The present invention provides a method of measuring the levels of APRIL in a biological sample, in a preferred embodiment, in serum. The diagnostic assays are useful in predicting an individual's likelihood of developing or currently suffering from an autoimmune disease, such as RA, predicting the future severity of the disease, and for methods for treating an individual clinically diagnosed with an autoimmune disease. This diagnostic test serves to predict a patient's likelihood to respond to a specific drug treatment, in particular treatment with APRIL antagonists, either singly or in combination with other immune suppressive drugs.
