What are ACR and Protein-to-Creatinine Ratio?
The albumin-to-creatinine ratio (ACR) is the preferred test for detecting and monitoring kidney damage, calculated by dividing urine albumin concentration (mg/dL) by urine creatinine concentration (g/dL) on a spot urine sample, while the protein-to-creatinine ratio (PCR) measures total protein instead of albumin using the same calculation method.
ACR: The Gold Standard
ACR is the ratio of urine albumin (in mg/dL) to urine creatinine (in g/dL), reported as mg albumin per gram creatinine 1. For adults and postpubertal children with diabetes, ACR measurement is explicitly preferred over total protein measurement 1.
Reference Ranges for ACR:
- Normal: ≤30 mg/g creatinine
- Microalbuminuria (Stage A2): 31-300 mg/g creatinine
- Macroalbuminuria (Stage A3): >300 mg/g creatinine 1
Collection Method:
Use a spot (untimed) urine specimen rather than 24-hour collections 1. First-morning specimens are best for children and adolescents to avoid orthostatic proteinuria 1. Patients should avoid vigorous exercise for 24 hours before collection 1.
PCR: The Alternative Measure
PCR uses the same calculation approach but measures total urine protein instead of albumin specifically. It is reported as mg total protein per gram creatinine, with a normal range <200 mg/g 1. PCR is more appropriate in children to identify both albuminuria and low-molecular-weight proteinuria 1.
At very high proteinuria levels (spot urine PCR 500-1,000 mg/g), measuring total protein instead of albumin is acceptable 1.
Key Differences and Clinical Implications
The relationship between ACR and PCR is nonlinear—the albumin-to-protein ratio increases from <30% in mild proteinuria to approximately 70% in severe proteinuria 2. This means you cannot simply convert between them using a fixed multiplier.
Sex significantly modifies the ACR-PCR relationship, with men generally having a higher albumin-to-protein ratio 2. The correlation between ACR and PCR is weaker in patients with low protein excretion or low urinary creatinine concentration 3.
When Each Test Performs Best:
ACR is superior for:
- Screening populations at risk for chronic kidney disease (diabetes, hypertension, family history) 1
- Early detection of kidney damage (microalbuminuria stage)
- Monitoring treatment response in diabetic kidney disease
- Risk prediction using validated kidney failure risk equations 4
PCR is acceptable for:
- Pediatric populations (captures broader spectrum of proteinuria) 1
- Very high levels of proteinuria (>500 mg/g) 1
- Situations where ACR is unavailable, though conversion equations can estimate ACR from PCR 2, 5
Clinical Performance
Both tests show similar associations with CKD complications (anemia, metabolic acidosis, mineral bone disease) when adjusted for kidney function 6. For predicting kidney failure and death, models using measured ACR versus PCR-calculated ACR demonstrate similar discrimination (AUC 83-89% for kidney failure, 69-75% for mortality) 4, 7.
Screening Recommendations
Populations requiring annual screening with ACR:
- Diabetes mellitus
- Hypertension
- Family history of CKD
- Elderly individuals
- US ethnic minorities 1
Confirm persistent albuminuria by repeating the test—2 of 3 samples must show values >30 mg/g creatinine 1. Retest within 6 months after initiating treatment for elevated blood pressure or lipids to assess response 1.
Common Pitfalls
The conversion from PCR to ACR underestimates true ACR at lower proteinuria levels (predicted ACR <50 mg/g) 7. This means PCR may miss early kidney damage that ACR would detect. Semi-quantitative dipstick ACR has lower sensitivity (78-90%) compared to quantitative ACR 8, making it suitable for screening but requiring confirmation with quantitative testing.
Laboratories should report ACR with a reference range of ≤30 mg/g creatinine and maintain interassay coefficients of variation <15% 1.