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 a reliable and practical alternative to 24-hour urinary protein excretion for clinical assessment, with a ratio of approximately 1000 mg/g corresponding to about 1 gram of protein in a 24-hour collection. This correlation is supported by the most recent and highest quality study, which suggests that spot urine protein measurements correlate reasonably well with 24-hour urine protein collections 1. The protein-to-creatinine ratio (PCR) from a random urine sample is the most reliable spot method, with values reported in mg/g or mg/mmol of creatinine.

Key Points to Consider

• First morning void samples are preferred as they minimize the effects of posture and activity on protein excretion.
• Correlation is less accurate at higher levels of proteinuria (>3.5 g/day), where 24-hour collections may be more appropriate.
• Factors affecting this correlation include hydration status, exercise, timing of collection, and kidney function variability.
• For routine monitoring of conditions like diabetic nephropathy or to screen for significant proteinuria, spot PCR provides sufficient accuracy while being more convenient for patients.
• When precise quantification is needed for conditions like nephrotic syndrome or when making critical treatment decisions, a complete 24-hour collection remains the gold standard, as noted in the study published in Hypertension in 2018 1.

Clinical Implications

The use of spot urine protein-to-creatinine ratio can simplify the diagnosis and monitoring of proteinuria, making it a valuable tool in clinical practice. However, it is essential to consider the limitations and potential factors that can affect the correlation between spot urine protein measurements and 24-hour urine protein collections. As stated in the study published in the American Journal of Kidney Diseases in 2003, a spot urine total protein to creatinine ratio of 500 to 1,000 mg/g is acceptable for measuring proteinuria at very high levels 1. Nevertheless, the most recent study from 2018 provides more comprehensive guidance on the use of spot urine protein-to-creatinine ratio in clinical practice 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 various research papers 2, 3, 4, 5, 6.
• A study published in 2006 found a significant correlation between spot morning urine protein creatinine ratio and 24-hour urinary total protein excretion rate in non-diabetic chronic renal disease patients 2.
• Another study published in 2010 found an excellent concordance between 24-hour protein excretion and spot urine protein-to-creatinine ratio, with a correlation coefficient of 0.931 3.
• A study published in 2009 found a very good correlation between spot urine protein-to-creatinine ratio and 24-hour urine protein excretion, with a correlation coefficient of 0.832 4.
• A study published in 2014 found a positive correlation between spot urine protein-to-creatinine ratio and 24-hour urine protein excretion, with a correlation coefficient of 0.7459 5.
• A study published in 2015 found that spot urine estimations are equivalent to 24-hour urine assessments of urine protein excretion for predicting clinical outcomes, including CKD progression, end-stage renal disease, and mortality 6.

Factors Affecting the Correlation

• The correlation between spot urine protein-to-creatinine ratio and daily urinary protein excretion may be affected by factors such as kidney function and level of proteinuria 3, 4.
• A study found that the difference between 24-hour protein excretion and spot urine protein-to-creatinine ratio was higher in patients with lower kidney function 3.
• Another study found that the limits of agreement between spot urine protein-to-creatinine ratio and 24-hour urine protein excretion became wider as protein excretion increased 4, 5.

Clinical Implications

• The use of spot urine protein-to-creatinine ratio as a reliable alternative to 24-hour urinary protein excretion has been suggested in several studies 2, 3, 4, 5.
• Spot urine protein-to-creatinine ratio can be a convenient and reliable method for estimating protein excretion in urine, especially in patients with low levels of proteinuria 4, 5.
• The results of these studies support the use of spot urine protein-to-creatinine ratio in clinical practice and research for assessing urine protein excretion and predicting clinical outcomes 6.