When a patient with arthritis presents to a rheumatologist, the most important thing to establish is a correct diagnosis. A careful history and physical examination are the most important tools. Laboratory tests and diagnostic procedures such as magnetic resonance imaging can also be helpful.
One other important element is the examination of joint fluid. This material, referred to as synovial fluid (SF), is an important constituent of a normal joint. It is a somewhat viscous liquid manufactured by synovial cells, the cells that form the lining of a joint. Under normal circumstances only a small amount of the fluid is produced. The purpose of SF is to provide nutrients for cartilage and also to provide lubrication for the joint.
When evaluating a patient with arthritis, if there is the opportunity to examine SF, it is essential that it be done. The fluid should be evaluated for appearance, total white blood cell count, the types of white blood cells, and also be cultured for bacteria.
Normal SF is slightly yellow and clear- clear enough to read newsprint through. SF that has a lot of white blood cells will be cloudy and turbid. Cloudy synovial fluid is usually due to an inflammatory form of arthritis. When it becomes extremely cloudy, infection should be considered.
Under normal circumstances, the total number of white blood cells in a SF specimen is less than 200 white blood cells per cubic milliliter. If there is infection, the total number of white blood cells will rise dramatically and the type of white blood cell changes. Joint fluid that contains more than 100,000 white blood cells per cubic milliliter is almost always due to infection. In special conditions, if suspected, the fluid should be cultured for fungus or tuberculosis.
Careful examination of the SF for crystals can be rewarding as far as establishing a diagnosis. The most common forms of crystal induced arthritis are gout and pseudogout. Using a polarizing microscope is key to making the diagnosis when crystal-induced arthritis is suspected. The appearance of crystals of monosodium urate in gout is very distinct from the appearance of calcium pyrophosphate, seen in pseudogout. Monosodium urate crystals are needle shaped and calcium pyrophosphate crystals are stubby rod-shaped objects.
One caveat: just because crystals are present doesn't mean there is no infection. Joint fluid should always be cultured.
On rare occasion, fat droplets are seen in SF. This can signify either a fracture or a disorder of the pancreas.
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