Complete blood count-derived inflammatory indices versus C-reactive protein for identifying complicated appendicitis in children: a retrospective cohort study.

The neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), and systemic immune-inflammation index (SII) are increasingly proposed for identifying complicated appendicitis in children, yet reported cut-offs vary widely and are seldom externally validated. We aimed to test the limits of these indices for identifying complicated appendicitis in a large pediatric cohort, using C-reactive protein (CRP) as the benchmark. This study evaluates the laboratory ceiling of these indices rather than complete diagnostic risk stratification. We retrospectively studied 422 children with histopathologically confirmed appendicitis (2012-2026), of whom 158 (37.4%) had complicated disease defined by intraoperative findings. Discrimination for complicated appendicitis was assessed by the area under the ROC curve (AUC) and compared with CRP (DeLong test). We additionally applied previously published pediatric cut-offs to our cohort, examined discrimination across developmental age strata and after age-normalization, assessed added value over CRP, and tested temporal stability across two consecutive periods. CRP discriminated complicated appendicitis better (AUC 0.812) than NLR (0.641), SII (0.641), or PLR (0.604; all p < 0.001 versus CRP), and the white cell count alone performed as well as the indices. None of five published cut-offs transported to a clinically useful accuracy (45.7-63.5%). In children aged 2-7 years-those with the highest proportion of complicated disease (52.7%)-the indices showed no evidence of discrimination (AUC 0.475-0.495), whereas CRP retained its highest discrimination (0.863); age-normalization did not help. Added to a CRP-based model, NLR and SII contributed statistically but slightly (ΔAUC + 0.018 and + 0.016), while PLR did not. Between the two periods, index values fell significantly despite an unchanged proportion of complicated disease, and an earlier-derived threshold lost approximately ten points of sensitivity in the later period; the CRP threshold instead lost specificity while retaining sensitivity, so no marker was fully stable, although discrimination was preserved for all and the median CRP was unchanged. Conclusion: NLR, PLR, and SII offer limited and unstable value for identifying complicated appendicitis in children. They are consistently outperformed by CRP, discriminate poorly in the youngest children in whom the distinction matters most, and depend on thresholds that transport poorly between settings and over time. These indices are best regarded as continuous adjuncts to CRP rather than standalone tests.