Uric acid and endogenous creatinine clearance studies in normal pregnancy and toxemias of pregnancy

High uric acid level during the first 20 weeks of pregnancy is associated with higher risk for gestational diabetes mellitus and mild preeclampsia

OBJECTIVE

To examine the association between uric acid (UA) level during the first 20 weeks of pregnancy and the development of gestational diabetes mellitus (GDM) and preeclampsia in the second half of pregnancy.

METHODS

The study population included registered births (n = 5507) between 2001 and 2007 in a tertiary medical center. The UA levels during the first 20 weeks of pregnancy were sorted by UA ≤ 2.4 mEq/L; UA = 2.5-4.0 mEq/L, UA = 4.1-5.5 mEq/L, and UA > 5.5 mEq/L. The linear-by-linear chi-square test and ROC curves were used to determine the association between UA level during the first 20 weeks and pregnancy complications. Multivariate analyses were performed to demonstrate whether UA level is an independent factor for the prevalence of preeclampsia and GDM.

RESULTS

Significant linear association was documented between UA level in the first 20 weeks and the prevalence of GDM and mild preeclampsia. The lowest and the highest prevalence of GDM were found in the UA ≤ 2.4 mEq/L group (6.3%) and in the UA > 5.5 mEq/L group (10.5%) (p < 0.001), respectively. Mild preeclampsia was diagnosed in 2.1% of the patients from the UA ≤ 2.4 mEq/L group, 3.3% from the UA = 2.5-4.0 mEq/L group, 5.3% from the UA = 4.1-5.5 mEq/L group, and 4.5% from the UA > 5.5 mEq/L group (p < 0.001). Three multiple logistic regression models controlling for maternal age showed that UA level is an independent risk factor for both GDM and mild preeclampsia.

CONCLUSIONS

UA levels in the highest quartile of the normal range during the first 20 weeks of pregnancy are associated with higher risk for the development of GDM and mild preeclampsia.

Mechanisms of Disease: pre-eclampsia

Pre-eclampsia, a syndrome of pregnant women, is one of the leading causes of maternal and fetal morbidity and mortality. Despite active research, the etiology of this disorder remains an enigma. Recent work has, however, provided promising explanations for the causation of the disorder and some of its phenotypes. Evidence indicates that the symptoms of hypertension and proteinuria, upon which the diagnosis of pre-eclampsia is based, have several underlying causes. Nevertheless, the treatment of pre-eclampsia has not changed significantly in over 50 years. This review describes the most recent insights into the pathophysiology of pre-eclampsia from both basic and clinical research, and attempts to provide a unifying hypothesis to reconcile the abnormalities at the feto-placental level and the clinical features of the maternal syndrome. The novel findings outlined in this review provide a rationale for potential future prophylactic and therapeutic interventions for pre-eclampsia.

Uric acid, endothelial dysfunction and pre-eclampsia

BACKGROUND

Pre-eclampsia is strongly associated with hyperuricemia, and in some studies the increase in serum uric acid has been found to correlate with both maternal and fetal morbidity. The hyperuricemia is believed to result primarily from the decreased renal excretion that occurs as a consequence of the pre-eclampsia, and as such is widely viewed as a marker for pre-eclampsia as opposed to having a role in the pathogenesis.

HYPOTHESIS

We present the hypothesis that hyperuricemia may also have a contributory role in the development of hypertension and renal disease in these patients, and we review recent experimental data that would support this hypothesis.

RECOMMENDATION

We suggest that studies addressing the role of uric acid in pre-eclampsia may provide new insights into both the pathogenesis and treatment of this condition.

Preeclampsia. Part 1: clinical and pathophysiologic considerations

Preeclampsia-eclampsia is still one of the leading causes of maternal and fetal morbidity and mortality. Despite active research for many years, the etiology of this disorder exclusive to human pregnancy is an enigma. Recent evidence suggests there may be several underlying causes or predispositions leading to the signs of hypertension, proteinuria, and edema, findings that allow us to make the diagnosis of the "syndrome" of preeclampsia. Despite improved prenatal care, severe preeclampsia and eclampsia still occur. Although understanding of the pathophysiology of these disorders has improved, treatment has not changed significantly in over 50 years. Although postponement of delivery in selected women with severe preeclampsia improves fetal outcome to a degree, this is not done without risk to the mother. In the United States, magnesium sulfate and hydralazine are the most commonly used medications for seizure prophylaxis and hypertension in the intrapartum period. The search for the underlying cause of this disorder and for a clinical marker to predict those women who will develop preeclampsia-eclampsia is ongoing, with its prevention the ultimate goal. This review begins with the clinical and pathophysiologic aspects of preeclampsia-eclampsia (Part 1). In Part 2, the experimental observations, the search for predictive factors, and the genetics of this disorder will be reviewed.

