The protocol study was approved by our institute’s review board. Written informed consent was obtained from all participants. Between March 2011 and December 2017, all women with singleton pregnancies diagnosed with CKD before o during pregnancy were recruited from the Department of Maternal-Fetal Medicine of a tertiary care level hospital. Patients with systemic lupus erythematosus were excluded from the study because lupus nephritis per se has been associated with worse pregnancy outcomes for multiple reasons [19].
CKD stage was defined according to Kidney Disease: Improving Global Outcomes (KDIGO) guidelines [20]. Given the lack of consensus regarding the best marker for renal function during pregnancy, glomerular filtration rate (GFR) was calculated using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formula [21] based on preconception data or serum creatinine measured at first control in pregnancy. This formula was chosen because of its wide use in several previous studies on CKD and pregnancy.
Hypertension was defined as systolic or diastolic blood pressure ≥ 140 mmHg or ≥ 90 mmHg, or antihypertensive therapy prior to 20 weeks’ gestational age. Preeclampsia (PE) was defined according to The American College of Obstetricians and Gynecology criteria [22]. PE without severe features was defined as hypertension and significant proteinuria (≥ 300 mg protein in a 24-h urine specimen or a protein to creatinine ratio ≥ 0.30 mg/mg in a random urine sample). PE with severe features was considered when either hemolysis, elevated liver enzymes, low platelet count (HELLP) syndrome, eclampsia, or PE with severe hypertension (systolic or diastolic blood pressure ≥ 160 mmHg or ≥ 110 mmHg, respectively) was present. Other parameters included, even in absence of significant proteinuria, were new-onset cerebral or visual disturbances, abnormal liver enzymes levels (to twice normal concentration), thrombocytopenia (< 100,000/µl), or pulmonary edema. Superimposed PE (i.e. with preexisting hypertension and/or proteinuria) was considered when HELLP syndrome, eclampsia, or other severe features were present; in their absence, a sudden increase in blood pressure and/or sudden increase in proteinuria accompanied by an angiogenic imbalance, as assessed by the maternal serum soluble fms-like tyrosine kinase-1/placental growth factor (sFlt-1/PlGF) ratio ≥ 85, measured by electrochemiluminescence [23, 24], was considered as superimposed PE.
Other APO included any of the following: preterm delivery (< 34 weeks of gestational age), the neonatal death (stillbirths [defined as death of a fetus] and neonatal death [defined as death of a newborn until hospital discharge]) and small-for-gestational age (SGA) infant, defined as an infant whose birth weight was below the 10th percentile.
A 24-h timed urine collection was obtained at enrollment to estimate GFR using standard creatinine clearance corrected for the body surface area and to measure total protein and IgM excretion. Samples were centrifuged, and the resulting sediment-free urine specimens were aliquoted and stored at − 80 °C until assayed. All samples were collected before clinical suspicion or diagnosis confirmation of any studied APO. Clinical and delivery outcomes were recorded to classify the patients with APO.
Urine analysis
IgM concentrations in urine were measured by an ultrasensitive enzyme immunoassay for human IgM developed in our laboratory to measure small amounts of IgM. Polystyrene 96-well plates (Nunc, Roskilde, Denmark) were coated with 100 µl/well of polyclonal antibodies specific for human IgM (goat anti-h IgM, American Qualex, San Clemente, CA, USA) at a concentration of 1 µg/ml in 0.1 M carbonate–bicarbonate buffer, pH 9.0. Plates were incubated for 2 h at 37 °C and stored at 4 °C until used. Optimum blocking conditions for nonspecific binding were achieved using 300 µl/well of 5% skimmed milk in phosphate-buffered saline (PBS), pH 7.4, containing Tween-20 at 0.05% (PBST), for 2 h at 37 °C. After washing five times with PBST, the samples were added in duplicate at different dilutions to ascertain parallelism with the standard curve (Bethyl Laboratories, Inc., Montgomery, TX, USA). The plates were left for 1 h at 37 °C, then washed five times with PBST, and subsequently incubated with monoclonal antibodies specific for human IgM (American Qualex, San Clemente, CA, USA) at a concentration of 1 µg/ml. Following 1 h of incubation at 37 °C, the plates were washed five times with PBST and subsequently incubated with a peroxidase-conjugated goat anti-mouse serum (Dako, Carpinteria, CA, USA), at a 1:2000 dilution for 1 h at 37 °C. Plates then were washed five times with PBST before the reactions were revealed with o-phenylene diamine (Sigma, St. Louis, MO, USA). Optical density was read at 492 nm using an ELISA reader (Emax, Molecular Device). The minimal detectable quantity was 3.0 ng/ml, and the intra- and interassay coefficients of variation were 3.5 and 5.6%, respectively. Urinary protein and creatinine were measured as previously described [25].
