How arthritis is tested on Step 1

Start with the joints involved and the stiffness pattern.

OsteoarthritisCartilage wear; not inflammatoryDIP, PIP, hips, kneesStiffness < 30 min; worse with useOsteophytes, asymmetric joint narrowingNo systemic symptomsRheumatoid arthritisAutoimmune synovitis (pannus)MCP, PIP, wrists; spares DIPStiffness > 1 hour; better with useErosions; ulnar deviation, swan neckNodules, anemia, lung diseasevs
OA vs RA. Methotrexate is the first-line disease-modifying drug for rheumatoid arthritis.

Crystal arthritis

Polarized light microscopy of joint fluid makes the diagnosis.

FeatureGoutPseudogout (CPPD)
CrystalMonosodium urateCalcium pyrophosphate
Shape and birefringenceNeedle-shaped, negative (yellow when parallel to the axis)Rhomboid, weakly positive (blue when parallel)
Typical jointFirst MTP (podagra)Knee, wrist
AssociationsAlcohol, thiazide and loop diuretics, tumor lysis, kidney disease, tophiOlder age, hyperparathyroidism, hemochromatosis, low magnesium
Acute treatmentNSAIDs, colchicine, corticosteroidsNSAIDs, colchicine, corticosteroids
PreventionAllopurinol, febuxostat (xanthine oxidase inhibitors); probenecidTreat the underlying cause

Aspirin in low doses reduces urate excretion and can worsen gout.

Seronegative spondyloarthritis

HLA-B27, axial disease, enthesitis, and eye involvement link this group.

HLA-B27 groupAnkylosingspondylitisSacroiliitis, bamboospine, uveitisReactivearthritisAfter Chlamydia orGI infectionPsoriaticarthritisDIP, dactylitis,pencil-in-cupIBD-associatedCrohn disease,ulcerative colitis
Spondyloarthritis. "Seronegative" means rheumatoid factor is negative. Back pain improves with exercise.

Lupus, autoantibodies, and septic arthritis

Match the antibody to the disease.

  • Systemic lupus erythematosus: mostly women of childbearing age; malar rash, arthritis, serositis, nephritis, cytopenias, Libman-Sacks endocarditis. ANA (sensitive); anti-dsDNA (specific, tracks nephritis); anti-Smith (specific); low C3 and C4.
  • Drug-induced lupus: anti-histone antibodies; hydralazine, procainamide, isoniazid; kidneys and CNS usually spared.
  • Other antibodies: anti-Ro/SSA and anti-La/SSB (Sjögren syndrome; neonatal heart block), anti-centromere (limited scleroderma, CREST), anti-Scl-70 (diffuse scleroderma), anti-Jo-1 (polymyositis and dermatomyositis).
  • Septic arthritis: a hot, swollen single joint with high synovial WBC; S. aureus most commonly, N. gonorrhoeae in sexually active young adults. Aspirate the joint urgently.

Worked example

Read the joint fluid.

functions Worked example — a painful big toe

A man taking hydrochlorothiazide wakes at night with a red, hot, very painful first MTP joint after drinking heavily. Joint fluid shows needle-shaped crystals that are yellow when parallel to the compensator axis. What is the diagnosis and acute treatment?

  1. Sudden monoarthritis of the first MTP is classic for gout.
  2. Thiazides reduce urate excretion; alcohol increases urate production and reduces excretion.
  3. Needle-shaped, negatively birefringent crystals are monosodium urate.
  4. Acute flares are treated with an NSAID (such as indomethacin), colchicine, or a corticosteroid.

Answer. Acute gout; treat with an NSAID, colchicine, or corticosteroid, and consider stopping the thiazide.