Plasma purine turnover metabolites in women with normal pregnancy and pregnancy complicated with induced hypertension as compared to fetal well-being indices

The purpose of this study was to determine the association between maternal plasma purine bases levels and fetal well-being indices. The research included pregnant women with pregnancy-induced hypertension (PIH) and women with physiologic pregnancy between 32nd and 41st week of gestation. To characterize the pregnant women, their age, number of gestations, and blood pressure values were used. To evaluate condition of the fetus, the values of biophysical profile and FHR tracing were assessed. The purine bases (hypoxanthine, xanthine and uric acid) concentrations in plasma were determined using high pressure chromatography. Hypoxanthine levels, oxypurine pool, hypoxanthine to xanthine molar ratio and the ratio of uric acid to the oxypurine pool were significantly different in patients with PIH in comparison with women with physiologic pregnancy. It was found that increased adenyl nucleotide catabolism in the PIH group can be related to fetal well-being indices, particularly to FHR tracings. Increased percentages of suspected and pathologic FHR tracings were found in patients with PIH in comparison with physiologic pregnancy. The unfavorable influence of increased metabolism of adenyl nucleotides on the condition of the fetus was further confirmed by significant negative correlation between the oxypurine pool and the FHR tracings. The multiple regression analysis choosing the optimal subgroup of independent variables showed significant correlation between the parameters describing the well-being of the fetus and newborn and the levels of inosine, uric acid and xanthine. In the group of women with physiologic pregnancy, the most significant correlation was found in the diastolic blood pressure and hypoxanthine levels.

Hyperuricemia and xanthine oxidase in preeclampsia, revisited

Hyperuricemia is associated with the severity of preeclampsia and with fetal outcome. Traditionally the high uric acid concentration in preeclampsia has been attributed soley to renal dysfunction. Preeclampsia is also characterized by increased free radical formation and elevated oxidative stress. Xanthine dehydrogenase/oxidase produces uric acid. Xanthine dehydrogenase/oxidase is present as two isoforms in vivo. Uric acid production is coupled with formation of reactive oxygen species when the enzyme is in the oxidase form. Several factors can increase the holoenzyme activity and the conversion of xanthine dehydrogenase/oxidase to its oxidase form. These factors include hypoxia-reperfusion, cytokines, and increased substrate availability (xanthine and hypoxanthine). Preeclampsia is characterized by hyperuricemia and signs of increased formation of reactive oxygen species and decreased levels of antioxidants. Preeclampsia is also characterized by shallow implantation, producing a relatively hypoxic maternal-fetal interface, and increased turnover of trophoblast tissue, which can result in higher xanthine and hypoxanthine concentrations and higher levels of circulating cytokines. These mechanisms can lead to increased production of uric acid and free radicals and contribute to the hyperuricemia and increased oxidative stress present in preeclampsia.

A prospective comparison of two endogenous creatinine clearance testing methods in hospitalized hypertensive gravid women

OBJECTIVE

Although 24-hour endogenous creatinine clearance testing is common in pregnancies complicated by hypertension, inaccuracies limit its usefulness. We controlled the conditions under which 4-hour endogenous creatinine clearance testing was performed and compared the results with outcomes of 24-hour tests from the same patients.

STUDY DESIGN

In 83 women hospitalized with mild hypertension in the third trimester, we measured endogenous creatinine clearance with a 4-hour urine collection during lateral recumbency and supervised oral hydration. This test was paired with a 24-hour test performed immediately thereafter. No restrictions or recommendations regarding ambulation or oral intake were imposed for the 24-hour test.

RESULTS

The 4-hour endogenous creatinine clearance value exceeded the 24-hour value in 133 of the 136 paired comparisons (p < 0.0001). Results of the tests from only the 29 patients with multiple paired tests showed more similarity (p < 0.005) among the 4-hour than among the 24-hour clearances.

CONCLUSION

The 4-hour endogenous creatinine clearance test, as described, provides a higher and less variable estimate of renal function in hypertensive pregnant women than does the 24-hour test.

Urinary excretion rates of calcium and magnesium in normal and complicated pregnancies

In this cross-sectional study calcium and magnesium metabolism was investigated in normal pregnancies (n = 34) and pregnancies complicated by either fetal growth retardation of hypertension with or without fetal growth retardation (SGA newborns) (n = 30). Special attention has been given to the renal excretion rates of calcium and magnesium and their relationship to creatinine and sodium clearances. No differences were noted in the third trimester of pregnancy between the normal and complicated pregnancies in calcium or magnesium metabolism except for an increased serum magnesium in the SGA group. Comparing the post-partum period to normal pregnancy the following results were observed: (i) serum ionic calcium levels showed no differences; (ii) urinary calcium excretion was increased as a result of increased calcium clearance. A striking feature was the fact that the fractional calcium clearance was not increased, in contrast to the increase in relative calcium clearance. The observed results can be explained by an increased GFR and a possible dissociation between the sodium and calcium handling in the cortical thick ascending Limb of Henle's Loop.

