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Make the Diagnosis: Septic Arthritis

Prior Probability

Among patients with an acute, swollen, and painful joint, the probability of septic arthritis is 8%-27%.1, 2

Population in Whom Septic Arthritis Should Be Considered

Many conditions can mimic septic arthritis; however, nongonoccocal bacterial arthritis is a very destructive form of septic arthritis that should be diagnosed promptly to prevent damage to the joint. The condition should be considered in all patients with an acutely swollen joint unrelated to trauma. Important risk factors include recent joint surgery, age >80 years, diabetes mellitus, rheumatoid arthritis, hip or knee replacements, or skin infection.3 Skin infections are particularly concerning in patients with joint replacements. Septic arthritis can complicate crystalline arthritides.

Assessing the Likelihood of Septic Arthritis

The presence of fever is associated with a decrease in the likelihood of septic arthritis (likelihood ratio [LR] 0.67, 95% confidence interval [CI] 0.43-1.0), while the absence of fever is associated with a slightly higher likelihood of septic arthritis (LR 1.7, 95% CI 1.0-3.0), However, the broad CIs for fever make it an unreliable finding.4 For all other symptoms and signs, only the sensitivity has been reported (Table 65-1). The majority of patients with septic arthritis will have joint pain and/or joint swelling, and most patients will not have sweats or rigors. Serum laboratory values are of limited usefulness because the LR+ are not sufficiently high, and the LR- have broad CIs. Because the history, physical examination, and serum laboratory tests do not substantially alter the pretest probability, arthrocentesis is required. The most valuable tests come from examination of the synovial fluid. Increasing numbers of leukocytes in the synovial fluid is associated with a higher likelihood of septic arthritis (Table 65-2).

Table 65-1Sensitivity of Symptoms and Signs for Septic Arthritis
Table 65-2Laboratory Diagnosis of Septic Arthritis

Reference Standard Tests

Synovial fluid culture is the generally accepted reference standard, but it is not perfect (sensitivity 75%-95%).5 Thus, a positive Gram stain, positive blood cultures in a patient with an acutely swollen and painful joint, aspiration of pus from the joint space, or response to antibiotics are sometimes used in clinical practice as the reference standard for septic arthritis.

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