When it looks like Behçet's syndrome but is something else

To investigate the differential diagnostic spectrum in patients with suspected Behçet's syndrome (BS) in low prevalence regions. In addition, the number of patients fulfilling the ICBD criteria despite not having BS was evaluated. This retrospective analysis was performed in two referral centers for BS. Patients with confirmed BS (clinical diagnosis with fulfilment of ISG criteria or a score of ≥ 5 points in the ICBD criteria) were excluded. The remaining patients were divided into eleven differential diagnosis categories. If no definitive alternative diagnosis could be established, patients were termed 'probable BS' in case of (1) relapsing orogenital aphthosis in the absence of other causes and either HLA-B51 positivity, origin from an endemic area or presence of an additional typical BS symptom that is not part of the classification criteria or (2) with 3-4 points scored in the ICBD criteria. In total 202 patients were included and categorized as follows: 58 patients (28.7%) as 'probable BS', 57 (28.2%) skin disease, 26 (12.9%) chronic pain syndrome, 14 (6.9%) eye disease, 11 (5.4%) spondyloarthropathy, 9 (4.5%) gastrointestinal disease, 7 (3.5%) neurological disease, 4 (2%) arthritis, 3 (1.5%) auto-inflammation, 3 (1.5%) connective tissue disease, 10 (5.0%) miscellaneous disease. HLA-B51 was positive in 55/132 (41.6%); 75/202 (37.1%) of the patients fulfilled the ICBD criteria. In a low disease prevalence setting the straightforward application of the ICBD criteria may lead to overdiagnosis of BS. The differential diagnosis of BS is enormously broad. Clinicians should be aware that HLA-B51 positivity is still not considered as a diagnostic feature in BS.