Normal Protein-to-Creatinine Ratio Values
The normal protein-to-creatinine ratio (PCR) in urine is less than 150 mg/g (<15 mg/mmol). 1
Reference Ranges for Protein-to-Creatinine Ratio
Protein-to-creatinine ratio measurements are categorized into three main ranges according to the Kidney Disease Improving Global Outcomes (KDIGO) guidelines:
- Normal to mildly increased (A1): PCR <150 mg/g (<15 mg/mmol) 1
- Moderately increased (A2): PCR 150-500 mg/g (15-50 mg/mmol) 1
- Severely increased (A3): PCR >500 mg/g (>50 mg/mmol) 1
Clinical Significance and Interpretation
- A PCR value less than 100 mg/g in untimed urine samples, obtained in the absence of exercise, fever, or other evidence of urinary tract disease, is considered a criterion of normal kidney function 2
- PCR values between 100-2000 mg/g may reflect various types of kidney disease 2
- PCR values exceeding 2000 mg/g typically indicate nephrotic-range proteinuria 2
- In pediatric patients, a PCR <0.2 g/g is considered normal, while values >3.5 g/g correlate with massive proteinuria 1, 3
Relationship to Other Protein Measurements
The PCR correlates with other measures of proteinuria:
- Albumin-to-creatinine ratio (ACR): Normal ACR is <30 mg/g (<3 mg/mmol) 1
- 24-hour protein excretion rate (PER): Normal PER is <150 mg/24h 1
- 24-hour albumin excretion rate (AER): Normal AER is <30 mg/24h 1
Clinical Applications
- PCR measurements in spot urine samples show excellent correlation with 24-hour urine protein measurements (r = 0.97), making them a reliable alternative to cumbersome timed collections 2, 4, 5
- For screening purposes, automated dipstick urinalysis can be used initially; if positive (≥1+), PCR testing should be performed 1, 6
- To confirm persistent proteinuria, repeat testing should show elevated values in at least 2 of 3 samples over a 3-month period 6
Special Considerations
- First morning void samples are preferred for PCR measurement, though random specimens are acceptable for initial screening 6
- The correlation between PCR and 24-hour protein excretion may be less reliable at higher levels of proteinuria (>2 g/day) 7
- PCR measurements may be affected by factors such as vigorous exercise, fever, and urinary tract infections 6, 2
- Creatinine excretion varies with age, sex, race, and diet; therefore, the relationship between PCR and actual protein excretion is approximate 1
Common Pitfalls
- Relying solely on dipstick testing without confirmation by quantitative PCR measurement 6
- Failing to refrigerate urine samples if they cannot be analyzed immediately 6
- Not accounting for factors that can temporarily increase proteinuria, such as exercise, fever, or urinary tract infections 6, 2
- Using PCR cutoffs without considering patient-specific factors like age, sex, and muscle mass that affect creatinine excretion 1