The Application of Serum Cystatin C in Estimating the Renal Function in Women With Preeclampsia

Abstract

Objective: Serum cystatin C (Cys-C) is known to reflect the glomerular filtration rate (GFR) more precisely in native kidney diseases and renal dysfunctions secondary to other diseases. This study investigated the serum Cys-C in estimating the renal function in preeclamptic women. Methods: A total of 96 patients with normal pregnancy (controls) and 48 cases of severe preeclampsia were recruited in this study. We measured the 24-hour creatinine clearance (CrCl), serum creatinine, Cys-C, uric acid (UA), and beta trace protein (BTP) concentrations on all the pregnant women in the second trimester and third trimester and in the postpartum of the patients with severe preeclampsia. Multiple comparisons and correlation analysis were used to analyze the indexes estimating the GFR. Results: In the normal pregnancies, the concentrations of serum creatinine, UA, and BTP were significantly higher in the third trimester compared to the second trimester, however with no significant differences in the serum Cys-C levels. Comparison between the second and third trimester in patients with severe preeclampsia indicated that significant difference existed in the serum Cys-C, with higher concentration in third trimester. Correlation analyses demonstrated that significant negative correlations could be detected between Cys-C and 24-hour CrCl in the second trimester and third trimester of all the 144 pregnant women and in the postpartum of the patients with severe preeclampsia, and better correlations in normal participants than in participants with preeclampsia. Conclusions: Serum Cys-C seems to reflect the GFR precisely in women with severe preeclampsia and can be a good marker to monitor the renal function from antepartum to postpartum.

Keywords

creatinine clearance glomerular filtration rate preeclampsia serum cystatin C

Introduction

Hypertensive disorders of pregnancy are one of the major causes of maternal death around the world. Routine prenatal care decreases the morbidity rate of pregnant-specific seizure disorder, eclampsia, however, preeclampsia, a common and major complication causing significant maternal and fetal morbidity and mortality worldwide,¹ is still evident among pregnant women in China. Widespread maternal endothelial dysfunction is the vital factor for preeclampsia, inevitably and sensitively influencing the glomerular dynamics and renal function first. Therefore, a thorough understanding of the renal function is critical for assessment of the possible clinical results in women with preeclampsia.

Uric acid (UA) was popularly used as a marker of glomerular filtration rate (GFR) when monitoring renal function. The elevation of the UA preceded hypertension and proteinuria, the clinical manifestations for diagnosing the disorder. Its serum concentration increased with the severity of preeclampsia and was assigned as a good predictor in clinical observation, even a pathogenic factor in the pathophysiology of preeclampsia.² ^,3 Despite knowledge that it had advantages, results of some investigations made UA assessment in the estimation of hypertensive disorders of pregnancy fallen into disfavor. Recent researches showed that UA was of little value in the prediction of renal function in pregnant women.^4–6 Beta trace protein (BTP), also known as prostaglandin D synthase, is a low-molecular-weight protein (a 23- to 29-kDa enzyme) that has been traditionally used as a marker for cerebrospinal fluid leakage and is another promising surrogate marker for GFR measurement. It is freely filtered by the glomerulus and its serum concentration depends on the GFR.⁷

Cystatin C (Cys-C) is a member of cysteine proteinase inhibitor superfamily, which has a molecular weight of 13.359 kDa, produced at a constant rate in all investigated nucleated cells. In the glomeruli, Cys-C can be filtrated freely and be degraded completely in the tuber cells, neither secreted nor reabsorbed. The serum concentration is stable, having no influence from inflammation, hemolysis, bilirubin, age, sex, food, diet, and so on. Its renal plasma clearance is virtually identical to that of creatinine clearance (CrCl) in animal experiments; therefore, Cys-C is the ideal endogenous marker of GFR.⁸ ^,9 A large number of studies favored Cys-C over serum creatinine (Scr) for the estimation of GFR in native kidney diseases.⁶ Later some researches began to apply Cys-C to reflect the GFR in normal pregnancy and hypertensive disorders of pregnancy. Results demonstrated that serum Cys-C levels can reflect GFR reliably in both healthy and hypertensive pregnant women, independent of age, height, weight, or other factors.¹⁰ For close supervision and early diagnosis of renal dysfunction in pregnant women, Cys-C was a useful and promising marker.^11–13 However, on the contrary, data from Akbari found that the Cys-C as a GFR marker in pregnancy was disappointing.¹⁴ Therefore, the current study was designed to investigate the reliability of Cys-C in estimating the renal function in normal pregnancy and severe preeclampsia from the second trimester to the third trimester, and in postpartum period, by comparing it to the 24-hour CrCl, Scr, UA, and BTP.

