Rheumatologic Disorders (Diagnosis and Evaluation)

Diagnosis and Evaluation of Rheumatologic Disorders

Physical Examination

Two helpful clinical clues for diagnosing arthritis are the joint pattern and the presence or absence of extra-articular manifestations.

The joint pattern is defined by the answers to three questions: (1) Is inflammation present? (2) How many joints are involved? and (3) What joints are affected?

Joint inflammation manifests as warmth, swelling, and morning stiffness of at least 30 minutes’ duration. Overlying erythema occurs with the intense inflammation of crystal-induced and septic arthritis.

Both the number of affected joints and the specific sites of involvement affect the differential diagnosis.

Some diseases—gout, for example—are characteristically monoarticular, whereas other diseases, such as rheumatoid arthritis, are usually polyarticular.

The location of joint involvement can also be distinctive. Only two diseases frequently cause prominent involvement of the distal interphalangeal (DIP) joint: osteoarthritis and psoriatic arthritis.

Extra-articular manifestations narrow the differential diagnosis further:

• fever (eg, gout, Still disease, endocarditis, vasculitis, SLE)
• rash (eg, SLE, psoriatic arthritis, inflammatory myositis)
• nodules (eg, rheumatoid arthritis, gout)
• neuropathy (eg, vasculitis)

Characteristic	Status	Representative Disease
Inflammation	Present	Rheumatoid arthritis, SLE, gout
	Absent	Osteoarthritis
Number of involved joints	Monoarticular	Gout, trauma, septic arthritis, Lyme disease, osteoarthritis
	Oligoarticular (2-4 joints)	Reactive arthritis, psoriatic arthritis, IBD
	Polyarticular (>4 joints)	Rheumatoid arthritis, SLE
Site of joint involvement	Distal interphalangeal	Osteoarthritis, psoriatic arthritis (not rheumatoid arthritis)
	Metacarpophalangeal, wrists	Rheumatoid arthritis, SLE, calcium pyrophosphate deposition disease (not osteoarthritis)
	First metatarsal phalangeal	Gout, osteoarthritis

Arthrocentesis and Examination of Joint Fluid

If the diagnosis is uncertain, synovial fluid should be examined whenever possible.

Most large joints are easily aspirated, and contraindications to arthrocentesis are few.

The aspirating needle should never be passed through an overlying cellulitis or psoriatic plaque because of the risk of introducing infection.

For patients who are receiving DOACs or long-term anticoagulation therapy with warfarin, joints can be aspirated with a small-gauge needle (eg, 22F); the INR should be < 3.0 for patients taking warfarin.

Measure	(Normal)	Group I (Noninflammatory)	Group II (Inflammatory)	Group III (Purulent)
Volume (mL) (knee)	< 3.5	Often > 3.5	Often > 3.5	Often > 3.5
Clarity	Transparent	Transparent	Translucent to opaque	Opaque
Color	Clear	Yellow	Yellow to opalescent	Yellow to green
WBC per mcL	< 200 (0.2 x 109/L)	< 2000 (2.0 x 109/L)	< 2000-75,000 (2.0-75.0 x 109/L)	> 100,0002 (100 x 109/L)
PMNs	< 25%	< 25%	50% or more	75% or more
Culture	Negative	Negative	Negative	Usually positive2

Types of Studies

Gross Examination — Clarity is an approximate guide to the degree of inflammation.

• Noninflammatory fluid is transparent
• Mild inflammation produces translucent fluid
• Purulent effusions are opaque
• Traumatic taps, trauma, and bleeding disorders are the most common causes of bloody effusions

Cell Count — Normal synovial fluid contains < 200 white cells/mcL (0.2 × 10⁹/L). Synovial fluid glucose and protein levels add little information and should not be ordered.

• Noninflammatory (< 2000 white cells/mcL [2.0 × 10⁹/L])
• Inflammatory (2000–75,000 white cells/mcL [2.0–75 × 10⁹/L])
• Purulent (> 100,000 white cells/mcL [100 × 10⁹/L])

Microscopic Examination — Compensated polarized light microscopy identifies and distinguishes crystal arthropathies; gram stains are useful for identifying septic arthritis.

• Monosodium urate crystals (gout, negatively birefringent)
• Calcium pyrophosphate crystals (pseudogout, positive birefringent)
• Gram stain has specificity but limited sensitivity (50%) for septic arthritis

Culture — Bacterial cultures and special studies for gonococci, tubercle bacilli, or fungi are ordered as appropriate.

Interpretation

Synovial fluid analysis is diagnostic in infectious or microcrystalline arthritis. Although the severity of inflammation in synovial fluid can overlap among various conditions, the synovial fluid white cell count is a helpful guide to diagnosis.

Noninflammatory (< 2000 white cells/mcL [< 2 x 109/L])	Inflammatory (< 2000-75,000 white cells/mcL [< 2.0-75.0 x 109/L])	Purulent (< 100,000 white cells/mcL [< 100 x 109/L])	Hemorrhagic
– Osteoarthritis – Traumatic arthritis – Osteonecrosis – Charcot arthropathy	– Rheumatoid arthritis – SLE – Polymyositis or dermatomyositis – Systemic sclerosis – Systemic necrotizing vasculitides – Polychondritis – Gout – Calcium pyrophosphate deposition disease – Hydroxyapatite deposition disease – Juvenile rheumatoid arthritis – Ankylosing spondylitis – Psoriatic arthritis – Reactive arthritis – IBD arthritis – Hypogammaglobulinemia – Sarcoidosis – Rheumatic fever – Indolent/low virulence infections (viral, mycobacterial, fungal, Whipple disease, Lyme disease)	– Septic arthritis (bacterial)	– Trauma – Pigmented villonodular synovitis – Tuberculosis – Neoplasia – Coagulopathy – Charcot arthropathy