Urinary Nephrin Concentrations in Preeclampsia (with and without Complications) vs Normal Pregnancies

Introduction

Preeclampsia is a complex hypertensive disorder of pregnancy, typically occurring after 20 weeks of gestation, and is characterized by new-onset hypertension with proteinuria or evidence of systemic involvement such as thrombocytopenia, renal insufficiency, impaired liver function, pulmonary edema, or cerebral/visual disturbances [1]. It remains a leading cause of maternal and perinatal morbidity and mortality worldwide, particularly when it presents as early-onset preeclampsia (<34 weeks of gestation), which often leads to severe complications, including fetal growth restriction, placental abruption, and preterm delivery [1].

Pathophysiologically, preeclampsia is believed to originate from inadequate trophoblast invasion and remodeling of the uterine spiral arteries. This results in poor placental perfusion and the release of anti-angiogenic factors such as soluble fms-like tyrosine kinase-1 (sFlt-1), which antagonizes vascular endothelial growth factor (VEGF) and placental growth factor (PlGF), causing systemic endothelial dysfunction [1–4]. In the kidneys, this dysfunction manifests as glomerular endotheliosis, a hallmark lesion of preeclampsia characterized by endothelial swelling, loss of fenestrations, and narrowing of capillary lumina, which collectively disrupt the glomerular filtration barrier and contribute to proteinuria [2,3,5].

One molecular consequence of this glomerular injury is the loss of nephrin, a crucial podocyte-specific transmembrane protein located at the slit diaphragm of glomerular podocytes. Nephrin plays a key role in maintaining the integrity of the filtration barrier, and its shedding into the urine – termed nephrinuria – reflects podocyte injury or detachment [6–9]. In the setting of preeclampsia, reduced nephrin expression and podocyte effacement can result in podocyturia and increased urinary nephrin excretion, both of which are associated with proteinuria and can precede clinical symptoms [8–10].

Several studies have explored the diagnostic potential of urinary nephrin as a non-invasive biomarker for early glomerular injury in pregnancy, particularly in women at risk of preeclampsia. Kandasamy et al highlighted the utility of nephrin as a sensitive biomarker of early glomerular injury before the onset of overt proteinuria [6]. In a subsequent study, Mesfine et al demonstrated elevated urinary nephrin levels in pregnant women with suspected renal impairment, supporting its potential role in routine antenatal screening for kidney-related complications in pregnancy [7].

Comparative studies also reinforce these findings. Oluwole et al reported significantly higher urinary nephrin levels in Nigerian women with preeclampsia compared to normotensive pregnant controls, underlining the biomarker’s potential utility across populations [11–13]. Similarly, Jung et al showed that both serum and urinary nephrin concentrations were elevated in women who developed preeclampsia compared to those with uncomplicated pregnancies [14].

Despite these insights, there remains a knowledge gap in differentiating urinary nephrin levels among women with preeclampsia with and without complications. Most studies have focused on comparing preeclamptic patients with healthy pregnancies but did not stratify preeclampsia by severity or presence of complications. To address this gap, this study aimed to evaluate urinary nephrin concentrations in preeclampsia cases with and without complications and normal pregnancies. This approach may provide further evidence for the role of nephrinuria, not only in early detection, but also in stratifying disease severity in preeclamptic patients.

Material and Methods

PARTICIPANTS:

The study used a cross-sectional design to measure urinary nephrin in 120 pregnant women: 40 with uncomplicated preeclampsia, 40 with complications, and 40 with normal pregnancies. Inclusion criteria for the uncomplicated preeclampsia group required a gestational age of 20–34 weeks, determined by either the first day of the last menstrual period (LMP) or first-trimester ultrasound confirmation. Inclusion criteria for the preeclampsia with complications group included the presence of severe manifestations, such as hemolysis, elevated liver enzyme, low platelet (HELLP) syndrome, impending eclampsia, eclampsia, pulmonary edema, or other complications. Pregnant women with a history of preeclampsia, chronic hypertension, diabetes mellitus, lupus nephritis, or pre-existing renal dysfunction were excluded. Ethics clearance was obtained from the Health Research Ethics Committee of Hasan Sadikin Hospital (Number: DP.04.03/D.XIV.6.5/222/2024).

