Abstract
This cross-sectional study was undertaken to compare 24-h urine protein estimation with spot urine protein-creatinine ratio (PCR). 72 patients having proteinuria of > 150 mg/day were included in this study. 24-h urine total protein (UTP) test were also asked to give their mid-morning spot urine sample the following day for urine protein-creatinine ratio. The subjects were grouped according to their UTP into Groups as described follows: Group A < 1 g/day, Group B 1–3.5 g/day and Group C > 3.5 g/day. Pearson’s correlation (r) and paired Student’s T test were done. The paired T test showed no difference in the UTP and PCR in the subgroup having qualitative protein of zero and in group A. There was no agreement between the two methods used for quantitation. In the overtly proteinuric patients, the correlation between the UTP and PCR was positive in group A which was statistically significant (p = 0.05) whereas it was not significant (p = 0.07) in group B. Though there was a negative correlation in the massive proteinuria group C it was not significant (p = 0.65). Our study demonstrates that spot PCR can be reliable parameter for initial diagnostic criterion of proteinuria but for follow-up measurements and for proteinuria > 0.5 gm/day, 24-h UTP is the single most accurate measurement of protein excretion.
Keywords: 24-h urine protein, Chronic kidney disease, Urine protein detection, Protein/creatinine
Introduction
Chronic kidney disease (CKD), usually secondary to diabetes mellitus or hypertension is asymptomatic for a long time and symptoms arise when the disease has progressed to end stage renal disease (ESRD) [1]. Early detection and treatment of CKD by the finding of albuminuria/proteinuria is very cost-effective and can decrease the risk of kidney failure progression and cardiovascular disease by up to 50% [2]. The gold standard for urine protein estimation is from a 24-h urine sample. As it is cumbersome and inconvenient to collect and delays management, protein-creatinine ratio (UPCR) was introduced as a rapid test from spot urine sample. According to the current Kidney Disease Initiative Global Outcomes (KDIGO) guidelines there is insufficient evidence of whether to estimate urine protein a 24 hr sample or timed or in a spot sample for protein or albumin is to creatinine ratio [3]. In non diabetic individuals, it is not established so far whether dipstick protein testing or estimation of either urine albumin or total protein is better for detecting CKD [4]. In clinical practice urine dipstick or reagent strips for albumin or total protein has been recommended for screening CKD [5]. They can be manually tested, which are more subjective and their specificity and sensitivity are significantly lower than the automated devices. However, neither of them can substitute laboratory measurement of protein [4]. As we receive many requests for spot urine protein creatinine ratio of patients diagnosed with renal pathology, we undertook this study to compare 24-h urine protein estimation with spot UPCR.
Materials and Methods
In this cross-sectional study conducted in a tertiary care medical college in south India, only patients having proteinuria of > 150 mg/day were included in this study.72 patients from various wards who were advised for a 24-h urine total protein (UTP) test, were also asked to give their mid-morning spot urine sample for spot urine protein–creatinine ratio as a routine follow-up test during their hospitalization. There were 45 males and 27 females, with an age range of 2–70 years. Qualitative urinary protein estimation was done by dipstick. Protein in urine was estimated using pyrogallol red method and creatinine by Jaffe’s method. The urine UPCR were calculated by dividing the urine protein concentration (mg/L) by urine creatinine concentration (mg/L) multiplied by 10 and expressed without units as it was mg/mg. The subjects were grouped according to their UTP into Groups as described follows: Group A < 1 g/day, Group B 1–3.5 g/day and Group C > 3.5 g/day. Pearson’s correlation (r) and paired Student’s T test was done to assess the relation between the two methods of urine protein estimation. Significance was set at 0.05.
Results
All our subjects were diagnosed with renal diseases like nephrotic syndrome, CKD or acute glomerulonephritis. Those urine samples of the study subjects which showed a qualitative protein of zero were of patients with renal pathology. The urine output ranged from 200 to 4200 ml/day (Table 1). The paired T test showed no difference in the UTP and UPCR in the subgroup having qualitative protein of zero (Table 2) and in group A which had UTP of < 0.5/day (Table 3). There was no agreement between the two methods used for quantitation in the overtly proteinuric patients who had a qualitative proteinuria of 1 to 4 which was statistically significant (p < 0.01) (Table 2). The correlation between the UTP and UPCR (Table 3) was positive in group A which was statistically significant (p = 0.05) whereas it was not significant (p = 0.07) in group B. Though there was a negative correlation in the massive proteinuria group C it was not significant (p = 0.65). The correlation between the methods was varied in 3 groups and the comparison also showed significant differences in the pathological ranges of total protein quantitation in 24 h.
Table 1.
General characteristics of the study population
| Mean | Std. Deviation | Minimum | Maximum | |
|---|---|---|---|---|
| Age (years) | 41.0 | 21.0 | 2.0 | 70.0 |
| 24 h Urine Total Volume(ml) | 1867 | 903 | 200 | 4200 |
| UTP (g/day) | 1.8 | 2.1 | 0.16 | 9.8 |
| PCR | 4.70 | 5.95 | 0.03 | 24 |
UTP-Urine Total Protein; PCR-Protein Creatinine Ratio
Table 2.
