What is the correlation between spot urine protein-to-creatinine ratio and daily urinary protein excretion?

From the Guidelines

The spot urine protein-to-creatinine ratio is the most reliable method to estimate daily urinary protein excretion, with a ratio of 500 mg/g approximating a daily protein excretion of 500 mg/day, as supported by the most recent evidence from 1. To estimate daily urinary protein excretion from a spot urine sample, the protein-to-creatinine ratio (PCR) is the most reliable method.

Key Points

• A spot urine PCR (measured in mg/g or mg/mmol) provides a reasonable approximation of 24-hour protein excretion (in mg/day or g/day).
• For practical purposes, a PCR value in mg/g can be considered roughly equivalent to the daily protein excretion in mg/day.
• The first morning void typically provides the most accurate results, though random samples are acceptable for clinical purposes.
• This method is preferred over 24-hour collections because it's more convenient for patients and less prone to collection errors.
• However, in certain situations like pregnancy or rapidly changing renal function, a 24-hour collection may still be necessary for precise measurements.
• The correlation is most accurate in the nephrotic range (>3.5 g/day) and may be less reliable at lower levels of proteinuria, as noted in 1 and 1.

Clinical Considerations

• The American Diabetes Association and the National Kidney Foundation recommend assessment of proteinuria to detect chronic kidney disease, with the ratio of protein or albumin to creatinine in an untimed urine specimen being the preferred method for measuring proteinuria, as stated in 1 and 1.
• A proposed algorithm for testing for proteinuria distinguishes persons at increased risk for chronic kidney disease from asymptomatic, healthy persons, with a sample of urine from the first voiding after awakening being preferred, but a random specimen being acceptable, as outlined in 1, 1, and 1.

From the Research

Correlation Between Spot Urine Protein-to-Creatinine Ratio and Daily Urinary Protein Excretion

• The correlation between spot urine protein-to-creatinine ratio and daily urinary protein excretion has been studied in several research papers 2, 3, 4, 5, 6.
• A study published in 2006 found a logarithmic relationship between the spot protein-to-creatinine ratio and 24-hour protein excretion, with a correlation of 0.92 (P < 0.0001) 2.
• Another study published in 2019 found a correlation coefficient of 0.819 between spot urine protein-to-creatinine ratio and 24-hour proteinuria, and derived a prediction equation to estimate daily proteinuria 3.
• A study published in 2009 found a very good correlation (r = 0.832, P < 0.0001) between spot urine protein-to-creatinine ratio and 24-hour urine protein excretion 4.
• A study published in 2004 found a good correlation between protein-to-creatinine ratio in spot urine specimens and urinary protein excretion in 24-hour collections, but only for patients with creatinine clearance > 10 ml/min 5.
• A study published in 2015 found that multiplying the spot urine protein-to-creatinine ratio by estimated daily urine creatinine excretion improved the accuracy of predicting daily proteinuria 6.

Prediction of Daily Proteinuria

• The spot urine protein-to-creatinine ratio can be used to predict daily proteinuria, but its accuracy may be limited at higher levels of protein excretion 2, 3, 4.
• The prediction equation derived by the 2019 study can accurately predict 24-hour proteinuria in patients with daily proteinuria below 3g 3.
• The spot urine protein-to-creatinine ratio can reliably predict protein excretion thresholds of 0.3,0.5,1.0, and 3.0 g/day 2.
• The protein-to-creatinine ratio in spot urine specimens is an accurate, convenient, and reliable method to estimate protein excretion in urine, but only when proteinuria is at reasonably low levels 4.

Limitations and Variability

• The agreement between spot and daily estimates of proteinuria was less pronounced with proteinuria > 3g than lower values in Bland-Altman analysis 2, 3.
• The correlation between spot urine protein-to-creatinine ratio and 24-hour protein excretion may be affected by factors such as creatinine clearance and daily urine creatinine excretion 5, 6.
• The spot urine protein-to-creatinine ratio may not be reliable in patients with high protein excretion and should not be used in the clinical setting unless 24-hour urine collection is unavailable 2.