Abstract
Introduction
Proteinuria is one of the classical criteria for the diagnosis of pre-eclampsia. The gold standard remains the measurement of 24-h urine protein which is time consuming and prone to preanalytical errors. Random urine protein creatinine ratio (UPCR) is endorsed by clinical practice guidelines as a faster alternative. The aim of this study was to evaluate the correlation between the 24-h urine protein excretion and UPCR in the identification of proteinuria in suspected preeclamptic patients.
Method
A total of 51 women with suspected pre-eclampsia from the maternal fetal clinic of our institution were retrospectively studied. The correlation between the UPCR in random urine samples and protein excretion in the 24-h urine collection was determined by Deming Regression analysis and Pearson correlation on EP evaluator and SPSS respectively.
Result
There was a significant positive correlation between the numerical values obtained by 24-h urine protein and the UPCR (R = 0.88, P < 0.001). Concordance analysis showed 81.1% positive agreement for proteinuria between methods (>300 mg/24hr and >0.3) and 71.4% negative agreement. The clinical sensitivity and specificity of the UPCR was 74% and 69% respectively.
Conclusion
Overall, UPCR was well correlated with 24-h urine protein and could be an effective and compliant screening tool to indicate proteinuria in preeclamptic patients.
Keywords: Pre-eclampsia, Protein-creatinine-ratio, Proteinuria, 24-H urine protein excretion
1. Introduction
Pre-eclampsia remains a leading cause of maternal and perinatal morbidity and mortality, affecting 5–18% of pregnancies worldwide and 5% in the United States [1]. The etiology of pre-eclampsia is not fully known; however, the pathophysiology likely involves maternal and fetal placental dysfunction during the early phase of pregnancy. Pre-eclampsia can be classified into two subtypes: early onset (<34 weeks of gestation) or late onset (≥34 weeks of gestation). However, early onset pre-eclampsia is often associated with adverse outcomes such as early preterm delivery and fetal growth restriction. Pre-eclampsia could also predispose affected women to higher risk of chronic hypertension, ischemic heart disease and stroke [2,3].
Over the years, criteria for the diagnosis of pre-eclampsia have evolved to distinguish it from other hypertensive disorders of pregnancy. The most recent American College of Obstetricians and Gynecologists (ACOG) criteria defines pre-eclampsia as 1) new onset hypertension (≥140/90 mmHg) on two occasions at least 4 h apart after 20 weeks of gestation in a woman with previously normal blood pressure and proteinuria (≥300 mg/24 h) or 2) the new onset of hypertension and significant end-organ dysfunction with or without proteinuria after 20 weeks gestation. Some other organizations include fetal growth restriction as part of the diagnostic criteria [[4], [5], [6]]. Although the newest ACOG guidelines do not require demonstration of proteinuria for the diagnosis of pre-eclampsia, in clinical practice, most patients presenting with gestational hypertension will be diagnosed with pre-eclampsia based on the presence of proteinuria.
The gold standard for assessment of proteinuria is 24-h urine collection. However, this method is time-consuming and sometimes inaccurate due to incomplete collection and other preanalytical variables. As an alternative, proteinuria may be identified by a urine protein/creatinine ratio (UPCR) ≥ 0.3 or a urine protein dipstick reading of 2+, only if other quantitative methods are unavailable due to its high false-positive and false-negative rates [5]. The UPCR is a calculated quantity, representing the ratio of random urine protein excretion to creatinine excretion. Thereby, UPCR serves as a means to normalize protein excretion to the glomerular filtration rate and minimize the effect of daily variations in protein excretion. UPCR is a convenient method, being quick to perform and available at the point of care, but various cutoff values have been reported and its clinical usefulness remains controversial [7].
Yet, several studies have demonstrated that there is a strong correlation between the 24-h urine protein and the UPCR in the identification of proteinuria. A recent meta-analysis of over 3000 individuals demonstrated high accuracy of random UPCR (sensitivity 91%; specificity 89%) compared to the standard 24-h urine collection for the diagnosis of significant proteinuria. Increased accuracy was further observed when the first morning urine void was not used [8].
The obstetrics and gynecology unit of our institution serves patients with high-risk pregnancy therefore we aimed to evaluate the correlation between UPCR and 24-h urine protein excretion in the diagnosis of patients with suspected pre-eclampsia in this high-risk population.
2. Method
We conducted a retrospective study of women with suspected pre-eclampsia who delivered at our institution between January 2015 and October 2021. This study was in accordance with a Baylor College of Medicine Institutional Review Board protocol.
