Why is Reiter syndrome not considered an infectious arthritis of the joint?

Reiter syndrome, also known as reactive arthritis, is not classified as an infectious arthritis because the joint space itself is free of bacteria or other infectious agents.1

While the condition is often triggered by an infection—such as a gastrointestinal illness or a sexually transmitted disease—the arthritis occurs as an indirect result. Rather than a direct invasion of the joint by a pathogen, the syndrome is believed to reflect an aberrant immune response to an infection located elsewhere in the body. In contrast, true infectious (or septic) arthritis involves the direct infiltration of microorganisms into the joint space, which can lead to rapid tissue destruction.2134