PROCEDURES:

A 10-mL urine sample was collected from each participant and delivered for analysis within 1–2 hours of collection. Samples were centrifuged for 20 minutes, transferred into storage tubes, and frozen at −20°C until analysis. Urinary nephrin concentrations were measured using the enzyme-linked immunosorbent assay (ELISA) method, with a sensitivity of 0.09 ng/mL and a detection range of 0.16–10 ng/mL (Elabscience). Results are expressed in ng/mL. The ELISA kits were sourced from, and assays were performed according to, the manufacturer’s protocol. Quality control samples with known concentrations were included in each assay batch to ensure accuracy and consistency. Validation of the assay was performed by assessing intra-assay and inter-assay coefficients of variation, which were found to be within acceptable limits (<10%).

STATISTICAL ANALYSIS:

Data were first assessed for normality using the Shapiro-Wilk test. Variables that demonstrated a normal distribution were analyzed using one-way analysis of variance (ANOVA) to compare mean differences among the 3 study groups: normal pregnancy, uncomplicated preeclampsia, and complicated preeclampsia. For variables that did not follow a normal distribution, the Kruskal-Wallis test, a non-parametric alternative to ANOVA, was employed. Post hoc comparisons were conducted using Bonferroni correction for ANOVA or Dunn’s test for Kruskal-Wallis when significant differences were identified. A P value <0.05 was considered statistically significant.

All data were recorded systematically on standardized data collection forms to ensure consistency and minimize data entry errors. Subsequent statistical analyses were performed using IBM SPSS Statistics version 26.0 for Windows (IBM Corp., Armonk, NY, USA). Descriptive statistics were used to summarize participant characteristics, with means and standard deviations reported for normally distributed variables and medians with interquartile ranges for non-normally distributed variables.

Results

SUBJECT CHARACTERISTICS:

Table 1 compares the demographic characteristics of participants across the 3 groups: normal pregnancy, preeclampsia without complications, and preeclampsia with complications. Variables included maternal age and gestational age. Maternal age is presented as mean±standard deviation, median, and range.

The mean maternal ages were similar across groups: 29.58±6.205 in normal pregnancy, 29.95±6.695 in preeclampsia without complications, and 29.98±6.265 years for preeclampsia with complications (P=0.952). Gestational ages ranged from 20 to 40 weeks, with preeclampsia groups predominantly in the 28–34 weeks range. Gestational age was categorized into 3 ranges (20–27 weeks, 28–34 weeks, and 35–40 weeks), with percentages provided for each group. A significant difference in gestational age distribution among the groups was observed (P=0.0001), while maternal age differences are not statistically significant (P=0.952).

URINARY NEPHRIN PROFILES:

Table 2 presents urinary nephrin concentrations across the 3 groups: normal pregnancy, preeclampsia without complications, and preeclampsia with complications. Data are shown as mean ± standard deviation, median, and range. A significant increase in urinary nephrin concentrations was evident in preeclampsia groups compared to normal pregnancy, with preeclampsia with complications exhibiting the highest concentrations. Urinary nephrin concentrations were significantly higher in preeclampsia (4.59±1.051 without complications and 9.12±1.225 with complications) compared to normal pregnancy (1.11±0.356; P<0.0001). The differences were statistically significant.

Table 3 compares urinary nephrin concentrations between 2 groups: preeclampsia without complications and preeclampsia with complications. Data are presented as mean±standard deviation, median, and range. The results show significantly higher urinary nephrin concentrations in the preeclampsia with complications group compared to the group without complications (P=0.0001).

Figure 1 illustrates the urinary nephrin concentrations across the 3 groups: normal pregnancy, preeclampsia without complications, and preeclampsia with complications. The graph visually demonstrates the progressive increase in nephrin levels, with normal pregnancy showing the lowest concentrations and preeclampsia with complications the highest, emphasizing the significant differences among the groups.

Discussion

In this study, the mean urinary nephrin concentration among the 40 women with normal pregnancies was 1.11±0.356 ng/mL, with a median of 1.01 and a range of 0.07–1.99 ng/mL. Urinary nephrin concentrations were significantly higher in preeclampsia (4.59±1.051 without complications and 9.12±1.225 with complications) compared to normal pregnancy (1.11±0.356; P<0.0001). Elevated urinary nephrin levels in uncomplicated preeclampsia suggest early glomerular injury, even in the absence of severe clinical symptoms or complications.