Plasma urate, urea and creatinine levels during pregnancy and after the puerperium in normal primigravid Nigerians

Plasma concentrations of urea, uric acid and creatinine were measured in 55 normal primigravid Nigerians during pregnancy and 6 weeks after delivery. Plasma urate concentrations were lowest in early pregnancy and increased with advancing gestation, achieving the highest values in the postnatal period. These findings are similar to those reported for European women. Plasma urea rose during the second trimester, fell in the third to almost first trimester levels and rose again in the postnatal period. Plasma creatinine generally followed the same pattern as urea. It is suggested that these changes are a reflection of fluid distribution rather than a change in urea and creatinine production. An improved nutritional status may explain the urea values which are higher than previously reported in African women.

Serum uric acid as a marker of pregnancy-induced hypertension

A prospective longitudinal study was conducted, looking at the changes in serum uric acid during pregnancy in women who were normotensive at initial presentation. In our sample of 78 women having a total of 88 singleton pregnancies, 13 developed pregnancy-induced hypertension during labour only, whilst a further 6 developed hypertension during pregnancy. Women who developed hypertension had significantly higher uric acid levels than women who remained normotensive throughout. However, there was an appreciable overlap between the groups. Women with essential hypertension showed similar changes. We conclude that the serum uric acid level is an unreliable indicator of developing hypertension in the individual woman. However, a rapidly rising uric acid level should be viewed with caution.

The effect of rest and ambulation on plasma urea and urate levels in pregnant women with proteinuric hypertension

Plasma urea and urate concentrations were determined daily for up to seven days in 40 pregnant women who had been admitted to hospital because of proteinuric hypertension and who were then allocated at random to either complete rest in bed or to being allowed to move freely in the ward. Neither management was superior to the other in improving renal function. The prognostic significance of plasma urea and urate concentrations to maternal and fetal outcome was confirmed.

Clinical relevance of coagulation and renal changes in pre-eclampsia

Changes in coagulation tests (fibrin/fibrinogen degradation products, factor-VIII activity, and platelet-count) and in renal function (creatinine clearance and serum concentration, clearance, and fractional reabsorption of urate) were measured in late pregnancy. 10 patients with severe preeclampsia showed changes in both coagulation and renal function when compared with 13 normotensive controls. 18 patients with mild pre-eclampsia had changes in renal function only. Results from 5 patients with essential hypertension did not differ from those of the normotensive group. When results from the patients with severe pre-eclampsia were arranged in order of decreasing protein excretion, only renal-function tests correlated significantly with this ranking. It is suggested that, in the management of patients with established preeclampsia, assessment of renal function may be of greater practical value than measurement of the degree of coagulopathy.

Transfer and dynamics of uric acid in the pregnant rhesus monkey. I. Transplacental and renal uric acid clearances

Data from the literature suggest that uric acid produced by the fetus could pass across the placenta and contribute to the observed increase in maternal plasma levels of uric acid in preeclamptic pregnancy. To investigate this hypothesis, fetal transplacental and renal uric acid clearances were estimated in 4 term pregnant rhesus monkeys by means of the steady infusion method using 14C-labeled uric acid. Allantoin clearances were determined in one pregnant monkey. Samples of maternal and fetal arterial blood were collected at regular intervals. The total amount of fetal urine produced during the experiment was collected at the end of the experiment. In addition, maternal endogenous renal uric acid and creatinine clearances were measured in 6 term pregnant monkeys. Fetal transplacental uric acid clearances appeared to be almost entirely limited by placental permeability and varied between 3.6 and 8.6 ml X min-1 X kg-1 of fetal weight; fetal renal clearances were between 0.11 and 0.20 ml X min-1 X kg-1. The allantoin clearances were found to be of the same magnitude. Maternal renal clearances of uric acid and creatinine were almost equal (mean 3.2 +/- 0.6 and 3.0 +/- 0.5 ml X min-1 X kg-1 of maternal weight, respectively). Extrapolation of these data to human preeclamptic pregnancy reveals that it is unlikely that fetal uric acid could significantly contribute to the maternal uric acid load.

The effect of normal pregnancy upon the renal handling of uric acid

Twenty-four women were investigated serially during and after normal pregnancy. Plasma uric acid concentration appeared to be inversely related to uric acid clearance under infusion conditions, and comparison with simultaneous inulin clearance suggested an alteration in renal function resulting in increased 'net tubular reabsorption' of uric acid as pregnancy progressed. No difference was detected between primigravidae and multigravidae. The implications of these changes are discussed with reference to the renal handling of uric in pre-eclampsia.