The most appropriate diagnostic test to perform next is HLA-B27 testing. The patient has uveitis and enthesitis at the Achilles tendon, which are suggestive of peripheral spondyloarthritis. The Assessment of SpondyloArthritis international Society classification criteria for peripheral and axial spondyloarthritis are primarily used for research purposes, although they include many of the common symptoms, signs, and tests that are useful in diagnosing these disorders. Classification criteria include several other manifestations, including psoriasis, inflammatory bowel disease, preceding infection, and sacroiliitis, which are absent in this patient. HLA-B27 is included in the criteria and, in the absence of other sufficient manifestations, can be helpful in supporting the diagnosis of peripheral or axial spondyloarthritis. HLA-B27 testing has limitations because of its approximately 5% prevalence in the general population, which can lead to false positives in diagnosis. Therefore, HLA-B27 is not a useful test in a patient in whom clinical suspicion for spondyloarthritis is low (for example, a 65-year-old patient with noninflammatory or mechanical back pain). Furthermore, it does not add anything in the setting of a high suspicion for spondyloarthritis when there are sufficient other findings to establish a diagnosis (for example, a 35-year-old man with a 12-month history of anterior uveitis, chronic inflammatory back pain, and radiographic evidence of sacroiliitis).

Testing for anti–cyclic citrullinated peptide antibodies can be useful in patients with suspected rheumatoid arthritis, which is characterized by an inflammatory polyarthritis of small joints. Rheumatoid arthritis is unlikely in this patient because he only has enthesitis, an uncommon presentation of rheumatoid arthritis.

The ANCA-associated vasculitides include granulomatosis with polyangiitis (formerly known as Wegener granulomatosis), microscopic polyangiitis, and eosinophilic granulomatosis with polyangiitis (formerly known as Churg-Strauss syndrome). Granulomatosis with polyangiitis is a systemic necrotizing vasculitis that predominantly affects the upper and lower respiratory tract and kidneys. More than 70% of patients have upper airway manifestations such as sinusitis or nasal, inner ear, or laryngotracheal inflammation. Microscopic polyangiitis is a necrotizing vasculitis that predominantly affects the lungs and kidneys. Eosinophilic granulomatosis with polyangiitis is an eosinophil-rich necrotizing vasculitis predominantly affecting the respiratory tract and other major organs. This patient does not have clinical evidence of ANCA-associated vasculitis; therefore, testing for ANCA is not indicated.

Antinuclear antibody testing is useful for patients with suspected systemic lupus erythematosus (SLE); however, SLE does not usually cause enthesitis or uveitis as seen in this patient.