Relationship between hematological parameters and bronchopulmonary dysplasia in premature infants

Abstract

Objective

To evaluate the association between hematological parameters on the first day of life and bronchopulmonary dysplasia (BPD) in preterm infants.

Methods

This retrospective study involved all premature infants admitted to our neonatal intensive care unit from January 2017 to June 2022. BPD was diagnosed based on hypoxia exposure for ≥28 days. The neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), platelet count (PLT), mean platelet volume (MPV), and platelet mass index (PMI) were compared between infants with and without BPD. Multivariate analysis was conducted to evaluate the association between hematological parameters and BPD.

Results

This study involved 124 premature infants (48 with BPD, 76 without BPD). The BPD group had a lower gestational age and lower weight. The NLR, MPV, and PLR were considerably higher and the PLT and MPI were lower in the BPD than non-BPD group. After adjusting for covariates, logistic regression analysis suggested that the NLR, PLT, and PMI were independent risk factors for BPD. Moreover, the receiver operating characteristic curve indicated that the NLR, PLT, and PMI were reliable predictors of BPD.

Conclusion

Our findings suggest that a higher NLR and a lower PLT and PMI on the first day may increase the risk of BPD.

Keywords

Neutrophil-to-lymphocyte ratio bronchopulmonary dysplasia platelet count platelet mass index hematological parameter premature infant

Introduction

Bronchopulmonary dysplasia (BPD) is the most common chronic lung disease in preterm infants.¹ Because of advances in perinatal medicine and the use of pulmonary surfactants and ventilators in recent years, the birth and survival rates of very low birth weight infants have significantly increased. However, the incidence of BPD has been increasing year by year,² seriously impacting the survival and prognosis of preterm infants. Effective measures to prevent and treat BPD are still lacking.^3,4 Some studies have indicated that the feeding of breast milk is a promising measure to prevent BPD.⁵ Bioactive compounds such as antioxidants, short-chain fatty acids, and breast milk-derived exosomes play key roles in this prevention.^6–8 Therefore, identifying markers to predict the risk of BPD and exploring the pathogenesis of BPD can help in the clinical prevention, diagnosis, and treatment of BPD at an earlier stage, thus improving the prognosis and quality of survival of preterm infants. Hematological parameters are widely used in clinical practice, and their measurement is a routine examination for all patients at hospital admission. Measurement of hematological parameters has received growing attention in recent years because of its advantages of being easily accessible and requiring no additional trauma to the child. Various blood parameters play significant roles in pulmonary inflammation and associated lung injury in preterm infants.⁹ Yang et al.¹⁰ reported that the eosinophil percentage was significantly higher in patients with than without BPD from the first postnatal week. However, the association between hematologic parameters on the first day of life and the risk of BPD in premature infants remains unclear. Therefore, this study was performed to investigate clinical hematologic parameters on the first day of life and their association with BPD in a cohort of preterm infants to provide early help in the diagnosis and treatment of BPD and thus improve the prognosis.

Patients and methods

Patients

This retrospective study involved premature infants with a gestational age of <32 weeks who were admitted to our hospital’s neonatal medicine unit from January 2017 to June 2022. The exclusion criteria were (i) admission at >1 day of age, (ii) death or abandonment of treatment within 4 weeks of birth, (iii) infants born with necrotizing enterocolitis (NEC) and hematologic diseases, (iv) congenital anomalies or genetic metabolic abnormalities, and (v) neonatal sepsis and severe asphyxia. The diagnostic criterion for BPD, namely the need for oxygenation for ≥28 days, was based on the diagnostic and grading criteria for BPD developed by the National Institute of Child Health and Human Development in 2000.¹¹ Children of <32 weeks of gestational age were assessed at 36 weeks of corrected gestational age. This study was approved by the medical ethics committee of the Linping Branch of the Second Affiliated Hospital of Zhejiang University (Approval number: LP-IRB2017051270). Because of the retrospective nature of this study, the requirement for informed consent was waived. The reporting of this study followed the relevant EQUATOR guidelines,¹² and all patient details have been de-identified.

