Measurement of urine protein concentration in a 24-hour urine collection is considered to be the gold standard for assessment of proteinuria. However, patients do not like to collect them, and they often submit an incomplete urine collection. Calculation of the protein to creatinine ratio (P/C) in a single random urine sample has been shown to be a reliable alternative.
In 2002, the National Kidney Foundation (NKF),in its recommendations for the evaluation, classification and stratification of chronic kidney disease, indicated that children and adults should be screened for proteinuria using a urine dipstick on a random urine sample. Patients with a positive dipstick test (1þ or greater) should have proteinuria confirmed by the P/C ratio or albumin to creatinine ratio (ACR) within a period of 3 months. A timed 24-hour collection was not necessary.
The 2012 National Kidney Foundation (USA) guidelines for defining the three categories of albuminuria and the corresponding values of proteinuria measured by various methods: albumin excretion rate (AER), albumin to creatinine ratio (ACR), P/C ratio, and protein excretion rate (PER).
| Test | Units | A1-Normal to Mildly Increased | A2-Moderately Increased | A3-Severely Increased |
| AER | mg/24h | <30 | 30-300 | >300 |
| PER | mg/24/h | <150 | 150-500 | >500 |
| ACR | mg/g | <30 | 30-300 | >300 |
| P/C ratio |
mg/mg mg/g |
<0.15 <150 |
0.15-0.50 150-500 |
>0.50 >500 |
| Dipstick | Negative to trace | Trace to 1+ | 1+ or greater |
P/C ratio may be expressed in different units including. mg/mg, mg/g, and mg/mmol. It is often reported simply as a numerical value without unit. The reference value for urine P/C is < 0.2 mg protein/mg creatinine (200 mg/g). A commonly used cutoff for proteinuria during pregnancy is 0.3 mg/mg (300 mg/g). Patients with nephrotic syndrome have P/C ratios of 3.5 mg/mg (3500 mg/g).
The National Kidney Foundation (NKF) defined proteinuria in children as normal to minimal when the P/C ration is 0.5 mg/mg (500 mg/g), increased when it is 0.5 – 2.0 mg/mg (500 to 2000 mg/g) and nephrotic when it exceeds 2.0 mg/mg (2000 mg/g). Adult cutoffs are recommended for children above 2 years of age.
Reference
Kaminska J et al. Diagnostic utility of protein to creatine ratio (P/C ratio) in spot urine sample within routine clinical practice. Crit Rev Clin Lab Sciences, published online February 14, 2020, https://doi.org/10.1080/10408363.2020.1723487
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