Materials and Methods

Study Population

Between January 2010 and January 2011, all the women, who gave their informed consent, with singleton pregnancies admitted to the antenatal ward at the Department of Obstetrics, Shenzhen Maternity and Child Healthcare Hospital affiliated to Southern Medical University, China, at their second trimesters were recruited into this study. Women with a history of hypertension, renal disease, heart disease, liver disease, diabetes, severe anemia, malignant tumor, and rheumatic disease were excluded. We gave detail observation and antenatal care for every pregnant woman including renal function examination during their second trimesters and third trimesters, and in postpartum period. Renal function examination included measurement of the 24-hour CrCl, Cys-C, Scr, UA, and BTP. Until delivery, 48 pregnancies developed severe preeclampsia in their 37 to 40 weeks of gestation, being as case group. Severe preeclampsia was defined as new onset of a blood pressure ≥160/110 mm Hg on 2 separate occasions at least 4 hours apart, accompanied by proteinuria ≥2000 mg/24 h or ≥2+ on dipstick, in the absence of a urinary tract infection. All the 48 patients with severe preeclampsia accepted cesarean section to terminate their pregnancy and renal function was determined again in these patients from day 1 to day 4 postoperatively. In addition, another 96 normal pregnancies during this study period were randomized into the control group using 2:1 ratio. Ethical approval for the study was obtained from the local research ethics committee.

Determination of Renal Function

For all the pregnancies, whether developed into severe preeclampsia or not during its later gestation, venous blood samples for the determination of the Cys-C, Scr, UA, and BTP were obtained when being instructed to collect the 24-hour urine properly. The 24-hour CrCl is assigned as the gold standard for GFR. Considering the radiation exposure to the pregnant women, we did not choose radio nucleotide markers to determine the GFR. The 24-hour CrCl was calculated from the 24-hour urine collection and Scr, being expressed in mL/min. The serum Cys-C, Scr, UA, and BTP were measured from blood samples, using Beckman-Coulter Synchron LX20 automated chemistry analyzer (Beckman Coulter, Inc, California), in Shenzhen Maternity and Child Healthcare Hospital clinical laboratories. Besides, for the 48 patients with preeclampsia, the 24-hour CrCl, Cys-C, Scr, UA, and BTP were determined 4 times more from day 1 to 4 postoperatively in the same manner.

Statistical Analysis

Data were presented as mean ± standard deviation. The 1-way analysis of variance (ANOVA) and further multiple comparisons were used for comparing the Cys-C, Scr, UA, and BTP between normal pregnancies and patients with severe preeclampsia in their second and third trimesters. Pearson correlation analysis was used to explain the relationships between 24-hour CrCl and the other renal function variables. A result was deemed significant when P < .05. SPSS (version 13.0) was used for statistical analysis.

Results

Demographic Characteristics

The demographic characteristics of the 48 patients with severe preeclampsia and 96 normal pregnancies are summarized in Table 1. There was no statistical difference for these clinical features between normal pregnancies and patients with severe preeclampsia except for the average arterial pressure and proteinuria. All patients came from one race.

Table 1.

Demographic Characteristics of Normal Pregnancies and Patients With Severe Preeclampsia( $\bar{x} \pm s$ )

	Normal Pregnancies (n = 96)	Severe Preeclampsia (n = 48)
Age (year)	33.41 ± 4.56	32.58 ± 4.43
Height (cm)	161.87 ± 15.37	163.21 ± 13.89
Gestation in second trimester (day)	131.27 ± 16.83	126.46 ± 20.03
Weight in second trimester (kg)	65.83 ± 13.67	68.32 ± 11.52
Gestation in third trimester (day)	277.75 ± 18.27	274.36 ± 20.15
Weight in third trimester (kg)	74.93 ± 16.22	78.76 ± 18.16
Gravidity	2.38 ± 1.27	2.58 ± 1.13
Parity	0.75 ± 0.17	0.68 ± 0.26
BMI in second trimester	25.40 ± 4.97	25.71 ± 6.07
BMI in third trimester	28.90 ± 7.35	29.64 ± 9.16
Mean arterial pressure (mm Hg)	136.56 ± 15.28^a	93.27 ± 10.52
Proteinuria (g/24 h)	0.18 ± 0.45^a	3.15 ± 0.99
Gestational ages of delivery (week)	38.76 ± 1.75	38.15 ± 2.36

^a Predicts P < .05 when compared with severe preeclampsia.

