Reference values of urine protein/creatinine ratio in healthy Dalian adults

Abstract Background The urine protein/creatinine ratio (UPCR) is commonly used in current clinical practice. However, there are only few published clinical data on UPCR from large cohorts of Chinese adults. This study aimed to determine the overall and age‐ and sex‐specific UPCR reference values for healthy Dalian adults. Methods According to the Clinical & Laboratory Standards Institute EP28‐A3c guidelines, 1321 healthy Dalian adults (646 men and 675 women) aged 20–69 years were enrolled. Urine protein and creatinine levels were analyzed in the random morning spot urine samples, and UPCR was calculated. The 95th percentile of the UPCR was used as the normal upper limit. The Mann–Whitney U test was used to test differences among groups. Results The UPCR reference value was 141.7 mg/g for the entire cohort, 128.7 mg/g for men, and 150.8 mg/g for women. In addition, women had relatively higher UPCR values than men in the same age group. We also compared the UPCR reference values between different estimated glomerular filtration rate (eGFR) groups and found that women had significantly higher UPCR values than men in the normal eGFR groups. Conclusions This study provides the overall and age‐ and sex‐specific UPCR reference values for healthy Dalian adults.

The National Kidney Foundation practice guidelines recommend that UPCR be measured instead of 24h-UP in most cases of children and adults with CKD, and the recommended reference value is lower than 200 mg/g. 9,10 Another widely used reference value is lower than 150 mg/g, which has been verified by many studies. 7,11,12 However, these reference values were determined using data largely from Caucasian populations and did not take sex and age discrepancies into account. Therefore, we aimed to determine the UPCR reference values of healthy Dalian adults and establish their age-and sex-specific reference values. and March 2021 were included in this study. They were screened by self-reporting, specialized physicians' examinations, and clinical laboratory examinations. The inclusion criteria were as follows: (1) age 20-69 years and apparently healthy adults; (2) no self-reported acute or chronic disease, pregnancy, and infections; (3) systolic blood pressure <140 mm Hg and diastolic blood pressure <90 mm Hg; (4) no hyperlipidemia, triglyceride <1.70 mmol/L, and total cholesterol <5.18 mmol/L; (5) negative dipstick testing for protein, glucose, blood cells, and white cells; and (6) biomarkers for coagulation, liver function, and renal function within the reference intervals used in our clinical laboratory.

Enrolment of healthy individuals and data analysis followed the
Clinical & Laboratory Standards Institute EP28-A3c guidelines. 13 A total of 1321 participants (646 men and 675 women) were included in the study.

| Ethical statement
This study was approved by the Human Ethics Committee of Dalian Municipal Central Hospital and performed in accordance with the Declaration of Helsinki.

| Urine protein and creatinine analysis
Random morning urine samples were collected after dipstick testing showed negative results for protein, glucose, blood cells, and white cells, stored at 4°C, and centrifuged at 212 g for 5 min. Total protein and creatinine levels in urine were measured within 2 h after collection using the colorimetric method (Randox Laboratories) enzyme colorimetric method on an ADVIA Chemistry XPT autoanalyzer (Siemens), respectively. To ensure that the results were reliable, low-and high-quality control of total protein and creatinine was conducted daily. We reviewed all available clinical and laboratory data on the subjects according to our inclusion criteria. We calculated the estimated glomerular filtration rate (eGFR) using the 2009 CKD-Epidemiology Collaboration (EPI) equation. 14

| Statistical analysis
Skew distributional data are expressed as median and interquartile range. The 95th percentile was used to identify the normal upper limit of UPCR for all participants and for men and women in different age groups separately. For skew distributional variables, the Mann-Whitney U test was used to test differences among groups.
Statistical analyses were performed using SPSS (version 23.0; IBM Corporation). All statistical analyses were performed using twosided tests, and statistical significance was set at p < 0.05.

| RE SULTS
Random morning urine samples were obtained from all eligible participants (N = 1321), including 646 men and 675 women. As shown in Table 1, there was no significant age difference between the sexes (p = 0.192), and men had higher serum creatinine (Scr), UP, and urine creatinine (Ucr) values but lower eGFR values than women (p < 0.0001). Table 2

| DISCUSS ION
Healthy individuals normally excrete very small amounts of protein in their urine; persistently high protein excretion is an indication of kidney damage. 9 Accurate assessment of pathological proteinuria is important in the diagnosis, disease activity monitoring, and prognosis of patients with CKD. 15 Owing to its simplicity and con- for the whole cohort was 141.7 mg/g which is obviously lower than that recommended by the National Kidney Foundation guidelines (200 mg/g), 9 and slightly lower than the commonly used value (150 mg/g) according to previous studies. 3,7,11,12,16 This discrepancy may be explained based on some aspects. First, the detection method and equipment for UP and Ucr used here have inevitable differences with those used in previous studies. Second, because the reported UPCR reference values were determined using data mainly obtained from Caucasian subjects, the racial differences in dietary protein intake, muscular mass, and physical exercise may result in the discrepancy. 17 Finally, the relatively stricter inclusion criteria used in our study may partly account for this discrepancy.
Similar to the findings of the study by James et al., we found that men excreted more creatinine than women, 18  results, but they mainly studied urine albumin to creatinine ratio in healthy Chinese adults. 19 We also observed that from the 40-49 to 60-69 age groups, an increase in age was related to an increase in the median and 95th percentile UPCR. The relative decrease in skeletal muscle mass and kidney function may partly account for this phenomenon. [20][21][22] Therefore, it is necessary to establish different reference ranges for UPCR for different age groups.
At present, GFR is considered the best measure of overall kidney function. 10 The eGFR was calculated using the 2009 CKD-EPI equation. Similar to the findings of Kosmadakis et al, 23 we found that a high eGFR corresponded to a lower median and 95th percentile of UPCR. In addition, we found that women had significantly higher UPCR than men did in the normal eGFR group (eGFR≥90 ml/

ACK N OWLED G M ENTS
The authors acknowledge all of the colleagues in the department of health physical examination center who had contributed to this study.

CO N FLI C T O F I NTE R E S T
The authors report that they have no conflict of interest.

DATA AVA I L A B I L I T Y S TAT E M E N T
All data included in this study are available upon request by contact with the corresponding author.