• Synovial fluid analysis and culture

• Blood tests

• Sometimes sputum, spinal fluid and urine samples

• Sometimes X-ray, 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 known to occur in people who have infectious arthritis.

Usually, a sample of synovial fluid is removed with a needle (called common aspirationor arthrocentesis) as soon as possible. It is examined for an increased white blood cell count and tested for bacteria and other organisms. The lab can usually grow and identify the infecting bacteria from the synovial fluid (called a culture) unless the person has recently taken antibiotics. However, the bacteria that cause gonorrhea, Lyme disease and syphilis are difficult to get from synovial fluid. If bacteria grow in culture, the lab then tests which antibiotics would be effective.

A doctor usually takes blood samples because bacteria that cause 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 if the infection is elsewhere.

If the doctors suspect that the infectious arthritis is caused by gonococciSamples are also taken from the urethra, cervix, rectum and throat. Tests for chlamydial infection of the genitals (another sexually transmitted infection) is also done because many people who have gonorrhea also have a chlamydia infection.

To determine the correct treatment, doctors can analyze the synovial fluid f to detect DNA from gonococci and mycobacteria.

Doctors can take x-rays of the affected joint to rule out other conditions. Doctors can do magnetic resonance imaging (MRI) if the joint cannot be easily examined or aspirated. MR or ultrasound is also done to identify accumulations of fluid or collections of pus (abscesses).