SPP1 and systemic lupus erythematosus: To assess whether OPN N-half can be used as a marker to identify the site of damage to the nephron, we examined the correlation of urine OPN N-half with markers of proximal tubule damage (ALB, B2M, cystatin C, KIM-1, and NGAL), distal tubule damage (calbindin and GST-π), and general inflammation (IL-18 and MCP-1) in the SLE patients.