Background/Objective <p>Obesity is linked to increased risks of cardiovascular disease, diabetes, and certain cancers. Bariatric surgery (BS) aids in weight management, significantly altering body composition. This study evaluates the predictive value of five complete blood count (CBC)-derived indices&#xa0;[C-reactive-protein-albumin-lymphocyte (CALLY), glucose-to-lymphocyte ratio (GLR), prognostic nutritional index (PNI), hemoglobin, albumin, lymphocyte, platelet (HALP), and controlling nutritional status (COUNT)]&#xa0;on body composition changes post-BS.</p> Method <p>A retrospective study was conducted on 240 patients undergoing BS at&#xa0;Sina Hospital,&#xa0;Tehran, Iran. Indices were calculated using routine laboratory tests, and body composition changes were measured using bioelectrical impedance analysis at 3 and 6&#xa0;months post-surgery.&#xa0;</p> Results <p>Higher pre-surgical GLR values positively correlated with increased fat-free mass (FFM) (<i>p</i> = 0.005 ¹, <i>p</i> = 0.003 ²), muscle mass (MM) (<i>p</i> = 0.011 ¹, <i>p</i> = 0.008 ²), and total body water (TBW) (<i>p</i> = 0.005 ¹, <i>p</i> = 0.005 ²) post-surgery.&#xa0;In contrast, higher PNI was negatively associated with changes in FM (<i>p</i> = 0.029 ¹, <i>p</i> = 0.015 ²), FFM (<i>p</i> = 0.002 ¹, <i>p</i> = 0.018 ²), TBW (<i>p</i> = 0.002 ¹, <i>p</i> = 0.015 ²) and MM (<i>p</i> = 0.003 ²), particularly after laparoscopic sleeve gastrectomy (LSG). Furthermore, there was a significant correlation between pre-surgical HALP score and changes in FFM (<i>p</i> = 0.002 ¹, <i>p</i> = 0.042 ²), TBW (<i>p</i> = 0.002 ¹) and MM (<i>p</i> = 0.011 ¹, <i>p</i> = 0.041 ²). In addition, the modified HALP score showed a more significant correlation compared to the HALP score to predict the changes FM (<i>p</i> = 0.002 ¹, <i>p</i> = 0.002 ²), FFM (<i>p</i> = 0.001 ¹, <i>p</i> = 0.006 ²), TBW (<i>p</i> = 0.001 ¹, <i>p</i> = 0.003 ²) and MM (<i>p</i> = 0.001 ¹, <i>p</i> = 0.023 ²) particularly, after 6&#xa0;months.</p> Conclusion <p>Our findings suggest that pre-surgical assessment of GLR, PNI, and HALP indices may provide valuable insights into predicting changes in body composition after bariatric surgery. Specifically, these indices could serve as tools for tailoring preoperative nutritional strategies and post-surgical interventions. However, as this study is retrospective, further prospective research with longer follow-ups is required to validate these findings and evaluate their utility in clinical practice. ¹ 3&#xa0;months after metabolic bariatric surgery. ² 6&#xa0;months after metabolic bariatric surgery.</p> Graphical Abstract <p></p>