How to Calculate a Protein to Creatinine Ratio in a Random Urine Sample
To calculate a protein to creatinine ratio in a random urine sample, divide the urine protein concentration (mg/dL) by the urine creatinine concentration (g/dL) to obtain the ratio in mg/g. This method provides a reliable estimate of 24-hour protein excretion without requiring timed collections.
Collection Method
- Preferred sample: First morning void midstream urine sample 1
- Alternative: Random untimed urine sample (less optimal but acceptable)
- Patient preparation: Patients should refrain from vigorous exercise for 24 hours before sample collection 1
Laboratory Measurement Process
Measure urine protein concentration in mg/dL using quantitative laboratory methods
- For albumin: Immunoassay methods are recommended 1
- For total protein: Pyrogallol red or similar colorimetric methods
Measure urine creatinine concentration in g/dL using standardized methods
- Jaffe reaction or enzymatic methods (enzymatic preferred for children) 1
Calculate the ratio:
- PCR = Urine protein (mg/dL) ÷ Urine creatinine (g/dL)
- Result expressed as mg/g creatinine 1
Interpretation of Results
- Normal PCR: <200 mg/g 1
- Microalbuminuria: 30-300 mg/g (for albumin-to-creatinine ratio) 1
- Macroalbuminuria: >300 mg/g (for albumin-to-creatinine ratio) 1
- Nephrotic-range proteinuria: >3500 mg/g 2
Important Considerations
Factors Affecting Interpretation
- Biological sex: Females have lower urinary creatinine excretion, resulting in higher PCR for the same protein excretion 1
- Body weight: Low weight can cause higher PCR relative to timed excretion 1
- Hematuria: Can falsely elevate protein measurements 1
- Exercise: Can temporarily increase protein excretion 1
- Urinary tract infection: Can increase protein measurements 1
Sample Handling
- Storage: Refrigerate urine samples for assay the same or next day 1
- Freezing: One freeze is acceptable if necessary; avoid repeated freeze-thaw cycles 1
- Shipping: If local laboratory cannot handle samples, ship overnight on ice 1
Clinical Applications
- Screening: PCR provides a convenient alternative to 24-hour collections for detecting proteinuria 3, 2
- Monitoring: Useful for following patients with known kidney disease 2
- Pregnancy: Valuable for evaluating preeclampsia (cutoff ≥0.19 has 90% sensitivity, 70% specificity) 4
- Pediatrics: Particularly useful in children where 24-hour collections are difficult 5
Quality Assurance
- Laboratories should implement external quality assessment schemes for urine protein and creatinine measurements 1
- For albumin assays, goal for interassay coefficient of variation should be <15% 1
- Laboratories should ensure quality control processes include the full range of expected values, especially important for pediatric samples 1
This method has shown excellent correlation with 24-hour urine protein measurements (r=0.96) 3 and offers significant advantages in terms of convenience, speed, and patient compliance compared to timed collections.