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A. Examination of the Patient

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 (Table 22–1). Some diseases—gout, for example—are characteristically monoarticular, whereas other diseases, such as rheumatoid arthritis (RA), 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 such as fever (eg, gout, Still disease, endocarditis, vasculitis, SLE), rash (eg, SLE, psoriatic arthritis, inflammatory myositis), nodules (eg, RA, gout), or neuropathy (eg, vasculitis) narrow the differential diagnosis further.

Table 22–1.Diagnostic value of the joint pattern.

B. Arthrocentesis and Examination of Joint Fluid

If the diagnosis is uncertain, synovial fluid should be examined whenever possible (Table 22–2). Most large joints are easily aspirated, and contraindications to arthrocentesis are few (eFigures 22–1, 22–2, 22–3, 22–4, 22–5, 22–6, 22–7). 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 less than 3.0 for patients taking warfarin.

Table 22–2.Examination of joint fluid.

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