A nested case-control study was conducted in very premature infants recruited in a preterm birth cohort from January 1, 2015, to August 31, 2019. Written informed consent was accepted from the guardian while the infants were enrolled in the cohort. The present study is the secondary analysis of the cohort study and the protocol was reviewed and approved by the institutional research ethics committee of the Affiliated Hospital of Southwest Medical University.
Inclusion Criteria:
(1) Gestational age (GA) at birth ≤ 30 weeks, (2) birth weight ≤1500 g, (3) admission to the hospital within 24h after birth, and (4) hospital stay more than 28 days.
Exclusion Criteria:
(1) Severe congenital constructional and/or chromosome malformations, such as congenital heart or lung disease, central nervous system malformation, diaphragmatic hernia, congenital deformity of the head, face or thorax, genetic metabolic disease, or chromosomal disease; (2) hematological disease or pulmonary disease; or (3) death in the first 7 days of life.
A total of 3,242 preterm infants were recruited in the preterm birth cohort, 185 of whom had a GA ≤ 30 weeks and a birth weight ≤ 1500 g, and all were admitted to the neonatal intensive care unit. Infants with congenital malformations (n = 6) or admission exceeding 24 h after birth (n = 19) were excluded. Infants with a hospital stay less than 28 days, including 11 infants who died and 15 infants discharged after recovery, were also excluded. The study subsequently enrolled 134 infants, including 64 infants in the BPD group and 70 infants in the non-BPD group (Figure 1).
Diagnostic Criteria:
According to the diagnostic criteria of the National Institute of Child Health and Human Development (NICHD), BPD was defined as a need for supplemental oxygen for 28 days and was categorized at 36 weeks postmenstrual age (PMA) or 56 days of age for infants > 32 weeks GA as “mild, moderate, or severe” based on oxygen use and/or respiratory support [9]. The patients were subsequently divided into the BPD group and the non-BPD group.
Thrombocytopenia was defined as a PLT count <150⋅109/L confirmed by peripheral blood smears [10]. Severe thrombocytopenia was defined as <50×109/L [11]. As thrombocytopenia in the first several days is more related to early onset infection, thrombocytopenia after the first week of life was used as an outcome in the present analysis.
Data Collection
A uniform questionnaire was used to extract the medical data of the eligible premature infants from the hospital information system. Epidata was used to record data by two researchers independently to ensure the accuracy of the data. Inconsistent data were assessed by a third researcher.
The potential risk factors and confounders were as follows.
(1) Maternal factors: premature rupture of membranes >18 hours, gestational diabetes mellitus, gestational hypertension, antenatal use of glucocorticoids and magnesium sulfate, chorioamnionitis, systemic antibiotics 48 h before delivery, infection during pregnancy, and thrombocytopenia during pregnancy.
(2) Infantile factors: GA, sex, birth weight, Apgar score at one minute and five minutes after birth, mode of delivery, presence of infection at admission, presence of intrauterine growth restriction, pulmonary surfactant replacement therapy, mechanical ventilation support and lasting time, nasal continuous positive airway pressure (nCPAP), noninvasive ventilator support and lasting time, reuse of noninvasive ventilator or invasive ventilator, duration of oxygen supplementation, and maximum oxygen fraction use.
(3) Blood cell counts: Based on the clinical guidelines of our department, the first blood cell counts were measured at patient admission. During hospitalization, blood cell counts were performed weekly. Tests for infection were performed immediately if infection was suspected. The tests were performed with an XS-800i automatic blood cell analyzer. The PLT count, mean platelet volume (MPV), plateletcrit (PCT) level, and platelet distribution width (PDW) data were extracted and used as outcomes in the analysis.
Statistical analysis
Statistical analysis was performed with SPSS 21.0 software. The normality of the continuous data was tested. Normally distributed variables are presented as the mean ± standard deviation (Mean ± SD), and abnormally distributed data are presented as the median and quartile range. Comparisons of the continuous data were made using an independent sample t-test or rank sum test. Categorical data are presented as the frequency and percentage and were compared by means of a chi-square test. Multivariate forward stepwise logistic regression was used to calculate the odds ratio (OR) of thrombocytopenia in the BPD group and adjust for potential confounders (P < 0.15 for inclusion, P> 0.1 for exclusion). The PLT parameters were subsequently compared between the BPD and non-BPD groups at each time point (two-way ANOVA). A generalized estimating equation (GEE) model for repeated measurement data was established to examine the effects of the interaction between BPD and age on the PLT parameters. The GEE model was also used to adjust for infection status or other potential confounding factors. Finally, receiver operating characteristic (ROC) curve analysis was performed to assess the predictive value of PLT count at each time point for BPD. After getting the cut-off value of PLT count, a multivariate ROC curve was used to test whether combining PLT count can improve the predictive value of GA and BW for BPD