Analysis and culture of joint fluid
Blood tests
Sometimes sputum, spinal fluid, and urine tests
Sometimes x-rays, magnetic resonance imaging (MRI), or ultrasound
Doctors typically suspect the diagnosis of infectious arthritis in people who have severe or unexplained arthritis and in people who have other combinations of symptoms that are known to occur in people who have infectious arthritis.
Usually, a sample of joint fluid is removed with a needle (called joint aspiration, or arthrocentesis) as soon as possible. It is examined for an increased number of white blood cells and tested for bacteria and other organisms. The laboratory can usually grow and identify the infecting bacteria from the joint fluid (called a culture), unless the person has recently taken antibiotics. However, the bacteria that cause gonorrhea, Lyme disease, and syphilis are difficult to recover from joint fluid. If bacteria do grow in culture, the laboratory then tests which antibiotics would be effective.
A doctor usually does blood tests because bacteria causing joint infections often appear in the bloodstream. Sputum, spinal fluid, and urine may also be tested for bacteria to help determine the source of infection and determine whether the infection is anywhere else.
If doctors suspect the infectious arthritis is caused by gonococci, samples are also taken from the urethra, cervix, rectum, and throat. Tests for chlamydial infection of the genitals (another sexually transmitted infection) are also done because many people who have gonorrhea also have a chlamydial infection.
To determine the correct treatment, doctors may analyze the joint fluid f to detect the DNA of gonococci and mycobacteria.
Doctors may take x-rays of the affected joint to rule out other conditions. Doctors may do magnetic resonance imaging (MRI) if the joint cannot be easily examined or aspirated. MRI or ultrasonography is also done to identify accumulations of fluid or collections of pus (abscesses).
