C-Reactive Protein Preferred Test for Septic Joints

A CRP test measures the amount of C-reactive protein in the blood. CRP measures general levels of inflammation in the body. High levels of CRP are caused by infections and many chronic diseases. The sedimentation rate blood test measures how quickly red blood cells settle in a test tube in one hour. The more red cells that fall to the bottom of the test tube in one hour, the higher the ESR.

When inflammation is present in the body, certain proteins cause erythrocytes to stick together and fall more quickly than normal to the bottom of the tube. These proteins are produced by the liver and the immune system under many abnormal conditions, such as an infection, an autoimmune disease, or cancer.

However, neither test is definitive enough to predict where in the body inflammation is occurring. In a study carried out at the University of New Mexico (Albuquerque, NM, USA), CRP and ESR tests were carried out on 163 patients, of whom 72 had inflammatory joints, 44 had septic joints, and 47 were normal. Fifteen admitted to drug use and 43 to alcohol consumption and there were 120 males and 43 females. Sensitivities, specificities, positive and negative predictive values were obtained using the laboratory's positive cutoffs of 15 mm/h for ESR and 0.8 mg/L for CRP. The mean CRP for septic joints was 13 mg/L, 8.5 mg/L for inflammatory joints, and 6 mg/L for normal. The mean ESR for septic joints was 57 mm/h, 48 mm/h for inflammatory joints, and 43 mm/h for normal joints.

By univariate analysis, drug use and elevated CRPs were significantly associated with septic joints while alcohol use, ESRs, and gender were not. A regression model with four variables indicated that drug use and CRP were predictive of septic joint; alcohol and ESR were not. The authors concluded that CRP is helpful in determining the presence of a septic joint, but ESR is not. Their findings were published in the June 2010 issue of the Southern Medical Journal.

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University of New Mexico