These low concentrations likely reflect optimal glomerular function and an intact filtration barrier. The detected nephrin may result from normal cellular turnover rather than pathological leakage or damage. This aligns with findings by Kostovska et al (2021), who reported no significant association between elevated urinary nephrin concentrations and increased glomerular filtration rate (GFR) [15–17]. Pathophysiologically, preeclampsia is believed to originate from inadequate trophoblast invasion and remodeling of the uterine spiral arteries. This results in poor placental perfusion and the release of anti-angiogenic factors such as soluble fms-like tyrosine kinase-1 (sFlt-1), which antagonizes vascular endothelial growth factor (VEGF) and placental growth factor (PlGF), causing systemic endothelial dysfunction [1–4]. In the kidneys, this endothelial dysfunction manifests as glomerular endotheliosis, a hallmark lesion characterized by endothelial cell swelling, loss of fenestrations, and narrowing of glomerular capillary lumina, leading to impaired filtration and subsequent proteinuria [2,3].

One key molecular event in this process is the loss of nephrin, a podocyte-specific transmembrane protein essential for the integrity of the slit diaphragm in the glomerular filtration barrier. Under pathological conditions such as preeclampsia, nephrin expression is reduced, and damaged podocytes may detach from the glomerular basement membrane (GBM), leading to nephrinuria and podocyturia [5–7]. Importantly, nephrinuria can precede the clinical onset of proteinuria, suggesting its potential utility as an early biomarker for glomerular injury [6–8].

Several studies have explored the potential of urinary nephrin as a non-invasive biomarker for early detection of renal injury in pregnancy. Kandasamy et al reported the clinical significance of nephrinuria in identifying early glomerular dysfunction [5], while Mesfine et al demonstrated elevated urinary nephrin levels in pregnant women undergoing routine antenatal care, suggesting its broader applicability in clinical settings [6]. Previous studies also reported that higher urinary nephrin levels in women with preeclampsia compared to healthy pregnancies reflect glomerular injury severity. Oluwole et al observed significantly elevated urinary nephrin levels among Nigerian women with preeclampsia, correlating nephrinuria with glomerular injury and proteinuria development [11]. Similarly, Jung et al showed increased serum and urinary nephrin levels in women who subsequently developed preeclampsia compared to those with normal pregnancies [12].

In the present study, mean urinary nephrin concentrations were notably higher in complicated preeclampsia cases (9.12 ng/mL) compared to uncomplicated preeclampsia (4.59 ng/mL). This suggests that even in the absence of complications, elevated urinary nephrin levels reflect early glomerular injury, while significantly higher levels in complicated cases indicate more severe glomerular damage and filtration dysfunction. These findings are consistent with Lee et al (2023), who reported a significant linear association between urinary nephrin concentrations and preeclampsia severity [18]. Moreover, Wang et al also demonstrated that elevated urinary nephrin was associated with severe preeclampsia, renal dysfunction, and adverse maternal and fetal outcomes, reinforcing its value as a predictive biomarker for disease severity [19,20].

Collectively, these observations support urinary nephrin as a promising non-invasive biomarker that not only detects early glomerular injury but may also differentiate between the severity of preeclampsia. This clinical utility is further supported by the work of Oluwole et al., who underscored nephrinuria’s role in identifying early renal dysfunction and its contribution to the development of proteinuria [11,21–24].

Despite the promising findings, several study limitations must be considered. First, urine samples were collected after the clinical diagnosis of preeclampsia, which limits the ability to assess urinary nephrin as a predictive biomarker before symptom onset. Second, the cross-sectional study design precludes establishing causality between nephrinuria and disease severity. Third, pre-existing medical conditions, such as chronic hypertension or diabetes, which could independently affect nephrin levels, were not analyzed as potential confounders. Fourth, there was no follow-up on urinary nephrin levels postpartum, preventing evaluation of whether nephrin concentrations normalize after delivery or persist, indicating potential long-term renal impairment. Finally, the broad gestational age range among participants may have introduced variability in nephrin concentrations, given that nephrinuria can be influenced by gestational age-specific physiological changes. Future prospective, longitudinal studies with pre-symptomatic sampling and postpartum follow-up are needed to fully establish the temporal relationship between nephrinuria and the development of preeclampsia and its complications.

Conclusions

This study found higher concentrations of urinary nephrin in women with preeclampsia compared to those with normal pregnancy. There was a correlation between elevated urinary nephrin concentrations and the severity of the disease. Women who had preeclampsia with complications showed higher urinary nephrin concentrations than those who had preeclampsia without complications.