Objective: To present platelet large cell ratio (P-LCR), reticulocyte, and immature reticulocyte fraction (IRF) values as novel parameters in diagnosis and response to treatment in patients developing sacroiliitis. Methods: Sixty-eight patients with clinical symptoms and Brucella standard tube agglutination (Wright) or Brucella Coombs agglutination test titers ≥1:160 were included in the study. Two groups were established, one developing sacroiliitis and another with no sacroiliitis development. P-LCR, reticulocyte, and IRF levels were measured using a Sysmex XN-9000 device (Japan). These were then compared between the two groups. Results: Reticulocyte (P=0.037) and IRF (P=0.026) levels were significantly lower among the patients developing sacroiliitis compared to the non-sacroiliitis group, while P-LCR (P=0.003) levels were significantly higher. P-LCR had the most powerful correlation with sacroiliitis development. Significant negative correlation was observed between reticulocyte, IRF levels and sacroiliitis. Conclusions: Elevated P-LCR levels were observed as a marker of persisting inflammation in patients developing sacroiliitis, while low reticulocyte and IRF levels secondary to bone marrow involvement were detected. These three parameters emerged as highly significant markers in terms of diagnosis and reflecting responses to treatment in organ involvement such as sacroiliitis in brucellosis. These are presented as inexpensive, and easily accessible novel parameters.