What is UACR (Urine Albumin-to-Creatinine Ratio)?
UACR is a laboratory test that measures the amount of albumin (a protein) relative to creatinine in a spot urine sample, serving as the gold standard screening and monitoring tool for kidney damage because it accurately detects albuminuria without requiring cumbersome 24-hour urine collections. 1, 2
Definition and Measurement
- UACR normalizes albumin excretion for variations in urine concentration by dividing urinary albumin (mg) by urinary creatinine (g), expressed as mg/g 1, 2
- The test eliminates the need for inconvenient and error-prone timed urine collections while providing an accurate estimate of albumin excretion rate 2
- A first morning void sample yields the lowest coefficient of variation (31%) and is the preferred collection method 2, 3
Clinical Categories and Risk Stratification
The American Diabetes Association and KDIGO guidelines classify UACR into three categories that directly correlate with cardiovascular and kidney disease risk: 1, 3
A1 (Normal to Mildly Increased): UACR <30 mg/g (<3 mg/mmol)
A2 (Moderately Increased Albuminuria): UACR 30-299 mg/g (3-29 mg/mmol)
A3 (Severely Increased Albuminuria): UACR ≥300 mg/g (≥30 mg/mmol)
At any level of kidney function, increased UACR independently raises the risk for adverse cardiovascular outcomes, chronic kidney disease progression, and all-cause mortality 2, 4
The risk increases continuously as UACR rises, even within the normal and moderately increased ranges 2, 4
Why Creatinine is Used in the Ratio
- Creatinine serves as a marker of urine concentration, allowing accurate normalization of albumin levels regardless of hydration status 2
- The ratio correlates strongly with 24-hour albumin excretion (Pearson's r = 0.87-0.88) while being far more convenient 5, 6
- Measuring albumin alone without creatinine is susceptible to false-negative and false-positive results due to variations in urine dilution 2
Screening Recommendations
For Type 1 Diabetes: Begin screening 5 years after diagnosis with annual monitoring thereafter 1, 2
For Type 2 Diabetes: Begin screening at diagnosis due to uncertain disease onset, with annual monitoring 1, 2
- First morning void samples should be collected at the same time of day, with no food ingestion for at least 2 hours prior 2, 3
Factors That Can Falsely Elevate UACR
The following conditions must be excluded before confirming chronic albuminuria: 2, 3
- Exercise within 24 hours
- Active urinary tract infection or fever
- Congestive heart failure exacerbation
- Marked hyperglycemia
- Menstruation
- Marked uncontrolled hypertension
Confirmation of Abnormal Results
- Due to high day-to-day variability (coefficient of variation 48.8%), elevated UACR requires confirmation with 2 out of 3 positive samples collected over 3-6 months before diagnosing persistent albuminuria 2, 7
- A single UACR increase from 2 to 5 mg/mmol has only a 50% probability of representing a true 30% increase, rising to 97% when 2 collections are obtained at each time point 7
Clinical Significance
- UACR is a continuous measurement where differences within both normal and abnormal ranges are associated with renal and cardiovascular outcomes 2, 4
- Even high-normal UACR values (>10 mg/g in men, >8 mg/g in women) predict increased risk of chronic kidney disease progression in patients with type 2 diabetes 8
- The presence of albuminuria markedly increases cardiovascular risk and healthcare costs in patients with diabetes 4