Statistical analysis
Differences between continuous variables were determined by the unpaired Student t test (or the Mann–Whitney U test for non-normally distributed variables). Differences between categorical variables were determined by the Chi square test with Yates continuity correction or the Fisher exact test for small samples (or the Mantel–Haenszel χ2 test with linear tendency for variables with > 2 categories). Association between uIgM and total protein levels, and CKD stages, and the subsequent risk of APO, was calculated. Test results were grouped into quartiles (urinary total protein and IgM excretion) and kidney disease was grouped via CKD stage; due to the clinical severity and sample size, the groups were collapsed into four groups, CKD stage 1, 2, 3 and CKD stage 4–5; and the odds ratios (ORs) and 95% confidence intervals (CIs) were calculated to assess the association between quartiles or CKD stage and the risk of APO. The lowest quartile was used as reference category for both proteinuria and IgM-uria, and for CKD stage, group 1 (CKD stage 1) was the reference group. Logistic-regression analysis was used to adjust the ORs for proteinuria, IgM-uria, and CKD stage because these variables were significantly different among the groups studied by univariate analysis, as well as for chronic hypertension considering that this variable may distinctly affect the adverse outcomes. In addition, the ORs for preterm delivery were also adjusted for PE, and for neonatal death were adjusted for gestational age at delivery. A two-tailed p < 0.05 was considered statistically significant.
General description of the population
Of the 171 CKD patients enrolled, 4 (2.3%) were excluded for the following reasons: two patients for congenital defects, one patient for molar pregnancy, and one patient for loss to follow-up at week 30. Therefore, a total of 167 CKD pregnancies were available for final analysis. Each patient was included only once in the study. The mean maternal age was 28.9 ± 6.2 years with a median gestational age at enrollment of 21 weeks (IQR 15–25). The majority of patients were multigravida (57.9%). In 130 of 167 cases (77.8%), the diagnosis of CKD was made during pregnancy. Glomerulonephritis was the main cause of CKD (69.4%), followed by diabetes (20.4%) and tubulointerstitial (10.2%). Chronic hypertension was present in 50.9% of patients. Of the 167 patients, 59 (35.3%) had CKD stage 1, 48 (28.7%) CKD stage 2, 38 (22.8%) CKD stage 3, 15 (9.0%) CKD stage 4, and 7 (4.2%) CKD stage 5. Hemodialysis was required in four patients with CKD stage 5.
Ninety patients (53.1%) had ≥ 1 APO, including PE 66 (39.5%), SGA infant 61 (36.5%), preterm birth 53 (31.7%), and neonatal deaths 21 (12.6%). Among the 53 patients who delivered at < 34 weeks of gestation, 47 of deliveries were indicated for PE, two for fetal distress, two for preterm premature rupture of membranes, and two for spontaneous preterm labor.
uIgM levels, proteinuria, and CKD stages are associated with the occurrence of APO
Tables 1 and 2 show the comparison between patients who developed combined APO or PE and those who did not. A significant trend towards higher frequency for combined APO was observed across CKD stages: CKD stage 1, 19/59 (32.2%); CKD stage 2, 25/48 (52.1%); CKD stage 3, 25/38 (65.8%); and CKD stages 4–5, 21/22 (95.5%) (p < 0.001). A similar trend was observed for PE: CKD stage 1, 14/59 (23.7%); CKD stage 2, 19/48 (31.3%); CKD stage 3, 17/38 (44.7%); and CKD stages 4–5, 16/22 (72.7%) (p < 0.001). Tubulointerstitial nephropathy patients had a lower frequency of combined APO and PE compared to patients with glomerular disease or diabetic nephropathy.