Comparison of urine total protein with protein creatinine ratio grouped by the urine qualitative protein test
| n | Qualitative for urine protein | Urine total protein (g/day)a | Protein creatinine Ratioa | Pb | P for Urine Protein qualitative 1–4 and PCR |
|---|---|---|---|---|---|
| 28 | 0 | 0.3 (0.2) | 0.3 (0.3) | 0.48 | |
| 18 | 1 | 1.6 (1.1) | 3.4 (3.1) | 0.01 | < 0.01 |
| 4 | 2 | 2.2 (1.1) | 5.0 (4.6) | 0.30 | |
| 4 | 3 | 3.4 (1.5) | 14.2 (2.5) | 0.01 | |
| 18 | 4 | 4.0 (2.8) | 10.7 (6.5) | 0.00 |
aExpressed as mean (Standard Deviation); bis for paired T test for UTP and PCR
Table 3.
Comparison and correlation of urine total protein (UTP) with protein creatinine ratio grouped according UTP
| Groups | Urine total protein (UTP) in (g/day) | n | Pa | ra | Pb |
|---|---|---|---|---|---|
| A | < 0.5 | 28 | 0.098 | 0.37 | 0.05 |
| B | 0.51–3.5 | 31 | 0.001 | 0.33 | 0.07 |
| C | > 3.5 | 13 | 0.009 | -0.14 | 0.65 |
aIs for paired T Test. ‘r’ Pearsons correlation; bis the p value for ‘r’
Discussion
Normal protein excretion is < 150 mg/day amounting to 0-14 mg/dl, and the minimum amount of protein that can be detected by dipsticks and other qualitative tests are 15–30 mg/dl [6]. With a normal urine volume, the total protein would be approximately 450 mg/day. In CKD, the urine volume is generally high further decreasing protein concentration in spot samples. Moreover, the dipsticks are mostly albumin specific [1]. Depending on the spot urine protein concentration there are false negative and false positive reactions [7]. All these limits the use of qualitative urine detection as there is limited correlation with quantitative estimation [8].
The presence of proteinuria is a sign of kidney disease and is a strong marker for progression of both acute and chronic kidney disease. Independent of the underlying cause of proteinuria, presence of high protein levels in the tubules may bring forth inflammatory and fibrotic effects that can add to chronic tubulointerstitial injury. As a consequence, there is progression in renal damage [9]. Hence, it is important not only to detect proteinuria but also monitor it during treatment. That can be done by quantitative measurement. Our results showed that there was a good correlation between UTP and UPCR at UTP levels < 0.5 gm/day (group A) and there was no difference between the two methods (p = 0.05). There was no agreement among the two methods at higher levels of UTP (groups B and C). Wahbeh et al. [10], Medina et al. [11], and Rydzewska-Rosołowska et al. [3] also obtained similar results like ours, but in contrast, Khan et al. [8] in their study concluded that protein creatinine index and UPCR are reliable and comparable to UTP. Montero et al. reported that both methods are comparable at lower UTP levels (our study groups A and B) but not in nephrotic ranges (groups C in our study) [12]. Patil et al. reported that both methods are comparable in groups B and C but not in A [13]. In a similar study conducted in children [14], they suggested that though the UPCR correlated well with 24-h urine protein, the cut-off for diagnosing nephrotic range proteinuria in children can be considered at UPCR > 2.0 instead of UTP of > 1gm/ m2/day.
The strength of our study is that the 24-h urine samples were collected from patients admitted in the ward, thereby ensuring accurate specimen collection. We collected the mid-morning spot urine sample the same day that the 24-h urine sample collection ended, in doing so we avoided the day to day variation in protein excretion. All samples were processed the same day. We also included subjects with a wide range of proteinuria: from 160 mg to 9.8 g/day.
The limitation of our study was that we did not compare the proteinuria with glomerular filtration rate (GFR) which is also an important measure of kidney function. We did not study any particular disease group like, nephrotic syndrome or glomerulonephritis or CKD. However, our estimation of urine total proteins would include albumin also. This embraces the spectrum of both glomerular and tubular diseases. Quoting Theodore Roosevelt Jr. “Comparison is the thief of joy”, the convenience of one test that is UPCR cannot be compared to the accuracy of the other that is, the UTP. However, the advice for a particular test depends on utilizing the benefit of the estimation with the goal of having a better outcome for the patient.
Conclusion
In conclusion, our study demonstrates that spot UPCR can be reliable parameter for initial diagnostic criterion of proteinuria but for follow-up measurements and for proteinuria > 0.5 gm/day, 24-h UTP is the single most accurate measurement of protein excretion.
Acknowledgements
This study was non-funded project. We acknowledge the support of our study participants for their consent and cooperation.
Author contributions
The authors SS, JJ, AA conceptualized the project, carried out the experiment and approved the final manuscript. SS did the data analysis and prepared the draft manuscript.
Funding
No funding was received for conducting this study.
Availability of data and material
The deidentified data is published in Mendeley Data, and is cited as Sahu, Suchanda (2020), “24 h urine protein vs UPCR”, Mendeley Data, V1, https://doi.org/10.17632/4c9z3n29pw.1
Compliance with ethical standards
Conflict of interest
The authors declare that they have no competing interest in the study conducted.
Ethical approval
Ethical approval was waived by the local Ethics Committee of Sree Narayana Medical College, Cochin in view of the retrospective nature of the study and all the procedures being performed were part of the routine care.
Footnotes
Publisher's Note
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Contributor Information
Suchanda Sahu, Email: biochem_suchanda@aiimsbhubaneswar.edu.in.
Joseph John, Email: peds_joseph@aiimsbhubaneswar.edu.in.
Asha Augusty, Email: asha.augusty@gmail.com.
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Associated Data
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Data Availability Statement
The deidentified data is published in Mendeley Data, and is cited as Sahu, Suchanda (2020), “24 h urine protein vs UPCR”, Mendeley Data, V1, https://doi.org/10.17632/4c9z3n29pw.1