time 3, 4.
Laboratory findings: Normal inflammatory markers (ESR/CRP) and negative RF/anti-CCP, though erosive OA can show elevated CRP 3.
Radiographic features: Joint space narrowing, osteophytes, subchondral sclerosis, and subchondral cysts; erosive OA shows subchondral erosions primarily at IP joints 3.
Inflammatory Arthritis (Broader Category)
Definition: Encompasses RA, spondyloarthritis (including psoriatic arthritis), and other immune-mediated joint diseases characterized by true synovitis 3.
Distinguishing features: Requires differentiation of localized versus generalized pain, assessment of inflammation adequacy, and identification of specific disease patterns 3.
Diagnostic Algorithm
Step 1: Clinical Assessment
Perform 28-joint count examination assessing PIPs, MCPs, wrists, elbows, shoulders, and knees for tenderness and swelling to quantify synovitis 1.
Document morning stiffness duration: >30 minutes suggests inflammatory arthritis; <30 minutes suggests OA 3, 1.
Identify joint distribution pattern: Symmetric small joint involvement suggests RA; DIP/thumb base involvement suggests OA; DIP involvement with dactylitis suggests psoriatic arthritis 1, 2.
Examine for specific signs: Soft tissue swelling and warmth indicate inflammatory arthritis; bony hard enlargement without warmth indicates OA 3.
Step 2: Laboratory Workup
Order simultaneously: RF, anti-CCP antibodies, CRP (preferred over ESR), complete blood count with differential, comprehensive metabolic panel 1.
Interpretation pitfalls: Do not dismiss RA diagnosis based on negative serology (seronegative RA accounts for 20-30% of cases) or normal inflammatory markers (can be normal even in active disease) 1.
Additional testing when indicated: ANA if connective tissue disease suspected; HLA-B27 if spondyloarthropathy considered; uric acid to exclude gout 1, 2.
Step 3: Imaging Strategy
Baseline radiographs: Obtain bilateral hand, wrist, and foot X-rays for all suspected inflammatory arthritis to assess for erosions and establish baseline 1.
Radiographic interpretation:
Advanced imaging when diagnosis uncertain: Ultrasound with Power Doppler detects subclinical synovitis; MRI with contrast is most sensitive for early bone marrow edema (osteitis), the best predictor of disease progression 1.
Step 4: Apply 2010 ACR/EULAR Classification Criteria (for RA)
Joint involvement scoring:
- 1 large joint = 0 points
- 2-10 large joints = 1 point
- 1-3 small joints = 2 points
- 4-10 small joints = 3 points
10 joints = 5 points 1
Serology scoring:
- Negative RF and anti-CCP = 0 points
- Low positive RF or anti-CCP = 2 points
- High positive RF or anti-CCP = 3 points 1
Acute phase reactants: Abnormal CRP or ESR = 1 point 1
Symptom duration: ≥6 weeks = 1 point 1
Diagnosis: Score ≥6/10 indicates definite RA; requires at least one joint with definite clinical synovitis not better explained by another disease 1.
Critical Differential Diagnoses
When to Consider Psoriatic Arthritis
Mandatory assessment: Thorough skin and nail examination for psoriatic plaques, nail pitting, or onycholysis 2.
Distinguishing features: DIP involvement, dactylitis (sausage digits), enthesitis, and negative RF/anti-CCP favor psoriatic arthritis over RA 2.
When to Consider Erosive Hand OA
Key features: Targets mainly IP joints with severe structural changes including subchondral erosion and ankylosis; can present with elevated CRP mimicking inflammatory arthritis 3.
Differentiation: Absence of systemic symptoms, negative serology, and characteristic radiographic pattern 3.
When to Consider Crystal Arthropathy
Gout consideration: May superimpose on pre-existing OA; requires synovial fluid analysis for urate crystals if monoarthritis or oligoarthritis present 2.
CPPD consideration: Radiographic changes extremely similar to OA; may coexist with other arthropathies 3.
Treatment Implications Based on Diagnosis
Rheumatoid Arthritis Management
Immediate action: Refer to rheumatology within 6 weeks of symptom onset; early treatment prevents irreversible joint damage 1.
First-line therapy: Methotrexate 15-30 mg weekly as anchor drug, combined with short-term low-dose glucocorticoids (10-20 mg prednisone daily) as bridge therapy 1.
Treatment target: Sustained remission (SDAI ≤3.3) or low disease activity (SDAI ≤11); reassess every 4-6 weeks using composite disease activity measures 1.
Escalation strategy: If inadequate response after 3 months, add biologic agent (TNF inhibitor) or triple DMARD therapy 1.
Osteoarthritis Management
Non-pharmacologic interventions: Physical activity and exercise show most uniformly positive effects on pain; weight management critical for weight-bearing joints 3.
Pharmacologic options: Acetaminophen or NSAIDs at lowest effective dose for shortest duration; avoid NSAIDs in cardiovascular/renal disease 5, 4.
Adjunctive therapies: Intra-articular glucocorticoid injections for localized joint inflammation; psychological interventions (CBT) demonstrate uniform positive effects 3, 5.
Inflammatory Arthritis (General Approach)
Biopsychosocial framework: Adopt patient-centered approach assessing pain characteristics, inflammation control, joint damage, psychological factors, and sleep quality 3.
Core interventions: Education complemented by physical activity/exercise (strongest evidence), psychological interventions, orthotics for specific disease groups 3.
Common Pitfalls to Avoid
Do not delay RA treatment waiting for positive serology: Seronegative RA has similar prognosis and requires equally aggressive treatment 1.
Do not dismiss inflammatory arthritis based on normal CRP/ESR: Acute phase reactants are poor predictors and can be normal even in active disease; prioritize clinical synovitis 1.
Do not overlook functional impairment in OA: Functional disability can be as severe as RA and requires careful assessment using validated outcome measures 3, 6.
Do not focus solely on pain intensity scores: Assess functional goals, quality of life, and psychological factors that perpetuate pain 5.
Do not miss psoriatic arthritis: Always perform thorough skin and nail examination when RF/anti-CCP negative with inflammatory arthritis pattern 2.