What does the urine albumin to creatinine (ACR) ratio measure?

Understanding Urine Albumin to Creatinine Ratio (ACR)

The urine albumin to creatinine ratio (ACR) measures the amount of albumin relative to creatinine in a urine sample, serving as a key marker for kidney damage and an essential tool for chronic kidney disease (CKD) screening, staging, and monitoring disease progression. 1, 2

Definition and Clinical Significance

• ACR is calculated by dividing the concentration of albumin by the concentration of creatinine in a urine sample, typically expressed in mg/g 1, 2
• ACR provides a standardized measurement of albumin excretion that accounts for variations in urine concentration, making it more reliable than measuring albumin concentration alone 2
• The test is primarily used to detect and monitor kidney damage, particularly in patients with diabetes, hypertension, or other risk factors for CKD 1, 2

Classification of Albuminuria

ACR values are categorized according to the following ranges:

• Normal to mildly increased (A1): ACR <30 mg/g 1, 2
• Moderately increased albuminuria (A2, formerly microalbuminuria): ACR 30-299 mg/g 1, 2
• Severely increased albuminuria (A3, formerly macroalbuminuria): ACR ≥300 mg/g 1, 2

Clinical Implications

• Higher ACR values indicate greater kidney damage and are associated with increased risk of CKD progression, cardiovascular events, and mortality 2, 3
• Even within the normal range (<30 mg/g), higher ACR values are associated with increased risk of CKD progression, with 10-year adjusted cumulative incidences of 8.7% for ACR 0-<5 mg/g, 11.5% for ACR 5-<15 mg/g, and 19.5% for ACR ≥15 mg/g 3
• A >30% sustained reduction in albuminuria is accepted as a surrogate marker of slowed kidney disease progression 1, 2

Measurement and Monitoring

• Morning spot urine samples are preferred for ACR measurement to minimize variability 2
• Annual ACR measurement is recommended for adults with diabetes 1
• If eGFR is <60 mL/min/1.73 m² and/or albuminuria is >30 mg/g creatinine, ACR should be repeated every 6 months 1
• Confirmation of elevated ACR requires at least 2 of 3 specimens collected within a 3-6 month period showing abnormal results 2

Advantages Over Other Methods

• ACR has better sensitivity and quantification capabilities compared to urine dipstick tests 4
• For detecting albuminuria (ACR ≥30 mg/g), dipstick tests show only moderate sensitivity (43.6%) but high specificity (93.6%) 4
• ACR-based CKD risk categories provide more accurate prediction of health outcomes compared to dipstick-based categories 4

Common Pitfalls and Considerations

• ACR measurements can have up to 40-50% variability, necessitating repeat testing for confirmation 1, 2
• False elevations in ACR can occur with hematuria, febrile illness, or vigorous exercise within 24 hours of collection 2
• Point-of-care ACR tests may have lower sensitivity (83.2%) compared to laboratory-based measurements, making them better for ruling out albuminuria (negative predictive value 95.0%) than confirming it (positive predictive value 51.2%) 5

Clinical Application

• ACR is used alongside estimated glomerular filtration rate (eGFR) to properly stage kidney disease according to the KDIGO classification system 1
• The goal of treatment should be to reduce ACR by at least 30-50% and ideally achieve an ACR of <30 mg/g 1
• Interventions for elevated ACR include blood pressure control and use of renin-angiotensin system blockers (ACE inhibitors or ARBs) 2