Among Five First-Trimester Complete Blood Count Inflammatory Indices, the Systemic Inflammation Response Index Rises...

Background/Objectives: Complete blood count inflammatory indices are widely studied in obstetrics but rarely compared directly. We compared five first-trimester indices, the systemic immune-inflammation index (SII), systemic inflammation response index (SIRI), pan-immune-inflammation value (PIV), neutrophil-to-lymphocyte ratio (NLR), and platelet-to-lymphocyte ratio (PLR), for their associations with preterm birth, examined across its severity. Methods: In a retrospective single-centre cohort of 5003 singleton pregnancies with a first-trimester complete blood count, late (34-36 weeks) and early (<34 weeks) preterm births were analysed separately. The association of each index with preterm birth was assessed by logistic regression, per standard deviation, and by quartile, with adjustments made for maternal age, body mass index, parity, fetal sex, and gestational age at sampling; discrimination was compared using the DeLong test. The birthweight category, classified against the INTERGROWTH-21st standards, was a secondary outcome. Results: After adjustment, all indices except for the PLR were associated with preterm delivery, with similar effects per standard deviation. Among the five indices, the SIRI showed the most consistent association with the severity of preterm birth: its values rose at every step from term to late to early birth. It discriminated these births best (area under the curve 0.600, significantly greater than the SII), and its quartile gradient was steepest, with the odds more than doubling from the lowest to the highest quartile. All five remained weak discriminators, with every area under the curve being below 0.70, and none was associated with the birthweight category. Conclusions: These indices are not standalone predictive tests, but among them, the SIRI, which, unlike the SII, captures the monocyte count, was most closely associated with early, more severe preterm birth, consistent with the role of monocytes in the inflammation of labour.