CCL13 and arthropathy: Patients with KL4 knee OA have significantly higher levels of CCL13 in serum and SF than those with KL2 and KL3, and patients with KL3 knee OA have significantly higher levels of CCL13 in SF than those with KL2 (The KL grading system is the most commonly used to assess the severity of joint disease in patients with OA, with grade 0 being the normal knee and grade 4 being the most severely affected) (56).