What is a normal protein-to-creatinine (P/C) ratio?

Normal Protein-to-Creatinine Ratio

A normal protein-to-creatinine ratio (PCR) in a random urine sample is less than 150 mg/g (or <15 mg/mmol). 1

Understanding Protein-to-Creatinine Ratio

The protein-to-creatinine ratio is a convenient and reliable method to estimate proteinuria without requiring a 24-hour urine collection. This ratio corrects for variations in urine concentration due to hydration status and is far more convenient than timed collections.

Normal Range Values

According to the KDIGO guidelines, normal to mildly increased proteinuria is categorized as:

• PCR: <150 mg/g or <15 mg/mmol 1
• Protein excretion rate (PER): <150 mg/24h 1

For albumin specifically (albumin-to-creatinine ratio or ACR):

• ACR: <30 mg/g or <3 mg/mmol 1, 2
• Albumin excretion rate (AER): <30 mg/24h 1, 2

Clinical Significance and Interpretation

Categories of Proteinuria

Proteinuria is classified into three categories based on PCR values:

1. Normal to mildly increased: PCR <150 mg/g (<15 mg/mmol) 1
2. Moderately increased: PCR 150-500 mg/g (15-50 mg/mmol) 1
3. Severely increased: PCR >500 mg/g (>50 mg/mmol) 1

Diagnostic Thresholds

Early research established that:

• PCR <100 mg/g in untimed urine samples indicates normal kidney function 3
• PCR between 100-2000 mg/g may reflect various types of kidney disease 3
• PCR >2000 mg/g typically indicates nephrotic-range proteinuria 3, 4

Clinical Application

Preferred Collection Method

• A first morning void sample is preferred to avoid orthostatic proteinuria 2
• Random samples are acceptable but may have more variability 1
• Patients should avoid vigorous exercise 24 hours before collection 2

Confirming Abnormal Results

For accurate diagnosis of chronic kidney disease:

• Persistent abnormalities should be confirmed with repeat testing over 3 months 1, 2
• Single measurements may not be reliable due to biological variability (>20% between measurements) 2

Special Populations

In children:

• Normal PCR is ≤0.2 g/g 1
• More frequent screening is required due to laboratory changes associated with growth and development 1

Advantages Over 24-Hour Collections

The PCR in random urine samples offers several advantages:

• Strong correlation with 24-hour protein excretion (r=0.91-0.97) 3, 5, 6
• Eliminates collection errors and patient inconvenience 4
• Provides immediate results for clinical decision-making 5

Common Pitfalls to Avoid

1. Relying solely on dipstick testing: Standard dipsticks have lower sensitivity for detecting low-grade proteinuria 2
2. Ignoring borderline results: Values near cutoff points should be confirmed with repeat testing 2
3. Failing to consider factors affecting results: Fever, exercise, and urinary tract infections can cause transient proteinuria 3
4. Not confirming persistence: A single elevated PCR is insufficient for diagnosing kidney disease; persistence over 3 months is required 1, 2

By understanding the normal range and proper interpretation of protein-to-creatinine ratios, clinicians can effectively screen for, diagnose, and monitor kidney disease while avoiding the inconvenience and potential inaccuracies of 24-hour urine collections.