The present invention provides methods for differentially diagnosing rheumatoid arthritis (RA) versus psoriatic arthritis (PsA) in an individual. Specifically, the method relates to detecting the presence or absence of distinct alleles of the PDE4D and PDE4B genes which are associated with either RA or PsA. Also provided herein are methods for selecting an individual who should receive or who is likely to respond to a treatment with a PDE4 inhibitor. The present invention also provides methods for treating an individual with RA or PsA.