Table 1 Clinical, laboratory, and demographic characteristics of pregnant women with CKD according to the occurrence of combined adverse pregnancy outcomes
Table 2 Clinical, laboratory, and demographic characteristics of pregnant women with CKD according to the occurrence of preeclampsia
Compared to patients without combined APO or PE, patients with these outcomes had higher serum creatinine levels, lower baseline creatinine clearance, as well as a higher baseline excretion of both total protein and IgM (p < 0.001). Patients who developed combined APO or PE had lower gestational age at delivery, delivered infants with lower birth weights, and had a greater proportion of preterm birth, SGA infants, and neonatal deaths than those patients without these APO (p ≤ 0.04). Although there was an overall increase in the frequency of cesarean section (88.6%), there was no significant difference between patients who developed combined APO or PE and patients who did not develop these outcomes (p ≥ 0.13).
uIgM levels, proteinuria, and CKD stages and the risk of APO
Logistic regression analyses were used to determine the association between quartiles of 24-h urinary excretion of proteins and IgM, and CKD stages and risk of combined and specific APO (Table 3). The risk of combined APO was progressively higher as uIgM level quartiles increased. Similarly, for women in the third and fourth quartiles of uIgM the ORs for PE, preterm birth, or SGA infant progressively increased, suggesting that the relationship between values of this biomarker and risk of these outcomes has a linear dose–response pattern. Women in the third and fourth quartiles for total proteinuria only exhibited higher risk for a combined APO and PE, with the ORs remaining similar across quartiles (ORs 5.7 and 5.2; and 3.3 and 2.9, respectively). For CKD stages, there was an increased risk of combined APO in patients with stage 2 and 3 (ORs 3.6 and 3.4), whereas this risk was markedly higher in those patients with stage 4–5 (OR 41.0). Women with stage CKD 4–5 only exhibited higher risk for PE and SGA infant (ORs 4.5 and 4.4, respectively). With regard to the risk of neonatal deaths, there were no significant differences according to quartiles of urinary excretion of IgM and proteins, or CKD stages.
Table 3 Risk of adverse pregnancy outcomes according to interquartile range of urinary IgM and proteinuria levels and CKD stages in patients with CKD
uIgM excretion and the risk of APO among patients with CKD stage 1
To determine the effect of eventual differences in uIgM excretion on the association with the development of APO, a further analysis was carried out, limited to stage 1 CKD patients. For this analysis, patients were divided into two groups according to quartiles based on the distribution of uIgM levels among all stage 1 CKD patients. The first group included 30 patients whose values for uIgM levels fell within the three lowest quartiles, and the second group comprised the 29 remaining patients in the highest quartile (> 8.95 µg/24 h). The demographic, clinical, and obstetric characteristics of the patients are shown in Table 4. As expected, serum creatinine levels and baseline creatinine clearance were similar in both groups. Proteinuria was significantly higher among patients in the higher quartile for uIgM levels in comparison to the lower three quartiles. Patients in the higher quartile for uIgM levels had lower gestational age at delivery, delivered infants with lower birth weights, and had a greater proportion of preterm birth, APO, and PE than patients in the three lower quartiles (p ≤ 0.015). Although the frequency of cesarean section, SGA infant and neonatal deaths was higher in patients in the higher quartile than in patients in the three lower quartiles, the differences were not statistically significant (p ≥ 0.051).
Table 4 Clinical, laboratory, and demographic characteristics of pregnant women with CKD stage 1 according to urinary IgM excretion
Additionally, ORs for quartiles of uIgM levels were calculated. A further adjustment for proteinuria levels was made because this variable differed significantly between groups at univariate analysis. The outcomes of SGA infant and neonatal deaths were not analyzed because of their very low frequency in both groups. Table 4 shows that the ORs were significantly high for combined APO, PE, and preterm birth (OR ≥ 4.8) among patients in the higher quartile of the CKD 1 stage patients distribution.