For the 24 h urine protein collection, patients were advised to pass their first morning urine and collect all urine, including the first morning urine of the next day (24 h period). Further instructions included storing the 24 h urine container in the refrigerator and keeping the container on ice if the transportation was more than 30 min. The urine protein was measured by the colorimetric pyrocatechol violet-molybdate dye-binding method. Briefly, protein binds to the pyrocatechol violet-molybdate complex in the presence of oxalate which causes an absorption shift. Urine creatinine was measured by a two-point rate enzymatic reaction assay where creatinine is converted to sarcosine and urea by creatinine amidinohydrolase. The final reaction involves the peroxidase-catalyzed oxidation of a leuco dye to produce a colored product which is then measured spectrophotometrically. Both assays were run on the Vitros 5600 analyzer (Ortho Clinical Diagnostics) [9,10].
Data were obtained from institutional electronic health records. Demographic variables included maternal age, Body Mass Index (BMI), gestational age, gestational age at diagnosis, evidence of gestational diabetes and laboratory parameters for pre-eclampsia such as uric acid, LDH, AST, ALT, 24-h urine protein excretion, random protein, and creatinine concentration for all 51 patients (Table 1). The laboratory parameters for pre-eclampsia and the 24-hr urine protein were obtained at the time each patient presented with clinical symptoms suspected to be pre-eclampsia. The study aimed to compare the accuracy of random UPCR to 24-h urine protein in our high-risk pregnancy population. Only patients with results for both 24-h urine protein and random UPCR during pregnancy were included in the study. The median time between 24-h protein excretion, random protein and creatinine was 7(IQR, 0–7) days. ‘True cases’ were defined as those that the obstetrician diagnosed as having pre-eclampsia and were treated accordingly. The correlation between the UPCR in random urine samples and 24-h urine protein excretion was determined by Deming Regression analysis and agreement between the two methods was assessed using Bland-Altman plot. Multivariate analysis of variance was used to compare the differences observed between the concordant and discordant groups.
Table 1.
Characteristics and clinical variables of patient population.
| Characteristics and clinical variables N = 51 | |
|---|---|
| Age (years) (IQR) | 30 (26–36) |
| Race, N (%) | |
| Asian | 5 (9.8) |
| Black non-Hispanic | 19 (37.2) |
| Hispanic | 13 (25.5) |
| Native Hawaiian and Other Pacific Islander | 1 (2) |
| Unknown | 1 (2) |
| White non-Hispanic | 12 (23.5) |
| BMI (Kg/m3) (IQR) | 35.1 (31–39.7) |
| Gestational diabetes, N (%) | 11 (21.6) |
| Gestational age at diagnosis (weeks) | 31.8 (29–34) |
| Gestational age at delivery (weeks) | 34.1 (32.6–36.6) |
| Mode of delivery, N (%) | |
| Vaginal | 18 (35.3) |
| Caesarean section | 33 (64.7) |
| Lactate Dehydrogenase (LDH) (U/L) (IQR) | 245 (178.5–490) |
| Alanine aminotransferase (ALT) (U/L) (IQR) | 19 (14–32) |
| Aspartate aminotransferase (AST) (U/L) (IQR) | 30.5 (24–42.5) |
| Uric acid (mg/dL) (IQR) | 4.7 (4–6.3) |
| Random urine creatinine (mg/dL) (IQR) | 71.4 (42–114) |
| Random urine protein (mg/dL) (IQR) | 22 (13–94) |
| Timed Urinary protein (mg/24hr) (IQR) | 519 (268–1695) |
| Protein/creatinine ratio (mg/mg) (IQR) | 0.44 (0.17–1.89) |
*BMI-Body Mass Index, IQR-Interquartile range.
3. Results
Patient characteristics and clinical variables are presented in Table 1. All 51 patients had results for 24-h urine protein, random urine protein, creatinine and other laboratory parameters. The median maternal age was 30.8 (IQR, 26–36) years, and the median body mass was 35.1 (IQR, 31–39.7) Kg/m3. Median gestational age at evaluation was 31.8 (IQR, 29–34) weeks. Thirty-six patients (70.6%) had proteinuria (≥300mg/24hr) identified by 24-h urine protein, and there was a significant correlation between the numerical values obtained by 24-h urine protein and UPCR (R = 0.88, P < 0.001) for all 51 patients. For patients with 24-h urine protein at the diagnostic cutoff of ≥0.3mg/24hr, UPCR also correlated well with 24-h urine protein (n = 37; R = 0.87, P < 0.001; slope = 1.1, Intercept = 0.1) (Fig. 1).
Fig. 1.
Scatter plot showing correlation between 24hr urine protein and UPCR (protein creatinine ratio) at 24 h urine protein excretion ≥0.3 mg/24hr using Deming regression. R = 0.87, slope = 1.1, Intercept = 0.1. Blue dotted line indicates the decision limit of >0.3 mg/24hr. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Concordance analysis indicated 81.1% positive agreement and 71.4% negative agreement for proteinuria between methods (Fisher's exact test, p < 0.001). Results were not concordant for 11 (21%) patients, with 7 (13.7%) being positive for proteinuria by 24-h urine but not UPCR, and 4 (7.8%) being positive by UPCR but not 24-h urine. Of note, there was poor correlation between both methods at lower urine protein concentration (<5 mg/mg) and sensitivity was reduced to 67%. No difference was observed in clinical variables such as age, BMI, AST, ALT, LDH and Uric acid for the discordant and non-discordant results (Table 2).
