How is the protein creatinine ratio (PCR) calculated?

How to Calculate Protein-to-Creatinine Ratio (PCR)

The protein-to-creatinine ratio (PCR) is calculated by dividing the urinary protein concentration by the urinary creatinine concentration from the same urine sample. 1

Formula and Units

• Formula: PCR = Urinary protein concentration ÷ Urinary creatinine concentration
• Common units:
  • mg/g (milligrams of protein per gram of creatinine)
  • mg/mmol (milligrams of protein per millimole of creatinine)

Conversion Between Units

• To convert from mg/g to mg/mmol, multiply by 0.113
• To convert from mg/mmol to mg/g, multiply by 8.84

Sample Collection Requirements

For optimal PCR calculation:

1. Preferred sample: First morning void midstream urine sample 1
2. Alternative: Random untimed urine sample (less preferred but acceptable) 1
3. Sample handling:
   • Can be analyzed fresh
   • Can be stored at 4°C for up to 1 week
   • Can be stored at -70°C for longer periods
   • Avoid freezing at -20°C as it may result in loss of measurable protein 1

Interpretation of Results

PCR values correlate with 24-hour protein excretion:

PCR Value (mg/g)	PCR Value (mg/mmol)	Interpretation
<150	<15	Normal to mildly increased [1]
150-500	15-50	Moderately increased [1]
>500	>50	Severely increased [1]
>3500	>350	Nephrotic range proteinuria [2]

Clinical Considerations

1. Advantages over 24-hour collection:

   • More convenient
   • Not affected by collection errors
   • Correlates well with 24-hour protein excretion 3

2. Factors affecting interpretation:

   • Sex: Females have lower urinary creatinine excretion, resulting in higher PCR values for the same protein excretion 1
   • Age: Elderly individuals have lower muscle mass and creatinine excretion, affecting the ratio 4
   • Muscle mass: Low muscle mass leads to lower creatinine excretion and higher PCR 1
   • Time of day: Morning samples show better correlation with 24-hour collections 5
   • Exercise: Can temporarily increase protein excretion 1
   • Infection: Urinary tract infections can increase protein excretion 1
   • Hematuria: Blood in urine increases protein measurement 1

3. Limitations:

   • Agreement with 24-hour collections decreases at higher levels of proteinuria 3
   • Different laboratory methods for measuring creatinine (Jaffe vs. enzymatic) may affect results 2

Best Practices

• Confirm positive screening results with quantitative PCR measurement 1
• For clinical monitoring of known proteinuria, use consistent collection timing and laboratory methods
• In children, obtain both PCR and albumin-to-creatinine ratio (ACR) from first morning urine samples 1
• When high precision is required (e.g., clinical trials, nephrotic syndrome), 24-hour collections may still be preferred 3

Relationship to ACR

• ACR is preferred for initial testing of albuminuria 1
• PCR measures all proteins, while ACR measures only albumin
• The albumin-to-protein ratio increases with severity of proteinuria (from <30% in mild to ~70% in severe proteinuria) 6
• In settings where only PCR is available, equations exist to estimate ACR from PCR 6

By following these guidelines for PCR calculation and interpretation, clinicians can effectively assess and monitor proteinuria in patients with suspected or confirmed kidney disease.