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TEXTBOOK PRESENTATION
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Septic arthritis usually presents as subacute joint pain associated with low-grade fever and progressive pain and disability. Because the infection is usually caused by hematogenous spread, a risk factor for bacteremia (such as injection drug use) is sometimes present. Disseminated gonorrhea is discussed separately below.
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Septic arthritis usually occurs via hematogenous spread of bacteria.
Joint distribution
The knee is the most commonly affected joint.
Monoarticular arthritis is the rule, with multiple joints involved in < 15% of patients.
Infection is most common in previously abnormal joints, such as those affected by OA or RA.
Staphylococcus aureus is the most common organism followed by species of streptococcus.
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EVIDENCE-BASED DIAGNOSIS
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Clinical findings
Fever is present in most patients.
One meta-analysis found that 57% of patients with septic arthritis had fever.
Recognize that this means that over 40% of patients with septic arthritis are afebrile.
Fever > 39.0°C is rare.
Findings predictive of a septic arthritis causing joint pain are recent joint surgery (LR+ 6.9) and the presence of a prosthetic knee or hip in the presence of a skin infection (LR+ 15.0).
Fever cannot distinguish septic arthritis from other forms of monoarticular arthritis. Patients with gout may be febrile while those with septic joints may not be.
Laboratory findings
WBC > 10,000/mcL is seen in only 50% of patients.
Definitive diagnosis is made by Gram stain and culture.
The Gram stain of synovial fluid is positive in about 75% of patients with septic arthritis.
The Gram stain is most likely to be positive when the infecting organism is S aureus; it is less likely to be positive when another organism is the infecting agent.
Elevated synovial fluid WBC count is predictive.
Synovial fluid WBC count > 100,000/mcL: LR+ 28; LR− 0.71.
Lower WBC cut offs are nondiagnostic.
Joint fluid culture is positive in about 90% of cases.
Blood (and sputum, when appropriate) should also be cultured
May help identify an organism if one is not isolated from the synovium
About 50% of patients will have positive blood cultures.
Because of the potential for septic arthritis to cause joint destruction, a single, acutely inflamed joint should be assumed infected until proved otherwise.
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Antibiotic therapy is directed by Gram stain findings.
Empiric therapy should cover S aureus.
The affected joint should also be drained, either with a needle, arthroscope, or arthrotomy (opening the joint in the operating room).
Small joints can usually be drained and lavaged with serial arthrocentesis.
Large joints usually require surgical drainage.
The knee is an exception, a large joint that, in many cases can be treated with serial arthrocentesis.
Patients who receive treatment within 5 days of symptom onset have the best prognosis.