Chapter 27-17: Septic Arthritis

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.

1. Septic arthritis usually occurs via hematogenous spread of bacteria.

2. Joint distribution

   1. The knee is the most commonly affected joint.

   2. Monoarticular arthritis is the rule, with multiple joints involved in < 15% of patients.

   3. Infection is most common in previously abnormal joints, such as those affected by OA or RA.

3. Staphylococcus aureus is the most common organism followed by species of streptococcus.

1. Clinical findings

   1. Fever is present in most patients.

      1. One meta-analysis found that 57% of patients with septic arthritis had fever.

      2. Recognize that this means that over 40% of patients with septic arthritis are afebrile.

      3. Fever > 39.0°C is rare.

   2. 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.

2. Laboratory findings

   1. WBC > 10,000/mcL is seen in only 50% of patients.

   2. Definitive diagnosis is made by Gram stain and culture.

      1. The Gram stain of synovial fluid is positive in about 75% of patients with septic arthritis.

      2. 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.

   3. Elevated synovial fluid WBC count is predictive.

      1. Synovial fluid WBC count > 100,000/mcL: LR+ 28; LR− 0.71.

      2. Lower WBC cut offs are nondiagnostic.

   4. Joint fluid culture is positive in about 90% of cases.

   5. Blood (and sputum, when appropriate) should also be cultured

      1. May help identify an organism if one is not isolated from the synovium

      2. 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.

1. Antibiotic therapy is directed by Gram stain findings.

2. Empiric therapy should cover S aureus.

3. The affected joint should also be drained, either with a needle, arthroscope, or arthrotomy (opening the joint in the operating room).

   1. Small joints can usually be drained and lavaged with serial arthrocentesis.

   2. Large joints usually require surgical drainage.

   3. The knee is an exception, a large joint that, in many cases can be treated with serial arthrocentesis.

4. Patients who receive treatment within 5 days of symptom onset have the best prognosis.