Role of hematological parameters including systemic immune–inflammation index and percentage of immature granulocytes in predicting acute pyelonephritis

Early differentiation of acute pyelonephritis (APN) from lower urinary tract infection (LUTI) and identification of children at risk for renal scarring remain clinical challenges. We aimed to evaluate the diagnostic and prognostic performance of complete blood count (CBC)-derived inflammatory markers, particularly systemic immune-inflammation index (SII) and immature granulocyte percentage (IG%). In this retrospective single-center study (January 2013-December 2023), 265 children with confirmed urinary tract infections (UTIs) were classified as APN (n = 132) or LUTI (n = 133). Patients with prior antibiotic use or conditions affecting inflammatory markers were excluded. CBC-derived indices and IG% were analyzed. Renal ultrasonography was performed in all patients, and Technetium-99m dimercaptosuccinic acid scintigraphy was used during follow-up to assess renal scarring. Diagnostic performance was evaluated using receiver operating characteristic analysis, and independent predictors were assessed with multivariable logistic regression. C-reactive protein (CRP), neutrophil count, SII, IG%, and neutrophil-to-lymphocyte ratio (NLR) were significantly higher in APN compared to those in LUTI (all p < .001). Renal scarring was detected in 48 of 115 (41.7%) APN patients. Patients with renal scarring had higher CRP, SII, IG%, neutrophil count, and NLR, and lower lymphocyte levels. SII showed the highest discriminative performance [area under the curve (AUC) = 0.941, 95% confidence interval: 0.902-0.980) with a cut-off of 750 × 10³/µL (sensitivity 91%, specificity 86%). IG% (AUC = 0.895) and CRP (AUC = 0.891) also performed well. SII [odds ratio (OR) = 2.87] and IG% (OR = 1.42) remained independent predictors. SII and IG% are inexpensive, readily available biomarkers that may improve early risk stratification for renal scarring in children with UTI. Given that these parameters are derived from routine CBC tests at no additional cost, they may offer a practical and scalable approach to risk assessment in low- and middle-income settings where access to advanced imaging is limited.