Septic Arthritis - Diagnosis

Diagnosis

Septic arthritis should be considered whenever one is assessing a patient with rapid onset of joint pain. Usually only one joint is affected (monoarthritis) however in seeding arthritis, several joints can be affected simultaneously; this is especially the case when the infection is caused by staphylococcus or gonococcus bacteria. Pain can be significant with any movement, therefore, patient will refuse to use extremity and prefers to hold joint rigidly. May have associated swelling, redness & warmth, often absent for deep joints such as hips & shoulders. In the case of gonorrhea the knee or wrist may be chronically affected. The pain may be chronic and the physician may inject steroids to reduce symptoms. Weeks later increased pain, redness and swelling- signs of inflammation- appear leading to drainage by needle puncture. Then the gram stain and cultures are typical of a Neisserian infection.

The diagnosis of septic arthritic can be difficult as no test is able to completely rule out the possibility.

A number of factors should increase one's suspicion of the presence of an infection. In children these are: fever > 38.5 C, non-weight-bearing, serum WBCs > 12 x 10^9, ESR > 40 mm/hr, CRP > 20 mg/dL, a previous visit for the same.

Diagnosis is by aspiration (giving a turbid, non-viscous fluid), Gram stain and culture of fluid from the joint, as well as tell-tale signs in laboratory testing (such as a highly elevated neutrophils (approx. 90%), ESR or CRP). The ESR and CRP are almost always raised on admission, CRP being faster in diagnostics. In the joint aspirate, the typical white blood cell count in septic arthritis is over 100,000/mm3. However, septic synovial fluid can have white blood cell counts as low as a few thousand in the early stages, and therefore, differentiation of septic joints from "merely" inflammatory ones is not truly possible based on cell counts alone.

The Gram stain can rule in the diagnosis of septic arthritis however cannot exclude it.

X-rays - may not be helpful early, but may show subtle increase in joint space tissue swelling.

Ultrasound - may reveal joint effusion.