Reference intervals of systemic immune-inflammation index, neutrophil-to-lymphocyte ratio, lymphocyte-to-monocyte ratio, and platelet-to-lymphocyte ratio during normal pregnancy in China.

To observe the changes in systemic immune-inflammation index (SII), neutrophil-to-lymphocyte ratio (NLR), lymphocyte-to-monocyte ratio (LMR), and platelet-to-lymphocyte ratio (PLR) during normal pregnancy and establish appropriate reference intervals (RIs) for healthy pregnant women.This retrospective study was conducted from March 2018 to February 2019. Blood samples were collected from healthy pregnant and nonpregnant women. The complete blood count (CBC) parameters were measured, and SII, NLR, LMR, and PLR were calculated. RIs were established using the 2.5th and 97.5th percentile of the distribution. Besides, the differences in CBC parameters between three pregnant trimesters and maternal ages were also compared to assess their influences on each indicator.SII and NLR in three pregnant trimesters increased in pregnant women, and the upper limit of SII and NLR in trimester 2 showed the highest value. On the contrary, LMR decreased in all three pregnant trimesters compared with nonpregnant women, and the values of LMR and PLR showed a gradual downward trend along with the trimesters. Besides, RIs of SII, NLR, LMR, and PLR during different trimesters in different age partitions showed that the values of SII, NLR, and PLR increased with age in a general trend, while LMR showed the opposite trend (p < 0.05).The SII, NLR, LMR, and PLR showed dynamic changes during pregnant trimesters. RIs of SII, NLR, LMR, and PLR for healthy pregnant women according to pregnant trimesters and maternal age were established and validated in this study, which will promote the standardization of clinical application.