Normal Urinary Albumin-to-Creatinine Ratio (ACR)
The normal urinary ACR is ≤30 mg of albumin per gram of creatinine (≤30 mg/g), as established by the National Kidney Foundation and National Institute of Diabetes and Digestive and Kidney Diseases. 1
Standard Reference Values
The urinary ACR classification system divides values into three distinct categories 1, 2:
- Normal (A1 category): ≤30 mg albumin/g creatinine 1, 2
- Microalbuminuria (A2 category): 30-300 mg albumin/g creatinine 1
- Macroalbuminuria (A3 category): >300 mg albumin/g creatinine 1
Unit Conversion
Laboratories should report albuminuria as milligrams of albumin per gram of creatinine (mg/g), which is equivalent to micrograms of albumin per milligram of creatinine (μg/mg) 1. The A1 normal category corresponds to a 24-hour albumin excretion rate of <30 mg/day 2.
Important Clinical Considerations
ACR as a Continuous Risk Marker
Even within the "normal" range of <30 mg/g, ACR functions as a continuous risk marker for cardiovascular and renal disease 2. Research demonstrates that higher values within the normal range carry increased risk 3, 4:
- Values >8-10 mg/g within the normal range are associated with increased risk of chronic kidney disease progression in patients with type 2 diabetes 3
- Community-based studies show that even high-normal values (10-30 mg/g) are associated with increased all-cause mortality compared to values <10 mg/g 4
Optimal Specimen Collection
First-morning spot urine collections provide the most reliable results with the lowest coefficient of variation (31%) and should be used preferentially, particularly in children and adolescents to avoid orthostatic proteinuria 1, 2, 5.
Confirmation Requirements
Due to high biological variability (>20% between measurements), abnormal values must be confirmed with 2 of 3 specimens collected within a 3-6 month period before diagnosing persistent albuminuria 1, 2, 5.
Factors That Can Falsely Elevate ACR
The following conditions may elevate ACR independently of kidney damage and should be excluded before confirming chronic kidney disease 5:
- Vigorous exercise within 24 hours 1
- Active urinary tract infection or fever 5
- Congestive heart failure 5
- Marked hyperglycemia 5
- Menstruation 5
- Marked hypertension 5
Sex-Specific Considerations
Future standardization may include sex-adjusted creatinine values (multiply male concentration by 0.68) to provide sex-independent reference ranges 1. Some studies suggest sex-specific cutoff values (>17 mg/g in men or >25 mg/g in women) may be more appropriate 5.