Factors for Long-Term Ritodrine Monotherapy in the Treatment of Preterm Labor: A Single-Center Retrospective Study

Ritodrine is used to manage threatened preterm labor (TPL) in Japan. While short-term administration is recommended internationally, long-term administration remains the customary practice in Japan, creating a gap in clinical practice between Japan and other regions. This retrospective study aimed to identify the maternal factors associated with long-term administration of ritodrine monotherapy. We reviewed the records of patients who received ritodrine infusion for TPL at the Yamaguchi University Hospital between April 2017 and March 2023. Patients were classified into the long-term monotherapy group (n = 37) if they received ritodrine monotherapy for more than 2 weeks. The non-long-term monotherapy group (n = 102) included patients who received ritodrine monotherapy for less than 2 weeks, required additional tocolytic agents, or discontinued ritodrine due to adverse events. The analysis showed that the white blood cell (WBC) count at treatment initiation was significantly lower in the long-term monotherapy group (p < 0.05). A receiver operating characteristic curve analysis identified a WBC count of 8710 as the cut-off value for predicting long-term administration. Multivariate logistic regression analysis identified a WBC count below 8710 at treatment initiation as an independent factor for the long-term administration of ritodrine infusion (odds ratio: 2.57; 95% confidence interval: 1.12-5.90). Although further validation is required, these findings suggest that the WBC count at the initiation of ritodrine infusion may serve as a potential indicator of long-term ritodrine monotherapy administration in pregnant women with TPL.