Data collection

The following data were gathered from the hospital’s electronic medical record system: maternal age, gestational diabetes mellitus, small for gestational age, delivery method, gestational hypertension, Apgar score, sex, neonatal respiratory distress syndrome, retinopathy of prematurity, birth weight, intraventricular hemorrhage, surfactant treatment, patent ductus arteriosus (PDA), and NEC. Whole blood was collected for testing within the first 24 hours of life. The neutrophil-to-lymphocyte ratio (NLR) was calculated as the neutrophil count divided by the lymphocyte count, and the platelet-to-lymphocyte ratio (PLR) was calculated as the platelet count (PLT) divided by the lymphocyte count. The platelet mass index (PMI) was equal to the PLT times the mean platelet volume. Blood tests were conducted using samples taken from the newborns’ umbilical veins within 24 hours of birth.

Statistical analysis

Statistical analyses were performed using SPSS for Windows Version 26 (IBM Corp., Armonk, NY, USA). Statistical data are expressed as n (%), and the χ² test or Fisher’s exact test was used for comparison between groups. Normally distributed measures are expressed as mean ± standard deviation, and the t-test for two independent samples was used for comparison between groups. Non-normally distributed measures are expressed as median (interquartile range), and the Kruskal–Wallis rank sum test was used for comparison between groups. In the multifactor logistic regression model, the risk factors with statistical significance in the univariate analysis were selected as covariates, and BPD and non-BPD subgroups were used as dependent variables to analyze the independent risk factors for BPD. A receiver operating characteristic curve (ROC) was used to analyze the predictive value of the NLR, PLT, and PMI on the first day of life for BPD; to determine the best cut-off point; and to calculate the sensitivity and specificity of the NLR, PLT, and PMI for predicting BPD. Differences were considered statistically significant at P < 0.05.

Results

In total, 124 premature infants with a gestational age of <32 weeks were enrolled during the study period. Of these infants, 48 were diagnosed with BPD whereas 76 did not have BPD (Table 1). The basic clinical characteristics of the premature infants in the BPD group and non-BPD group are summarized in Table 1. Univariable analysis showed that the BPD group had a higher rate of PDA (45.8% vs. 11.8%, P < 0.05) (Table 1). In addition, infants with BPD had a lower gestational age (28.7 vs. 30.6 weeks, P < 0.05) and birth weight (1222 vs. 1684 g, P < 0.05) (Table 1). However, there were no statistically significant differences in terms of sex, small for gestational age, Apgar score, retinopathy of prematurity, neonatal respiratory distress syndrome, intraventricular hemorrhage, NEC, or gestational diabetes mellitus.

Table 1.

Clinical characteristics by BPD status.

	BPD (n = 48)	No BPD (n = 76)	P value
Gestational age, weeks	28.7 ± 1.9	30.6 ± 1.4	<0.05
Birth weight, g	1222 ± 283	1684 ± 271	<0.05
Male sex	23 (48.0)	43 (56.6)	0.37
Vaginal delivery	22 (45.8)	39 (51.3)	0.35
SGA	4 (8.3)	13 (17.1)	0.17
1-minute Apgar score	8 (6–9)	8 (7–9)	0.14
5-minute Apgar score	9 (8–10)	9 (8–10)	0.06
ROP	6 (12.5)	5 (6.6)	0.11
PDA	22 (45.8)	9 (11.8)	<0.05
NRDS	32 (66.6)	37 (48.7)	0.21
Pulmonary arterial hypertension	5 (10.5)	3 (3.9)	0.28
IVH grade 3 or 4	9 (18.8)	6 (7.9)	0.42
Maternal age	30.1 ± 4.0	29.9 ± 5.0	0.79
GDM	13 (27.1)	22 (28.9)	0.82
Gestational hypertension	5 (10.4)	15 (19.7)	0.17
Premature rupture of membranes	25 (52.1)	43 (56.6)	0.62
NEC	4 (8.3)	3 (3.9)	0.15

Data are presented as mean ± standard deviation, n (%), or median (interquartile range).

BPD, bronchopulmonary dysplasia; SGA, small for gestational age; ROP, retinopathy of prematurity; PDA, patent ductus arteriosus; NRDS, neonatal respiratory distress syndrome; IVH, intraventricular hemorrhage; GDM, gestational diabetes mellitus; NEC, necrotizing enterocolitis.

The hematological parameters on the first day of life are compared between infants with and without BPD in Table 2. The NLR was significantly higher in infants with than without BPD (1.2 ± 0.9 vs. 0.76 ± 0.7, P < 0.05). The PLT and PMI were significantly lower in infants with than without BPD (213.4 ± 38.8 vs. 258.9 ± 43.2 ×10⁹/L, P < 0.05 and 2119 ± 410 vs. 2358 ± 405, P < 0.05, respectively).

