Figure 5. Proposed subdivisions within arthritis: PRINTO and the four-cluster model.

A. Provisional PRINTO JIA categories for childhood-onset arthritis⁵⁹. B. Four-cluster model defining mechanistic subgroups within arthritis across the age spectrum (modified from ¹⁷). These two models represent hypotheses with differing methodology and purposes. The PRINTO model reflects an ongoing effort based on consensus methodology to provide preliminary criteria defining clinically homogeneous entities to enable structured validation studies as well as biological research. The four-cluster model integrates clinical and biological data to define groups that exhibit pathophysiologic similarity, irrespective of age of onset. These models exhibit more resemblance than difference, recognizing seropositive arthritis, spondyloarthritis, and systemic/Still’s disease as distinct entities. Early-onset ANA-positive children may or may not form a distinct subset within seronegative arthritis.