Arthritis is one of the most common chronic conditions and affects almost 70 million Americans.
The term “arthritis” is derived from the Greek “arthron” meaning joint, and “itis” meaning inflammation and is used to refer to more than 100 different conditions.
One difficult situation that confronts a physician is whether a patient's symptoms are located within a joint or are due to disorders of areas around the joint, which is referred to as “periarticular” pain. Examples of these problems may be bursitis, tendonitis, ligament injuries, or skin infections.
Once that situation is resolved, it is important then to move on and see whether there is one joint involved or many joints. These clues all lead to the early diagnosis.
One very important tool is the ability to examine joint fluid (synovial fluid). Sometimes patients with arthritis will have fluid inside a joint that is accessible by needle. This fluid is referred to as an “effusion.”
To help establish a diagnosis, it's critical to examin synovial fluid if it is available. Various stains and other examinations can lead the physicist diagnostically. Not a lot of fluid is needed. Perhaps a few cc's is all that is usually required. The procedure is done using sterile technique. Local anesthetic can be administered to make the aspiration of fluid less uncomfortable. Diagnostic ultrasound to help with needle localization is mandatory.
Normal synovial fluid contains fewer than 200 white blood cells per cubic milliliter.
Higher counts should raise suspicion that infection is present. Effusions where there are more than 100,000 white blood cells / cubic milliliters are almost always due to an infection. On rare occasion “pseudoseptic” reactions from injections of lubricants can do this. Also, crystal induced arthritis such as gout or pseudogout can also cause synovial fluids to have markedly elevated white blood cell counts.
All synovial fluid must be cultured. I learned this as a fellow from my chief. He said, “Every time you get joint fluid, it must be cultured.” And this was at the National Institutes of Health, the forensic biomedical research center in the world.
As mentioned earlier, crystal-induced arthritis such as gout and pseudogout can cause elevated white blood cell counts. Examination of the fluid using polarized microscopy can determine if crystals are present and what kind they are.
The presence of crystals does not exclude infection. Gout sometimes predisposes to infection. All synovial fluid must be cultured!
The presence of fat in joint fluid should raise suspicion for fracture.
So why is the joint fluid so important? Because once the diagnosis is established, treatment becomes much clearer.