Role of Urinary Calcium to Creatinine Ratio in The Prediction of Pre-Eclampsia Among Pregnant Women at Usmanu Danfodiyo University Teaching Hospital, Sokoto

Background: Pre-eclampsia is a multi-system disorder of pregnancy that contributes considerably to the maternal morbidity and mortality worldwide. The unpredictable nature of the disease further confounds this problem. Researchers have found an association between hypocalciuria and the development of preeclampsia. Hence, the need to test the predictive value of decreased urinary calcium to creatinine ratio in pre-eclampsia.

Materials and Methods: This was a prospective study carried out on pregnant women between 16 to 20 weeks of gestation to determine the predictive values of Urinary calcium creatinine ratio for pre-eclampsia. A pretested structured, interviewer-administered questionnaire was completed for eligible participants. Urine sample was then taken for quantification of urinary calcium using O-cresolphthalein complexone reaction and urinary creatinine using the Jaffe’s method. They were then grouped group A: women who developed pre-eclampsia, and group B - women who did not develop pre-eclampsia till delivery. 

Results: Incidence of pre-eclampsia was 10.71%. The mean level of maternal urinary calcium creatinine ratio in the pre-eclamptic group was significantly lower (0.09 ± 0.05 vs. 0.13 ± 0.69), (t= 2.41, p=0.02). The sensitivity and specificity for diagnosis of pre-eclampsia was 66.7% and 57.0% respectively. The area under the curve was 0.66 with 95% confidence interval (0.50-0.82). 

Conclusion: Decreased level of urinary calcium creatinine ratio in early pregnancy may be a useful marker to identify women who are at risk of developing pre-eclampsia.

Pre-eclampsia (PE) is a multisystem hypertensive disorder of pregnancy that typically occurs after 20 weeks of gestation in previously normotensive women. It is a major contributor to maternal and perinatal morbidity and mortality, particularly in low-resource settings. The World Health Organization (WHO) estimates that PE complicates 2-8% of pregnancies and accounts for 10-15% of maternal deaths globally [1]. In Nigeria, hypertensive disorders complicate up to 17% of pregnancies, with the prevalence of PE ranging from 3.3% in Enugu to 4.6% in Sokoto [2-4]. Despite decades of research, the exact etiology of PE remains incompletely understood. The central mechanism is thought to involve abnormal placentation, particularly inadequate trophoblastic invasion and poor remodeling of uterine spiral arteries. This results in a high-resistance, low-flow uteroplacental circulation and subsequent ischemia-reperfusion injury. The hypoxic placenta releases anti-angiogenic factors such as soluble fms-like tyrosine kinase-1 (sFlt-1) and soluble endoglin (sEng), which antagonize VEGF, PlGF, and TGF-β signaling, leading to widespread maternal endothelial dysfunction [5,6]. Other contributing mechanisms include maladaptation of maternal immune tolerance, imbalance between prostacyclin and thromboxane favoring vasoconstriction, and autoantibodies against angiotensin II receptors [5,7]. Together, these abnormalities explain the multisystem nature of PE, which may present with renal, hepatic, neurologic, hematologic, and uteroplacental dysfunction, even in the absence of proteinuria.

Emerging evidence suggests that disturbances in calcium metabolism play a role in the pathophysiology of PE. In normal pregnancy, urinary calcium excretion increases due to enhanced intestinal absorption and glomerular filtration. In contrast, hypocalciuria-a consistent feature of PE-has been observed, although the underlying mechanism remains unclear. Proposed explanations include impaired vitamin D metabolism, increased tubular calcium reabsorption, intrinsic renal tubular dysfunction, or reduced levels of insulin-like growth factor 1 (IGF-1) [8,9]. Renal function markers are also altered in PE: while glomerular filtration rate normally rises by up to 65% in early pregnancy, this increase is blunted in PE due to elevated arteriolar resistance [10]. Serum creatinine is a relatively late marker of renal compromise, whereas urinary creatinine clearance and the urinary calcium-to-creatinine ratio (CCR) offer more sensitive assessments of renal function and have shown potential as predictive markers [9,10]. Several studies from different settings support the predictive utility of urinary CCR in preeclampsia. Munge et al. (2016) reported that a CCR ≤0.04 had a sensitivity of 63.6% and specificity of 94.9% for predicting PE among normotensive women on calcium supplementation [11]. Similarly, Prasad et al. (2016) found that 89% of preeclamptic women had CCR ≤0.04 compared with only 6% of controls, highlighting its discriminatory value [12]. Prospective studies by Aherwar and Vahdat et al. further demonstrated high sensitivity (83%) and specificity (96%) of CCR as a cost-effective, non-invasive screening tool [13,14]. The WHO also recommends calcium supplementation for the prevention of PE, though this may influence CCR values and confound study outcomes [15].