Table 2.

Patients’ laboratory measurements.

	BPD (n = 48)	No BPD (n = 76)	P value
PLT, ×10⁹/L	213.4 ± 38.8	258.9 ± 43.2	<0.05
MPV, fL	9.9 ± 0.6	9.1 ± 0.5	<0.05
PDW, %	16.7 ± 0.4	16.5 ± 0.4	0.45
WBC count, ×10⁹/L	9.6 ± 2.8	10.2 ± 3.7	0.32
Neutrophil count, ×10⁹/L	4.7 ± 3.3	5.1 ± 3.6	0.11
Lymphocyte count, ×10⁹/L	4.2 ± 2.1	6.1 ± 3.3	0.45
NLR	1.2 ± 0.9	0.76 ± 0.7	<0.05
PLR	51.2 ± 18.6	42.9 ± 22.8	<0.05
PMI	2119 ± 410	2358 ± 405	<0.05

Data are presented as mean ± standard deviation.

BPD, bronchopulmonary dysplasia; PLT, platelet count; MPV, mean platelet volume; PDW, platelet distribution width; WBC, white blood cell; NLR, neutrophil-to-lymphocyte ratio; PLR, platelet-to-lymphocyte ratio; PMI, platelet mass index.

The logistic regression analysis showed that the NLR (odds ratio (OR), 1.15; 95% confidence interval (CI), 1.04–1.28; P < 0.05), PLT (OR, 0.65; 95% CI, 0.47–0.89; P < 0.05), PMI (OR, 0.90; 95% CI, 0.81–1.0; P < 0.05), gestational age, and birth weight were independent risk factors for BPD (Table 3). The optimal cut-off value of the NLR was 1.3, with 68.5% sensitivity, 72.2% specificity, and an area under the ROC curve of 0.79 (95% CI, 0.72–0.86). The optimal cut-off value of the PLT was 212 × 10⁹/L, with 64.2% sensitivity, 66.5% specificity, and an area under the ROC curve of 0.69 (95% CI, 0.59–0.78). The optimal cut-off value of the PMI was 2135, with 73.2% sensitivity, 76.3% specificity, and an area under the ROC curve of 0.82 (95% CI, 0.74–0.89). Combination of the PMI, NLR, and PLT showed an area under the curve of 0.86 (Figure 1).

Table 3.

Logistic regression analysis showing independent predictors of bronchopulmonary dysplasia.

Factors	β	S_b	Wald	OR	95% CI	P value
Gestational age	−0.26	0.36	0.54	0.77	0.48–0.97	<0.05
Birth weight	−0.07	0.12	0.37	0.93	0.88–0.98	0.04
MPV	0.96	0.26	13.27	2.62	1.56–4.40	0.34
PDA	1.09	0.31	12.76	2.97	1.64–5.40	0.16
NLR	0.14	0.17	0.69	1.15	1.04–1.28	<0.05
PLR	0.80	0.24	10.81	1.33	1.14–1.55	0.26
PMI	−0.11	0.05	3.87	0.90	0.81–1.00	<0.05
PLT	−0.44	0.16	7.39	0.65	0.47–0.89	<0.05

OR, odds ratio; CI, confidence interval; MPV, mean platelet volume; PDA, patent ductus arteriosus; NLR, neutrophil-to-lymphocyte ratio; PLR, platelet-to-lymphocyte ratio; PMI, platelet mass index; PLT, platelet count.

Figure 1.

Receiver operating characteristic curve of hematological parameters.

Discussion

In recent years, the incidence of BPD has increased secondary to the increased morbidity and survival of very early preterm infants. Previous studies have shown that gestational age, birth weight, and PDA are risk factors for BPD,¹³ but the use of hematological parameters to predict the risk of BPD is less commonly reported. In this study, we analyzed the relationship between hematological parameters and BPD in preterm infants on the first day of life and found that the NLR, PMI, and PLT on the first day of life were independent risk factors for BPD and that the NLR, PMI, and PLR after birth were valuable in predicting the development of BPD.

