Blood chemistries are tests designed to assess electrolytes, liver and kidney function, muscle integrity, uric acid, and other areas. This article discusses why chemistries are an important part of an arthritis evaluation.
Blood chemistries are used in the field of arthritis care for a number of reasons. A blood chemistry panel is most often used by a rheumatologist to assess for drug toxicity. Most medications are metabolized by the liver and excreted (expelled from the body) through the kidneys. The chemistry panel is a good way to monitor for drug toxicity. Side effects from drugs may result in a decrease in serum albumin, an elevation of liver enzymes, or abnormally increased blood urea nitrogen and creatinine which are measures of kidney function.
A good example would be a patient who takes methotrexate or a similar drug for their rheumatoid arthritis and develops abnormally elevated liver enzymes. This could be an indication of potentially significant liver toxicity which would require a reduction in dosage or even discontinuation of the drug altogether.
Non-steroidal anti-inflammatory drugs (NSAIDS) are used to help with arthritis inflammation and pain. On occasion, they can cause kidney damage. In addition, they can cause abnormalities in electrolyte levels. An example would be a patient with diabetes taking an NSAID who develops an elevated blood potassium.
Elevated blood uric acid is useful in the diagnosis of gout. However, the only certain way to establish the diagnosis is to aspirate fluid from an inflamed joint and see crystals of monosodium urate under the microscope. The higher the level of blood uric acid is above the normal limits, the more likely a patient who presents with an acutely inflamed joint has gout. One should keep in mind though that there are many other conditions that can cause joint pain and also be associated with elevations in uric acid including psoriatic arthritis and pseudogout.
In addition, occasional elevations of serum uric acid can also be seen in otherwise healthy people. Many people with elevated blood uric acid never get clinical gout. A normal blood uric acid can also occur during a real gout attack. Once the diagnosis of gout has been established by finding monosodium urate crystals inside an inflamed joint, checking the serum uric acid is a good way to monitor the effectiveness of treatment. In addition, many gout medicines have side effects including toxicity related to the liver and kidneys so that chemistries are useful for toxicity monitoring.
Measures of muscle enzymes such as creatine phosphokinase (CPK) and aldolase can help establish the presence of muscle injury. Diagnosis and monitoring inflammatory muscle disease such as dermatomyositis and polymyositis requires frequent monitoring of these muscle enzymes. As the disease improves, the level of muscle enzymes goes down. Muscle enzymes in the blood can also be elevated due to intramuscular injections, vigorous exercise, and heart attacks. Some medicines such as anti-cholesterol drugs (statins) can lead to muscle enzyme elevation.
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