Final answer:
Arthrocentesis or joint aspiration is the test used to check for excess fluid build-up on the knee by draining the fluid for analysis, which can include PCR testing. Additional imaging tests like MRI, ultrasound, and arthrography can also be employed to visualize fluid accumulation.
Step-by-step explanation:
To check for excess fluid build-up on the knee, a medical test called arthrocentesis, also known as a joint aspiration, is commonly performed. During this procedure, a sterile needle and syringe are used to drain the excess fluid from the joint space. This fluid can then be analyzed in a laboratory, often involving a PCR (Polymerase Chain Reaction) test, to determine the cause of the effusion, such as infections like Lyme disease or other inflammatory conditions. Furthermore, imaging tests such as MRI, ultrasound, and arthrography can help visualize the soft tissue and fluid in the joint and are useful in the diagnosis and management of conditions resulting in knee effusion.