Novel inflammatory indices compared with CRP for diagnosing pelvic inflammatory disease: a case-control study.

BACKGROUND AND

objectivesPelvic inflammatory disease (PID) is a common upper genital tract infection associated with serious long-term sequelae. Its clinical diagnosis is challenging due to non-specific symptoms, creating an urgent need for accurate, accessible biomarkers. While C-reactive protein (CRP) is commonly used, its specificity is limited. Novel, composite inflammatory indices derived from routine complete blood counts (CBC) have emerged as promising biomarkers in other fields. This study aims to evaluate the diagnostic performance of CBC-derived inflammatory indices against the conventional marker CRP in differentiating PID from non-inflammatory gynecological conditions and to exploratorily assess their association with descriptive ultrasound categories.

methodsIn this retrospective case-control study, 135 PID patients were compared with 135 age-matched non-PID controls. Key hematological indices-including the systemic immune-inflammation index (SII), systemic inflammation response index (SIRI), neutrophil-to-lymphocyte ratio (NLR), monocyte-to-lymphocyte ratio (MLR), and platelet-to-lymphocyte ratio (PLR)-were calculated from routine CBCs obtained at initial clinical presentation. Diagnostic accuracy was evaluated using receiver operating characteristic (ROC) curve analysis, and independent associations were assessed via multivariable logistic regression. As an exploratory analysis, the PID cohort was also subcategorized according to a novel, purely descriptive classification of ultrasound findings to investigate patterns of systemic inflammation across different anatomical presentations.

resultsPID patients exhibited significantly elevated levels of all novel inflammatory indices compared to controls (all p < 0.001). In multivariable analyses adjusted for clinical and laboratory confounders, SII, SIRI, NLR, MLR, and PLR remained strong and independent factors associated with PID (all p < 0.01). ROC analysis revealed that SII possessed the highest diagnostic accuracy (AUC = 0.870, 95% CI: 0.830-0.911), significantly outperforming CRP (AUC = 0.648, p < 0.001) and other single ratios. SIRI demonstrated comparable efficacy to SII (AUC = 0.832). The inflammatory indices showed a distinct pattern of strong intercorrelations specifically within the PID group. In the exploratory ultrasound analysis, the highest median levels of SII, SIRI, and NLR were observed in patients with isolated pelvic effusion, with lower levels seen in groups with findings such as tubo-ovarian abscess.

conclusionsThe CBC-derived indices SII and SIRI showed higher diagnostic accuracy for PID compared to CRP. As inexpensive, routine tests, they represent highly promising tools to aid timely and accurate PID diagnosis in clinical practice. The pattern of their association with different ultrasound phenotypes suggests they may reflect distinct inflammatory states, a hypothesis requiring validation in future studies.