Elevated levels of inflammatory cytokines have recently been found to be associated with BPD.^14,15 Premature infants who are exposed to hyperoxia or intrauterine infection, require mechanical ventilation, and develop BPD have visible infiltration of inflammatory cells and inflammatory factors in their lungs. Within such infiltrations, neutrophils and macrophages play a key role in systemic inflammation in the lung and extrapulmonary tissues.¹⁶ Compared with children who do not have BPD, children with BPD have a higher concentration and greater persistence of inflammatory cells in the bronchoalveolar lavage fluid.¹⁷ These previous findings demonstrate a link between inflammatory responses and the pathogenesis of BPD.

Inflammation is usually accompanied by relative changes in the absolute values of circulating peripheral blood leukocyte subpopulations such as neutrophils and lymphocytes. In recent years, leukocyte and leukocyte subpopulation counts have been used as markers of the degree of inflammation in several diseases, such as acute appendicitis,¹⁸ allergic rhinitis,¹⁹ chronic obstructive pulmonary disease,²⁰ and acute pulmonary embolism.²¹ Neutrophils are important cells in the immune defense system and regulate the functions of mast cells, epithelial cells, and macro cells.²² The NLR is a marker of inflammation and is used in conjunction with other inflammatory markers to determine the severity of the clinical condition in patients with certain inflammatory diseases. In one study, the NLR was significantly higher in patients with severe pneumonia and acute respiratory distress syndrome than in controls.²³ The present study showed that the NLR was higher in infants with than without BPD at birth. We confirmed that the NLR has the capacity to predict BPD.

In recent years, research has shown that alveolation and abnormal pulmonary vascular development play an important role in the occurrence and development of BPD.²⁴ Many cell growth factors are involved in the development of alveoli and pulmonary blood vessels, such as platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), and transforming growth factor. These growth factors play an important role in lung development and can promote the generation of alveoli and pulmonary blood vessels.²⁵ Platelets play a role in repairing damaged blood vessels. When the vascular endothelium is damaged, subcutaneous matrix components such as von Willebrand factor are exposed. Platelets bind with von Willebrand factor to activate platelets, and the activated platelets release substances in internal particles (including adhesive proteins, growth factors, and procoagulation factors) to promote platelet aggregation. The aggregated platelets participate in the repair of damaged blood vessels and promote the formation of lung blood vessels. In this study, the PLT on the first day of life was significantly lower in infants with than without BPD, which was consistent with the results of Chen et al.²⁶ This suggests that the risk of thrombocytopenia is higher in children with than without BPD. Additionally, it was speculated that decreases in secreted growth factors might be caused by thrombocytopenia. Effects on alveolation and pulmonary vascular development were further confirmed in a study by Oak and Hilgendorff,²⁷ who found that PDGF, VEGF, transforming growth factor, and other factors were reduced in children with BPD and that these reductions were associated with abnormal pulmonary vascular development.

Compared with the PLT alone, the PMI uses both the PLT and mean platelet volume, allowing it to more accurately reflect platelet function. Okur et al.²⁸ reported that preterm infants with ventricular hemorrhage, retinopathy of prematurity, neonatal NEC, and BPD had a lower PMI than preterm infants without these diseases. Studies have also shown that the growth factors of blood lamella, such as VEGF and PDGF, increase as the PMI increases.^29,30 The present study also showed that the PMI was lower in infants with than without BPD, and the clinical value of the PMI in predicting the occurrence of BPD was higher than that of the PLT. We speculate that children with BPD who have a low PMI also have low levels of VEGF and other cell growth factors, which are related to the occurrence and development of alveolation and abnormal pulmonary vascular development. Therefore, a low PMI may promote the occurrence of BPD.

The main limitation of this study is the small patient population. Furthermore, because this was a retrospective observational study, it was challenging to ensure consistency of the clinical data because not all patients were treated by the same physicians. Finally, this study only included Chinese patients, which prevents us from assessing the impact of ethnic diversity. Multicenter, large-sample studies are needed to increase the level of evidence for the relationship of BPD with the NLR, PMI, and PLT.

In conclusion, as new inflammatory markers, the NLR, PLT, and PMI have the advantages of being simple, economical, and rapid; having a certain sensitivity and specificity for predicting the occurrence of BPD; and having important clinical application value.

Footnotes

Author contributions

J. Jiang wrote the main manuscript text. Y. Mao prepared the figure. J. Wu collected the information from the medical records. Q. Zhou prepared the tables and searched the literature. All authors reviewed the manuscript.

Declaration of conflicting interest

The authors declare that there is no conflict of interest.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

ORCID iD

Junsheng Jiang

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