Septic arthritis laboratory tests


 * Associate Editor(s)-In-Chief: Jumana Nagarwala, M.D., Senior Staff Physician, Department of Emergency Medicine, Henry Ford Hospital;

Overview

 * Aspiration of joint fluid for cell count, examination of crystals under the microscope, gram stain, and culture


 * Blood culture


 * X-ray of affected joint

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

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

Joint Fluid Aspiration
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). A proportion of patients with septic arthritis have little in the way of fever or raised ESR, although the CRP is usually raised

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