Comparison of Cys-C, Scr, UA, and BTP

When observing the changes in the renal function variables from second trimester to third, data showed a similar change trend in serum UA, Scr, and BTP (seen in Figures 1, 2, and 3) in both normal pregnancies and patients with severe preeclampsia. The serum UA, Scr, and BTP in their third trimesters were significantly higher than that of their second trimesters (P < .05). Comparisons between normal pregnancies and patients with severe preeclampsia illustrated no significant differences in serum UA, Scr, and BTP in the second trimester (P > .05). Whereas, when comparing with normal pregnancies, there were a relatively higher levels of serum UA and Scr in patients with severe preeclampsia in their third trimesters (P < .05), but no difference was detected in BTP (P > .05). Regarding the serum Cys-C, the only difference from serum UA and Scr, there was no significant increase between second and third trimesters in normal pregnancies, but with higher concentration in third trimester of patients with severe preeclampsia (seen in Figure 4).

Correlation Between 24-Hour CrCl and Cys-C, Scr, UA, and BTP

The results of Pearson correlation analyses between 24-hour CrCl and Cys-C demonstrated significant negative correlations in both normal pregnancies and patients with severe preeclampsia from second trimester to third trimester, even until day 1 to 4 postpartum for severe preeclampsia (all Ps < .004). And better correlations in normal participants were found than that in preeclampsia both in the second (Coefficient −0.985 vs −0.762) and third trimesters (Coefficient −0.912 vs −0.778). No correlation was found between 24-hour CrCl and Scr, UA, and BTP, excepting a weak, but significant positive correlation (P = .042) in the second trimester of normal pregnancy and negative correlation (P = 0.044) in the third trimester of severe preeclampsia for BTP (seen Table 2).

Table 2.

Correlation Between 24-Hour CrCl (mL/min) and Cys-C (mg/L), Scr (µmol/L), UA (µmol/L), BTP (mg/L) in Normal Pregnancy and Severe Preeclampsia

24-Hour CrCl	Second Trimester of Normal Pregnancy				Third Trimester of Normal Pregnancy
24-Hour CrCl	Cys-C	Scr	UA	BTP	Cys-C	Scr	UA	BTP
Coefficient	−.985	.297	.290	.436	−.912	−.163	.042	.118
P	<.001	.180	.190	.042	<.001	.448	.847	.582
24-hour CrCl	Second trimester of severe preeclampsia				Third trimester of severe preeclampsia
24-hour CrCl	Cys-C	Scr	UA	BTP	Cys-C	Scr	UA	BTP
Coefficient	−.762	−.050	.085	.256	−.778	−.153	−.457	−.589
P	.004	.878	.792	.422	.003	.635	.136	.044
24-hour CrCl	Post-day 1 of severe preeclampsia				Post-day 2 of severe preeclampsia
24-hour CrCl	Cys-C	Scr	UA	BTP	Cys-C	Scr	UA	BTP
Coefficient	−.966	−.062	−.139	−.484	−.963	.052	−.017	.084
P	<.001	.848	.668	.111	<.001	.874	.958	.796
24-hour CrCl	Post-day 3 of severe preeclampsia				Post-day 4 of severe preeclampsia
24-hour CrCl	Cys-C	Scr	UA	BTP	Cys-C	Scr	UA	BTP
Coefficient	−.986	−.167	−.045	.088	−.995	−.571	−.129	−.278
P	<.001	.605	.889	.785	<.001	.140	.761	.505

Abbreviations: Post-day 1, day 1 postoperatively; post-day 2, day 2 postoperatively; post-day 3, day 3 postoperatively; post-day 4, day 4 postoperatively.

