A doctor must rule out other possible knee problems, such as arthritis, gout, pseudogout (CPPD disease), tendinitis, a fracture, or a tumor, before making a diagnosis of prepatellar bursitis (knee bursitis). If prepatellar bursitis is the diagnosis, the doctor must then distinguish between septic bursitis and aseptic bursitis.
Knee Bursitis Diagnostic Process
During a medical office visit, the doctor or other medical staff will conduct a patient interview and physical exam. The doctor may also take a fluid sample from the inflamed bursa.
Patient interview. During the doctor office visit, the patient will be asked questions about his or her medical history and describes the onset of symptoms, the pattern of knee pain and swelling, and how symptoms affect lifestyle. A patient's reported symptoms are important to diagnosis and treatment.
Physical exam. A doctor will examine the patient’s knee joint, noting swelling, tenderness and pain points, and range of motion.
Aspiration and lab tests. Using a needle and syringe to remove fluid from a joint capsule or bursa is called aspiration or arthrocentesis. A doctor may aspirate the prepatellar bursa to remove excess fluid. In addition to relieving pressure and making the patient more comfortable, the aspiration provides a fluid sample that can be tested for infection.
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Common tests for infection include:
- Gram stain. A lab test called a Gram stain is used to determine if certain troublesome bacteria are present. Not all bacteria can be identified with a Gram stain, however, so even if the test comes back negative, septic bursitis cannot be completely ruled out.
- White blood cell count. An elevated number of white blood cells in the prepatellar bursa’s synovial fluid indicates an infection.
- Glucose levels test. Glucose levels that are significantly lower than normal may indicate infection.
Occasionally lab tests to detect infection are negative even when an infection is present. Therefore, a doctor who strongly suspects septic bursitis may prescribe antibiotics without definitive positive lab tests.
Imaging tests such as X-rays, MRIs, and ultrasounds. Imaging tests are not usually needed to diagnose knee bursitis. However, if it is unclear whether symptoms are caused by knee bursitis or another condition, a doctor may request an imaging test. MRIs are not effective in determining if bursitis is septic or aseptic.1
Knee bursitis can be relatively easy to diagnose and often does not require lab tests or imaging. However, a doctor may order lab tests and imaging to determine whether or not the bursa is infected, or if the doctor wants to rule out an underlying medical condition.