The literatures indicate that hypocalciuria and reduced CCR are reproducible biochemical markers of PE across diverse populations. However, most studies have recruited women after 20 weeks of gestation or included those on calcium supplementation, which may limit predictive accuracy. There remains a critical need for prospective studies evaluating urinary CCR as an early screening tool among women recruited before 20 weeks of gestation and not receiving calcium supplementation, especially in high-burden, resource-limited settings such as Nigeria. This study aimed to evaluate the predictive value of the urinary calcium-to-creatinine ratio for preeclampsia among asymptomatic pregnant women between 16 and 20 weeks of gestation. It sought to examine the association between urinary calcium-to-creatinine ratio and subsequent development of preeclampsia, to assess its ability to identify women at increased risk, and to determine the optimal cut-off value and diagnostic accuracy of this ratio in predicting preeclampsia.

Study Design and Setting

This was a prospective cohort study conducted at the antenatal clinic of Usmanu Danfodiyo University Teaching Hospital Sokoto. From November 2019 to July 2020.

Study Population: Pregnant women at 16–20 weeks gestation who attended the antenatal clinic during the study period. Gestational age was confirmed via last menstrual period or first trimester ultrasound.

Inclusion Criteria

This involve consenting pregnant women within the gestational age of 16 - 20 weeks that are normotensive, with no significant proteinuria.

Exclusion Criteria

Pregnant women with hypertension, renal disease, diabetes, multiple pregnancy, those on supplement containing calcium ion or who did not give consent.

Sample Size Determination

Formula for comparing proportions; n = (Z2Pq) / d

assuming a 6% prevalence of pre-eclampsia in the region7, with a 95% confidence interval and 5% precision.

The calculated sample was 96, increased to 120 to account for attrition.

Sampling Technique

Non-probability consecutive sampling.

Data Collection and Variables

A structured, interviewer-administered questionnaire was used. Anthropometric measurements, blood pressure, and urinalysis were conducted at baseline.

Study Population and Recruitment

Participants were consecutively recruited until the calculated sample size was reached. 

Outcomes

The primary outcome was the development of pre-eclampsia among women with reduced urinary calcium-to-creatinine ratio (CCR).

Secondary outcomes included gestational age at delivery, birth weight, and maternal or neonatal complications related to pre-eclampsia.

Sample Collection and Measurements

Blood pressure was measured using a mercury sphygmomanometer following standard technique. Midstream clean-catch urine samples were collected for urinalysis, calcium and creatinine assessment.

Urinary calcium concentration was determined by the O-cresolphthalein complexone (OCPC) method, and urinary creatinine by Jaffe’s method, both using ChemCHECK™ (Agappe Diagnostics LTD) kits. CCR was calculated from these values.

Follow-up and Diagnosis of Pre-Eclampsia

Routine antenatal visits were continued. At each visit, symptoms suggestive of pre-eclampsia were elicited, blood pressure measured, and urinalysis performed if blood pressure was ≥140/90 mmHg. Preeclampsia was defined as new-onset hypertension (≥140/90 mmHg on two occasions) with significant proteinuria after 20 weeks of gestation. Women were classified into those who developed pre-eclampsia and those who remained normotensive.

Data analysis

Receiver-operator curve (ROC) analysis was performed to determine the optimal CCR cut-off for predicting pre-eclampsia. Sensitivity, specificity, positive predictive value, and negative predictive value were calculated.

Ethical Approval

This followed the ethical standards of Helsinki Declaration of the 1975, as revised in 1983. This was cross checked and scrutinized by the Ethical committee of the Usmanu Danfodiyo University Teaching Hospital Ethics and Research. The approval was duly given with the number (UDUTH/HREC/ 2019/ No828). Written informed consent was obtained from all participants after diligent counselling on the protocol and ethical guidelines, including risks and benefits.

Study Population

The study was conducted over 8 months. A total of 120 women were recruited, of whom 112 (93.3%) completed follow-up until delivery. Eight participants (6.7%) were excluded (7 lost to follow-up, 1 miscarriage). Among the 112 women analyzed, 12 (10.7%) developed pre-eclampsia (Group A), while 100 (89.3%) remained normotensive (Group B). This corresponds to an incidence of pre-eclampsia of 10.7%.

Socio-Demographic Characteristics

The mean age of participants was 27.4 ± 5.3 years (range 17-40), with no significant difference between groups (p=0.91). Most participants were Hausa/Fulani (79.0%), Muslim (85.7%), housewives (65.8%), and had secondary or tertiary education. Parity distribution was similar between groups. None of the socio-demographic variables was significantly associated with pre-eclampsia (Table 1).

Table 1: Socio-demographic characteristics of study participants (n=112).

Clinical Characteristics

Women with pre-eclampsia had significantly higher blood pressures at delivery and delivered earlier than normotensive women. Mean gestational age at delivery was 37.8 ± 2.5 weeks in the pre-eclamptic group versus 38.8 ± 1.5 weeks in the normotensive group (p=0.04). Mean arterial pressure at delivery was significantly higher in pre-eclampsia (112.5 ± 20.3 vs 85.8 ± 9.6 mmHg, p less than 0.001). Birth weight was slightly lower among pre-eclamptics, though not statistically significant (3.08 ± 0.75 vs 3.23 ± 0.38 kg, p=0.20) (Table 2).

