Albumin-to-Creatinine Ratio Is Measured in Urine, Not Blood, for Kidney Function Assessment
The albumin-to-creatinine ratio (ACR) is measured in urine samples, not blood, and is a critical marker for assessing kidney damage and function. 1
Why ACR Is Measured in Urine
- ACR measures urinary albumin excretion, which indicates glomerular damage when elevated
- Albumin in urine is an early marker of kidney damage, often preceding decreases in GFR
- The ratio to creatinine helps standardize for urine concentration, making spot samples reliable
Proper Collection and Measurement
- First morning void urine samples are preferred due to lower variability 1
- Spot urine samples are acceptable and more practical than 24-hour collections 1
- Laboratory standards require:
- Samples analyzed fresh or stored at 4°C for up to 7 days
- Avoiding freezing at -20°C (causes loss of measurable albumin)
- Reporting ACR in addition to albumin concentration 1
Clinical Interpretation of ACR Values
ACR categories and thresholds:
- Normal to mildly increased: <30 mg/g creatinine
- Moderately increased (formerly "microalbuminuria"): 30-299 mg/g creatinine
- Severely increased (formerly "macroalbuminuria"): ≥300 mg/g creatinine 1
Monitoring Frequency
- Annual ACR measurement is recommended for all adults with CKD 1
- More frequent monitoring (every 6 months) is needed for:
Factors Affecting ACR Measurement
Several factors can cause variability in ACR results:
- Transient elevations: exercise, upright posture, fever, urinary tract infection 1
- Biological variability: age, sex, race, muscle mass 1
- Analytical factors: sample storage conditions, assay methods 1
Clinical Applications of ACR
- Risk stratification: ACR is a strong predictor of kidney disease progression and cardiovascular outcomes 1
- Treatment monitoring: A >30% sustained reduction in albuminuria indicates slowed kidney disease progression 1
- Treatment decisions: ACR helps determine the need for nephrology referral and guides medication choices 1
Common Pitfalls to Avoid
- Relying on blood measurements for albumin-to-creatinine ratio (ACR is a urine test)
- Using the outdated term "microalbuminuria" (now called moderately increased albuminuria) 1
- Misinterpreting small fluctuations in ACR (a doubling of ACR exceeds laboratory variability and warrants evaluation) 1
- Failing to account for non-renal factors that affect creatinine excretion when interpreting ACR 1
For accurate kidney function assessment, combine ACR measurements with estimated glomerular filtration rate (eGFR) from serum creatinine and/or cystatin C to provide a complete picture of kidney health.