A biomarker is something that can be measured (relatively) easily which tells us information about the presence, severity or progress of a disease process. It is usually thought of as a chemical that can be measured in blood or urine, but it could be other things. A common biomarker used in rheumatology is blood measurement of CRP (C-reactive protein). The blood concentration increases when there is more inflammation in the body. Most biomarkers provide only a rough indication of the process they are being used to measure, and very few are effective at predicting the future progress of disease.