Table 2: Clinical characteristics of study participants.

Laboratory Parameters

The mean urinary CCR was significantly lower among women who developed pre-eclampsia (0.09 ± 0.05) compared with normotensive women (0.13 ± 0.07) (p=0.02). Urinary calcium levels were also lower in pre-eclamptics, though not statistically significant, while urinary creatinine levels were higher but not significant (Table 3).

Table 3: Laboratory parameters of study participants.

*Significant at p less than 0.05

Correlation and Predictive Accuracy

Urinary CCR showed significant positive correlations with systolic BP (r=0.67, p=0.04), diastolic BP (r=0.67, p=0.04), and mean arterial pressure (r=0.86, p=0.02) at delivery. The ROC analysis for urinary CCR in predicting pre-eclampsia yielded an AUC of 0.66 (95% CI: 0.50-0.82, p=0.04). The optimal CCR cut-off was 0.094, with sensitivity of 66.7%, specificity of 57.0%, positive predictive value of 15.7%, and negative predictive value of 93.4%.

The ROC curve demonstrated an AUC of 0.66 (95% CI: 0.50-0.82, p=0.041), indicating moderate discriminative ability of CCR for predicting pre-eclampsia. The optimal cut-off was 0.094, which provided the best balance between sensitivity (66.7%) and specificity (57.0%).

Table 4: Receiver Operating Characteristic (ROC) analysis of urinary calcium-to-creatinine ratio (CCR) for prediction of pre-eclampsia.

*Statistically significant at p less than 0.05.

This prospective study demonstrated that a reduced urinary calcium-to-creatinine ratio (CCR) was significantly associated with the development of pre-eclampsia among pregnant women in Sokoto, Nigeria. The mean CCR was lower in women who developed pre-eclampsia compared with normotensive women (0.09 vs. 0.13, p=0.02). Using a cut-off of 0.094, CCR achieved a sensitivity of 66.7% and specificity of 57.0%, with a high negative predictive value (93.4%), indicating its potential utility as a screening tool to rule out pre-eclampsia.

The incidence of pre-eclampsia in this study was 10.7%, which is comparable to earlier reports from Nigeria and other developing countries where the burden of hypertensive disorders in pregnancy is disproportionately high [7-9].

Our finding of significantly reduced CCR among pre-eclamptic women aligns with the work of Munge et al. in India, who reported a CCR ≤0.04 as a predictor of pre-eclampsia with high specificity (94.9%), and Prasad et al., who found that 89% of pre-eclamptic women had CCR ≤0.04 compared to 6% of controls [11,12]. Similarly, Aherwar in India and Vahdat et al. in Iran observed that CCR had good predictive value with sensitivities and specificities exceeding 80% [13-16]. More recently, studies from Nigeria reinforce our findings. Jagu et al. in Delta State reported that CCR measured as early as 8-18 weeks predicted subsequent pre-eclampsia, while Osuji et al. in Lagos also demonstrated significantly reduced urinary calcium levels among pre-eclamptic women [16,17]. Together, these studies highlight the reproducibility of hypocalciuria as a biochemical feature of pre-eclampsia across diverse settings.

Although urinary calcium excretion was lower and creatinine higher in pre-eclamptic women in our study, these differences did not reach statistical significance. This contrasts with some earlier reports, but may be due to sample size limitations or population-specific metabolic variations [10].

The high negative predictive value of CCR observed in this study suggests that it could be a useful, inexpensive screening tool in low-resource settings to identify women at low risk of developing pre-eclampsia. This is particularly important in Nigeria, where access to advanced biomarkers such as sFlt-1/PlGF ratio or uterine artery Doppler studies is limited. While angiogenic biomarkers have shown superior predictive accuracy in high-income settings, their cost and availability remain prohibitive in most African contexts [18-20]. In contrast, CCR is simple, non-invasive, and affordable, making it attractive for routine antenatal care in resource-limited environments.

This study concludes that a reduced urinary calcium-to-creatinine ratio is associated with the development of pre-eclampsia in Nigerian women. CCR demonstrated moderate sensitivity and specificity but high negative predictive value, supporting its use as a simple screening tool to exclude pre-eclampsia risk in low-resource settings. Larger, multicenter studies recruiting women before 20 weeks of gestation and accounting for calcium supplementation are needed to validate its predictive utility and define optimal cut-off values for African populations.

The strength of this study is that it is a prospective design and complete follow-up in over 90% of participants. Standardized laboratory methods were employed, and outcomes were clinically defined according to internationally accepted criteria [1,6].

However, the study has some limitations. First, the relatively small sample size may have limited statistical power to detect differences in some variables. Second, participants were recruited after 18 weeks of gestation; earlier recruitment (before 16 weeks) may enhance predictive accuracy, as demonstrated in other studies [17]. Third, calcium supplementation was not controlled for, though participants were encouraged to maintain their usual diet.

The study was self-funded by the researcher with institutional support from Usmanu Danfodiyo University Teaching Hospital.