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Our four-year old started limping and complaining of hip pain. Blood tests were done and a diagnosis of septic hip arthritis was made. I understand the final diagnosis was made on the basis of a test called C-reactive protein. This is the same test I had done to diagnose heart disease. How does it work to test for both these things?

C-reactive protein is a measure of inflammation. During the acute phase of an infection or injury, the C-reactive protein is elevated in the blood. In fact, the C-reactive protein levels rise in the first six to eight hours after the start of inflammation, injury, or infection.

In the past, doctors relied on another test called the sedimentation (sed) rate. The sed rate is another measure of inflammation or cell death. It is the rate at which red blood cells fall to the bottom of the test tube in one hour. The faster the red cells fall, the higher the sed rate.

Studies show that C-reactive protein is a better measure of inflammation than the sed rate. It is increased in collagen diseases, infections, inflammatory diseases, and cancer. Its use to help diagnose septic hip arthritis has also been confirmed by doctors at The Children's Hospital of Philadelphia (CHOP).

Recently, scientists have found elevated C-reactive protein may be useful as a predictor of heart disease as well. This remains very controversial and under continued study.

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