Discussions

Previous researches^2–4 believed that hyperuricemia was one of the earliest and most consistent observations manifested in preeclamptic pregnancies. The observation of Roberts et al² indicated that the UA concentration elevated as early as 10 weeks of gestation in whom go on to develop the preeclampsia, much earlier than that of clinical features. Whereas, other findings demonstrated that the elevation of serum UA concentrations started from 28 weeks of gestation or just before delivery. Our data seemed to reflect that there were higher levels of serum UA and Scr in the third trimester not only in patients with severe preeclampsia, but also in the normal pregnancies, with no changes in their second trimesters either in normal pregnancies or in patients with severe preeclampsia. Nontheless, these changes had historically been attributed to a series of changes in the renal system and blood volume, including the increased renal plasma volume, increased GFR, and hypovolemia, causing increased reabsorption of UA and the increased secretion of Scr. Therefore, the elevation of serum UA and Scr in patients with preeclampsia may not be the real reflect of renal function, besides, no correlation was found between serum UA, Scr, and 24-hour CrCl indicated from correlation analyses.

Recently, many investigators have given great interests on serum Cys-C in evaluating the renal function in both native renal diseases and renal injury secondary to other diseases. In pregnant women, good renal function still plays important role in maintaining the whole pregnancy. In many pregnant-related diseases, kidney always was the first target organ, like hypertensive disorders of pregnancy. But there were discrepancies in the role of serum Cys-C in prediction of renal injury in patients with preeclampsia. Some authors¹⁴ believed that serum Cys-C was of little use, elevated Cys-C concentrations can also be seen at the end of gestation in some normal women with the possibility of small degrees of endotheliosis. Maybe it is reasonable, a small increase in serum Cys-C from 0.77 mg/L in second trimester to 0.82 mg/L in third in normal pregnancies were found in our study, however with no significant difference. As we all know, endotheliosis, the key pathogenic mechanism, was more prominent with injury to the vascular endothelial cells in preeclampsia. Our results favored the view, with the serum Cys-C increasing from 0.76 mg/L in second trimester to 1.17mg/L in third trimester in patients with preeclampsia, similar to the results of Strevens.¹² Furthermore, the correlation analyses demonstrated significant negative correlations in both normal pregnancies and patients with severe preeclampsia from second trimester to third trimester only between 24-hour CrCl and serum Cys-C, even until day 1 to 4 postpartum for severe preeclampsia. Recent investigation¹⁵ found that the serum Cys-C did not correlate with the neonatal Cys-C level, suggesting the elevation of Cys-C concentration in the third trimester of patients with preeclampsia in our study independent of its production of placenta. More interesting, the serum Cys-C concentration gradually decreased (1.12-0.89 mg/L) with the elevation of 24-hour CrCl from day 1 to 4 postoperatively in the patients with preeclampsia, with significant negative correlation between them in postpartum period. It would be logical that the vascular injury recovered in patients with preeclampsia while terminating the pregnancy.

Like Cys-C, BTP had been evidenced to be better than Scr in nonpregnant populations as one of the endogeneous markers of GFR, but not better than Cys-C.¹⁶ Later, BTP had been studied in pregnancy by Akbari,¹⁸ showing little promise in predicting GFR, unfortunately. In this study, higher serum BTP can be found in the third trimester in both normal pregnancies and patients with severe preeclampsia than that of second trimester, respectively, even more with similar concentrations in these 2 populations in both second and third trimesters, which would not be expected to be a good marker of GFR.

In conclusion, serum Cys-C increased in the third trimester of severe preeclampsia higher than that of second trimester and third trimester of normal pregnancies, and correlated negatively with 24-hour CrCl from antepartum to postpartum. In severe preeclampsia, serum Cys-C was of good prospect in reflecting the GFR precisely and then monitoring the renal function. Nonetheless, one big limitation of our study was that we failed to compare the results with those of a gold standard measurement of GFR such as inulin clearance or using radio nucleotide markers. Although nonradioactive markers like iohexol can decrease the radiation to the pregnant women, the validity and safety of its application in estimating the GFR in pregnancy need further confirmation.¹⁹ In both pregnant women and the nonpregnant women, CrCl usually was accepted to measure the GFR for its value does not deviate much from inulin clearance. Another important limitation lies in the failure of including the other types of hypertensive disorders of pregnancy, such as gestational hypertension, mild preeclampsia, eclampsia, and even hemolysis, elevated liver enzymes, and low platelets (HELLP) syndrome. In future, more healthy and unhealthy pregnancies from multiple clinical centers will be included for further study using the inulin clearance as gold standard of GFR, eliminating the generalizability of this sample coming from a single center, although the women entered into this study are socioeconomically diverse.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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