Normal Urine Creatinine Ratio (UCR) Values
Albumin-to-Creatinine Ratio (ACR) - The Preferred Standard
Normal albumin-to-creatinine ratio is defined as less than 30 mg albumin per gram of creatinine (≤30 mg/g). 1, 2
Standard Reference Ranges
- Normal ACR: <30 mg/g creatinine 1, 2, 3
- Moderately increased albuminuria (formerly "microalbuminuria"): 30-299 mg/g creatinine 1, 2, 3
- Severely increased albuminuria (formerly "macroalbuminuria"): ≥300 mg/g creatinine 1, 2, 3
The National Kidney Foundation and NIDDK established these cutoffs in their position statement, which remains the foundation for current clinical practice. 1 These values apply to spot urine specimens, which have replaced 24-hour collections as the gold standard. 1, 2
Sex-Specific Considerations
Some evidence suggests sex-specific cutoff values may be more accurate due to differences in creatinine excretion: 2
- Men: >17 mg/g may indicate abnormal albuminuria
- Women: >25 mg/g may indicate abnormal albuminuria
However, the universal cutoff of 30 mg/g remains the standard recommendation across major guidelines. 1, 2
Total Protein-to-Creatinine Ratio (PCR)
Normal total protein-to-creatinine ratio is less than 200 mg/g. 1
When to Use PCR Instead of ACR
At very high levels of proteinuria (spot urine total protein-to-creatinine ratio 500-1,000 mg/g or higher), measurement of total protein instead of albumin is acceptable. 1 Research demonstrates that PCR values less than 100 mg/g in untimed urines obtained without exercise, fever, or urinary tract disease indicate normal kidney function. 4
PCR Reference Ranges Based on Research
- Normal: <100 mg/g 4
- Intermediate kidney disease: 100-2,000 mg/g 4
- Nephrotic range: >2,000 mg/g (corresponding to ≥4,000 mg/day protein excretion) 4
Critical Implementation Details
Specimen Collection Requirements
First-morning spot urine collections are optimal because they have the lowest coefficient of variation (31%) and minimize the confounding effect of orthostatic proteinuria, particularly in children and adolescents. 1, 2
To minimize variability further: 2
- Collect at the same time of day
- Patient should not have ingested food for at least 2 hours prior to collection
- Avoid vigorous exercise for 24 hours before sample collection 1, 5
Factors Causing False Elevations
The following conditions can temporarily elevate urine albumin or protein levels independently of kidney damage: 2, 5
- Exercise within 24 hours
- Infection or fever
- Congestive heart failure
- Marked hyperglycemia
- Marked hypertension
- Menstruation
Confirmation Requirements
Due to high biological variability (>20%), elevated values must be confirmed with 2 of 3 specimens collected over 3-6 months before diagnosing persistent albuminuria. 1, 2, 3, 5
Important Caveats About Urine Concentration
A critical pitfall is that urine concentration significantly affects the accuracy of protein-to-creatinine ratios. 6
- Dilute urine (specific gravity ≤1.005, creatinine ≤38.8 mg/dL): PCR tends to overestimate actual daily protein excretion 6
- Concentrated urine (specific gravity ≥1.015, creatinine ≥61.5-63.6 mg/dL): PCR tends to underestimate actual daily protein excretion 6
This is particularly problematic in dilute samples, where overestimation may lead to erroneous diagnosis of proteinuric renal disease or incorrect CKD staging. 6 The correlation between PCR and actual 24-hour protein excretion is also inconsistent for PCR values less than 50 mg/g. 7
Agreement Limits
Research shows that differences and variability between 24-hour protein excretion and spot urine ratios increase with higher proteinuria levels. 8, 9 For protein excretion <2.0 g/day, the limits of agreement are reasonable (+1.48 to -1.2 g/day), but these limits widen substantially as protein excretion increases. 9
Clinical Application Algorithm
- Use ACR as the primary test for kidney damage assessment 1, 2
- Collect first-morning void when possible 2
- If ACR ≥30 mg/g: Repeat 2 additional times over 3-6 months 1, 2
- If 2 of 3 tests are elevated: Confirm persistent albuminuria 1, 5
- For very high proteinuria (PCR 500-1,000 mg/g or higher): Total PCR is acceptable instead of ACR 1
- Interpret with caution if: Urine is very dilute (creatinine <38.8 mg/dL) or very concentrated (creatinine >61.5 mg/dL) 6