Peripheral Spondyloarthritis
Classification Criteria

Peripheral SpA encompasses SpA subtypes presenting predominantly with peripheral joint, entheseal, or digital manifestations rather than axial (spinal/sacroiliac) symptoms. This includes forms of reactive arthritis, psoriatic arthritis, enteropathic arthritis, and undifferentiated SpA. The 2011 ASAS criteria apply when peripheral features are dominant and axial symptoms are absent or secondary.

Arm A includes 6 features that carry high diagnostic specificity for SpA on their own, so only 1 is required: psoriasis, IBD, preceding infection, HLA-B27, uveitis, and sacroiliitis on imaging. Arm B includes 5 features that are more common but less specific in isolation - arthritis, enthesitis, dactylitis, prior IBP, and family history of SpA - requiring 2 to achieve adequate specificity. A patient may meet both arms simultaneously.

Yes. Dactylitis (and arthritis and enthesitis) appear both as entry criteria and as SpA features in Arm B. A patient who qualifies via dactylitis as an entry criterion can also count dactylitis as one of the 2 features needed for Arm B. This was deliberate in the criteria design - the entry criterion confirms the peripheral phenotype, while the feature in Arm B contributes to the classification threshold.

A documented genitourinary or gastrointestinal infection within 4-6 weeks prior to the onset of peripheral arthritis, enthesitis, or dactylitis. Common pathogens include Chlamydia trachomatis (urogenital), Salmonella, Shigella, Yersinia, and Campylobacter (enteric). This is essentially the reactive arthritis trigger, though the ASAS criteria do not require identification of a specific organism.

Patients with predominant inflammatory back pain should be evaluated with the ASAS Axial SpA criteria, not peripheral SpA criteria. Patients can have overlapping features. If a patient has both inflammatory back pain (onset before age 45, duration ≥3 months) and peripheral manifestations, apply the axial criteria first. The two criteria sets are complementary and designed to classify the entire SpA spectrum without overlap.

In the peripheral SpA criteria, "IBP in the past" counts as a feature even if the patient no longer has back pain as a dominant complaint. IBP is defined by the ASAS 2009 IBP criteria: age at onset <40 years plus at least 4 of 5: insidious onset, improvement with exercise, no improvement with rest, nocturnal pain improving on getting up, alternating buttock pain.

The ASAS peripheral SpA criteria cast a wider net, covering the full spectrum of peripheral SpA (reactive arthritis, enteropathic arthritis, undifferentiated SpA, and psoriatic arthritis). CASPAR criteria are designed specifically for psoriatic arthritis and have higher specificity (98.7%) but require psoriasis or psoriatic nail disease as a core feature. For patients with psoriasis and peripheral arthritis, CASPAR is the preferred tool. For patients without psoriasis, ASAS peripheral criteria are more appropriate.