Table 2.
Results of multivariate analysis of variable in the concordant and discordant groups.
| Variables | Mean for concordant values of 24hr urine and UPCR N = 40 | Mean for discordant values of 24hr urine and UPCR N = 11 | p-value |
|---|---|---|---|
| Age (years) | 30.4 (7.1) | 32.3 (5.2) | 0.168 |
| BMI (Kg/m3) | 35.8 (7.8) | 35.6 (5.8) | 0.462 |
| AST (U/L) | 34.5 (20.3) | 37.36 (9.2) | 0.248 |
| ALT (U/L | 31.0 (34.7) | 28.9 (19.7) | 0.400 |
| LDH (U/L) | 354.4 (227) | 295 (158.9) | 0.187 |
| Uric acid (mg/dL) | 5.2 (1.7) | 4.72 (1.2) | 0.97 |
4. Discussion
Pre-eclampsia is a complex disease to manage. Prompt diagnosis is often required to provide optimal management; however a 24-h urine collection may cause delay in patient management and warrant unnecessary hospital admissions. 24-h urine collection is especially challenging from a preanalytical perspective, with incomplete collections or omitted voids resulting in the underestimation of protein excretion. Consequently, the diagnosis of pre-eclampsia may be missed. A random UPCR of ≥0.3 has been established as an alternative marker for proteinuria in pre-eclampsia and offers substantial benefits in both simplicities of collection and time to results.
This study demonstrated a good correlation between UPCR and 24hr urine protein excretion (R = 0.88) which is congruent with other studies. Concordance analysis showed 81.1% positive agreement and 71.4% negative agreement between methods. For the 11 discordant results, there was no difference observed in clinical variables such as maternal age, gestational age at diagnosis, gestational age at delivery, BMI and laboratory parameters. Of the 7 patients that were positive for proteinuria by 24-h urine but not UPCR, the 24-h protein concentration ranged from 349 to 1143 mg/24hr and the random protein concentrations ranged from 9 to 30 mg/dL. Multivariate analysis did not reveal any difference between the concordant and discordant groups and there was no pattern observed to explain the discrepancy.
The correlation observed in this study is similar to the findings of several other studies. Previous studies have also reported good correlation between 24-h urine protein excretion and random UPCR, with correlation coefficients ranging between 0.76 and 0.93 [[11], [12], [13], [14], [15], [16]]. However, few studies have suggested limitations to the utility of UPCR. Lamontagne et al. observed that the diagnostic accuracy of the UPCR was significantly reduced in first morning samples relative to those obtained during the rest of the day [17]. Another study of 220 women with no concurrent diagnosis of chronic hypertension, diabetes mellitus or preexisting renal disease showed a poor correlation between UPCR and 24-h urine protein excretion (r2 = 0.41). Of note, most of the participants in the study of 220 women, had 24-h urine collection obtained via Foley catheter, which suggests that they may not be representative of ambulatory or outpatient patient populations [18]. Our study also demonstrated that at lower urine protein concentrations, there is poor correlation of both methods and the sensitivity of the UPCR method is 67%.
This study also reports an 81.1% positive agreement and a 71.4% negative agreement between both methods. Similarly, larger studies have also reported sensitivity values of 83.6% to100% and specificity values of 76.3% to100%. There are some limitations to this present study. We did not limit the duration between the 24-h urine protein and UPCR sample collection and had no way to identify if the UPCR was based on first morning samples or otherwise. We also did not differentiate nulliparous women from multiparous ones. However, the primary goal of this study was to assess the correlation of both tests as it is being utilized within our institution.
In conclusion, urine protein/creatinine ratio was well correlated with 24-h urine protein and could be an effective tool to increase compliance with screening for proteinuria in suspected pre-eclampsia. However, in doubtful situations, it is recommended to assess proteinuria by 24-h urine protein excretion.
Credit author statement
All have seen and approved final version.
Lily Olayinka: Conceptualization, Methodology, Validation, Writing-Original Draft,
Sridevi Devaraj, Emily Garnett, Brian Burnett: Conceptualization, Writing-Original Draft, Supervision.
Research funding
None.
Declaration of competing interest
Lily Olayinka: None.
Emily Garnett: None.
Brain Burnett: None.
Sridevi Devaraj: None.
Acknowledgements
LO is supported by Ching Nan-Ou endowment for clinical chemistry.
Data availability
Data will be made available on request.
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Associated Data
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Data Availability Statement
Data will be made available on request.