What is the protein-to-creatinine (P/C) ratio in urine?

From the Guidelines

The protein-to-creatinine (P/C) ratio in urine is a crucial diagnostic tool for detecting and monitoring kidney disease, with a recommended cutoff value for an albumin-creatinine ratio greater than 30 mg/g in men and women, as suggested by the National Kidney Foundation practice guidelines 1.

Key Points

• The P/C ratio is used to measure the amount of protein being excreted in the urine relative to creatinine levels, which is essential for detecting kidney damage.
• A normal P/C ratio is generally less than 0.2 mg/mg (or less than 20 mg/mmol in SI units), and values above this threshold suggest abnormal protein excretion.
• The test is particularly useful for screening for conditions like diabetic nephropathy, preeclampsia in pregnant women, and various forms of glomerulonephritis.
• The ratio works because creatinine is excreted at a relatively constant rate, so it serves as a reference to normalize protein excretion regardless of urine concentration.
• Higher ratios correlate with greater protein excretion and often indicate more severe kidney damage.

Clinical Application

• Regular monitoring of the P/C ratio can help track disease progression and response to treatment in patients with known kidney disorders.
• The National Kidney Foundation practice guidelines recommend using the albumin-creatinine ratio or total protein-creatinine ratio to monitor proteinuria in adults with chronic kidney disease 1.
• The guidelines also suggest that adults at increased risk for chronic kidney disease with a negative result for protein on a standard dipstick test should undergo testing with either an albumin-specific dipstick or an untimed urine measurement for the albumin-creatinine ratio 1.

Important Considerations

• The P/C ratio is a convenient and practical diagnostic tool, as it requires only a single urine sample, making it more accessible for patients compared to 24-hour urine collection.
• The ratio is an accurate estimate of the protein or albumin excretion rate, and it corrects for variations in urinary protein concentration due to hydration 1.

From the Research

Protein-to-Creatinine (P/C) Ratio in Urine

The protein-to-creatinine (P/C) ratio in urine is a measure used to assess proteinuria, which is the presence of excess proteins in the urine. This ratio is calculated by dividing the amount of protein in a urine sample by the amount of creatinine in the same sample.

Calculation and Interpretation

• The P/C ratio is highly correlated with 24-hour urine protein excretion, with a correlation coefficient of 0.91 2.
• A P/C ratio of 0.25 g/g can reliably predict 24-hour urine total protein equivalent "thresholds" at 0.2 g/day, while a ratio of 0.66 g/g can predict thresholds at 1.0 g/day, and a ratio of 2.55 g/g can predict thresholds at 3.5 g/day 3.
• A spot urine P/C ratio >20 mg/mmol (0.2 mg/mg) is commonly reported as a cutoff value for detecting proteinuria, while a ratio >350 mg/mmol (3.5 mg/mg) confirms nephrotic proteinuria 4.
• The International Society for the Study of Hypertension in Pregnancy recommends a P/C ratio of 30 mg/mmol (0.3 mg/mg) for the classification of proteinuria in pregnant women at risk of preeclampsia 4.

Diagnostic Accuracy

• The P/C ratio has been shown to have a high diagnostic accuracy in estimating 24-hour proteinuria, with areas under the curve of 0.99 (95% CI: 0.97-1.00) and 0.99 (95% CI: 0.99-1.00) for cut-off levels of 0.20 g and 3.5 g, respectively 5.
• The P/C ratio can be used to estimate the urine albumin-to-creatinine ratio (ACR), with equations developed to estimate ACR from P/C ratio allowing for modification by specified covariates 6.
• The estimated ACR from P/C ratio can be used to predict kidney failure risk, with similar estimates obtained using measured ACR and estimated ACR derived from P/C ratio 6.