Rationale: Gout is a disease with growing incidence and complexity due to increased life expectancy, co-morbidity and medication. The disease can be diagnosed by microscopy, demonstrating monosodium uric acid (MSU) in synovial fluid of the affected joint or in tophi (subcutaneous or peritendinous MSU depositions). In daily practice, however, the diagnosis is difficult to ascertain due to sampling error (no synovial fluid acquired because the needle was not exactly placed in the affected joint, or the location of the gout might have been extra-articular e.g. around tendons) or to a different cause of acute arthritis (e.g. infection, reactive arthritis). Recently, Dual Energy CT scan has become available. This technique allows the visualization and quantification of MSU. Although imaging modalities such as DECT show promise in the classification of gout, the studies to date have been small and have primarily involved people with established disease.

A study with cross-sectional design in which patients for whom the clinical questions "does this patient have gout?" are referred for participation may contribute to assess the value of DECT scan in diagnosing acute arthritis caused by gout.

Objective: Assessment of value of DECT scan in diagnosing acute arthritis, caused by gout.

Study design: Prospective Study population: Patients with acute mono or oligo arthritis without prior diagnosis, the rheumatologist has an indication for diagnostic needle aspiration.

Nature and extent of the burden and risks associated with participation, benefit and group relatedness: In current daily practice, patients with acute mono- or oligo-arthritis without prior diagnosis undergo a diagnostic aspiration of the affected joint. This can be done by blind aspiration or ultra sound guided aspiration depending on the judgement of the rheumatologist. The aspirated synovial fluid is then assessed by polarized microscopy to detect MSU crystals. The diagnostic value of DECT in acute gout attacks had not yet been established and is therefore not used in daily practice. In this study all patients undergo DECT scan to assess the value of DECT scan in diagnosing acute arthritis caused by gout. If the DECT scan demonstrates MSU depositions and the diagnosis of gout was not ascertained prior to DECT scanning by MSU crystals in the synovial fluid, then additional ultrasound guided aspiration will take place, with knowledge of DECT results, followed by repeat microscopy

Inclusion Criteria:

• Age > 18 years
• Mono or oligo arthritis (2-3 swollen joints)
• Indication for diagnostic aspiration of an inflamed joint in which gout is one of the possibilities

Exclusion Criteria:

• Polyarthritis ( up to 4 swollen joint);
• Chrystal proven gout in history
• Patient is on uric acid lowering therapy (Allopurinol, Benzbromaron, Febuxostat)
• Hip arthritis*
• Metal or prosthesis of the inflamed joint
• Highly suspicion of infectious arthritis
• Pregnancy
• Contra indication of joint aspiration (skin infection, hemophilia)
• No informed consent