The most appropriate diagnostic test to perform next is a DNA amplification urine test for Chlamydia trachomatis. This patient has a presentation consistent with reactive arthritis (oligoarticular lower extremity dactylitis, keratoderma blennorrhagicum). Up to 30% of patients with reactive arthritis develop keratoderma blenorrhagicum, a hyperkeratotic rash found on the soles and palms that may be indistinguishable from pustular psoriasis. Chlamydia infection is a common cause of reactive arthritis and is often asymptomatic. Patients with reactive arthritis, even if asymptomatic, should be tested for chlamydia because they may have persistent infection or carriage of this organism. If chlamydia is identified, these patients, as well as their partners, should receive treatment to prevent recurrence or transmission of infection. More studies are needed to determine whether antibiotics are helpful in treating arthritic symptoms. Antibiotics are not indicated for non–chlamydia-related reactive arthritis unless there is documentation of persistent infection because they have not been shown to alter the course of arthritis.
Anti–cyclic citrullinated peptide antibodies are associated with rheumatoid arthritis, which is characterized by a symmetric small joint polyarthritis typically without dactylitis. This patient does not have this typical presentation; therefore, testing for these antibodies is not indicated.
Antinuclear antibodies are associated with systemic lupus erythematosus (SLE); with the exception of arthritis, this patient has no clinical evidence of SLE, including alopecia, aphthous ulcers, pericardial and pleural serositis, kidney disease, rash, and cytopenias, and does not need to be tested for these antibodies.
Although many patients with reactive arthritis are positive for HLA-B27, it will not add any further value to the diagnosis or management. This patient has several typical features to establish a diagnosis of reactive arthritis, and management with anti-inflammatory medication for arthritis will be effective in the presence or absence of HLA-B27.