Albumin-to-Creatinine Ratio (ACR) vs Protein-to-Creatinine Ratio (PCR)
Albumin-to-creatinine ratio (ACR) is the preferred test for detecting and monitoring kidney disease in adults and children, particularly in high-risk populations including patients with diabetes, hypertension, or suspected chronic kidney disease. 1
Primary Recommendation
The 2025 KDOQI guideline explicitly prioritizes ACR over PCR for initial kidney disease assessment 1. When measuring urine albumin, the hierarchy is:
- Urine ACR (first-line)
- Reagent strip urinalysis for albumin with automated reading
- Only if measuring total protein: use PCR 1
Why ACR is Preferred
ACR is more sensitive for detecting early kidney damage, particularly in conditions where albuminuria precedes total proteinuria 1, 2. This is critical because:
- Albumin is the earliest and most specific marker of glomerular damage in diabetic nephropathy, hypertensive kidney disease, and most forms of CKD 1
- ACR detects microalbuminuria (30-300 mg/g), which PCR may miss since standard dipstick tests for total protein are insensitive at this range 1
- Early detection with ACR allows intervention before irreversible kidney damage occurs, directly impacting morbidity and mortality 3
When to Use PCR Instead
PCR has specific but limited roles 1:
- When ACR is very high (>500-1000 mg/g), PCR may be used for monitoring since the distinction between albumin and total protein becomes less clinically relevant 1
- In suspected tubular disease or paraproteinemia, where non-albumin proteins predominate and ACR would miss the diagnosis 4
- When ACR testing is unavailable, though this should prompt efforts to obtain ACR subsequently 1
Clinical Algorithm for Testing
For patients at increased risk (diabetes, hypertension, cardiovascular disease, family history of kidney disease):
- Start with ACR on a random untimed urine sample 1
- If ACR ≥30 mg/g, confirm with first morning void within 3 months 1
- Two of three positive tests over 3 months establishes persistent albuminuria requiring CKD evaluation 1
For children, obtain first morning urine and measure both PCR and ACR when possible, with ACR taking priority 1
Critical Thresholds
- Normal: ACR <30 mg/g 1
- Moderately increased albuminuria (A2): 30-300 mg/g 5
- Severely increased albuminuria (A3): >300 mg/g 5
These ACR-based thresholds are used for CKD staging and risk stratification, directly influencing treatment decisions including RAAS blockade initiation 1, 5.
Performance Comparison
Research demonstrates that while PCR correlates well with 24-hour protein (rho=0.91), ACR also performs adequately (rho=0.84) 6. However, PCR outperforms ACR only at higher proteinuria levels (>1 g/day) 6. For the clinically crucial range of early kidney disease detection (<500 mg/g), ACR's superior sensitivity for albumin makes it the better screening tool 1, 6.
Common Pitfalls to Avoid
- Do not use PCR alone in diabetic patients—you will miss early diabetic nephropathy when ACR is 30-300 mg/g but total protein is still normal 1
- Do not dismiss the need for ACR confirmation after positive dipstick—biological variability exceeds 20% between measurements 5
- Avoid testing during menstruation, after vigorous exercise, or during acute illness—these cause transient elevations 4, 5
- In kidney donor evaluation, measure both ACR and PCR to avoid missing non-albumin proteinuria that could indicate tubular disease 4
Point-of-Care Testing Considerations
When using POCT devices for albuminuria screening, ensure the device can produce ACR (not just albumin alone) and achieves 85% sensitivity for detecting ACR ≥30 mg/g 1. POCT is acceptable when laboratory access is limited, but the same quality standards must apply 1.
Monitoring Strategy
For established CKD with confirmed albuminuria, continue monitoring with ACR unless levels exceed 500-1000 mg/g, at which point either ACR or PCR is acceptable 1. Collect samples at the same time of day with similar activity levels to minimize variability when tracking individual patients over time 4, 7.