What is Tested in a UACR (Urine Albumin-to-Creatinine Ratio)
A UACR test measures two substances in a single urine sample: albumin (a protein that leaks into urine when kidneys are damaged) and creatinine (a waste product used to normalize for urine concentration), with the ratio providing an accurate assessment of kidney damage without requiring cumbersome 24-hour urine collections. 1
Components Measured
Albumin
- Albumin is a protein that should normally be retained in the bloodstream by healthy kidneys 1
- When kidney filtration barriers are damaged (particularly in diabetes, hypertension, or kidney disease), albumin leaks into the urine 1
- Measured using spectrophotometric dye-binding assays (such as tetrabromophenol blue) or immunoassays 1, 2
Creatinine
- Creatinine is a waste product from muscle metabolism that is consistently excreted in urine 3
- Serves as a normalizing factor to account for variations in urine concentration due to hydration status 1
- Prevents false-positive or false-negative results that would occur if measuring albumin alone 1
- Measured using colorimetric Jaffe assay or enzymatic methods 2
Why the Ratio is Used
The albumin-to-creatinine ratio eliminates the need for inconvenient and error-prone 24-hour urine collections while providing accurate estimates of albumin excretion rate 3
- The ratio correlates well with timed albumin excretion measurements 3
- Provides the best method to predict renal events in people with type 2 diabetes 3
- Using first morning void samples yields the lowest coefficient of variation (31%) compared to other collection methods 3
Clinical Interpretation Categories
Normal UACR: <30 mg/g creatinine 1, 4
Moderately Increased Albuminuria (Stage A2): 30-299 mg/g creatinine 1, 3
Severely Increased Albuminuria (Stage A3): ≥300 mg/g creatinine 1, 3
Important Caveat on "Normal" Values
- Even within the normal range (<30 mg/g), UACR functions as a continuous risk marker 4
- Higher values within the normal range are associated with increased cardiovascular and renal risk 4
- Research suggests UACR values >10 mg/g in diabetic patients may predict CKD progression despite being technically "normal" 5
Factors That Can Falsely Elevate UACR
Several transient conditions can elevate UACR independently of true kidney damage and should be excluded before confirming chronic kidney disease: 1, 3
- Exercise within 24 hours 1
- Active urinary tract infection or fever 1, 3
- Congestive heart failure exacerbation 1, 3
- Marked hyperglycemia 1, 3
- Menstruation 1, 3
- Marked or uncontrolled hypertension 1, 3
Confirmation Requirements
Due to high biological variability (coefficient of variation exceeding 20-48%), two of three specimens collected within a 3-6 month period should be abnormal before diagnosing persistent albuminuria 1, 3, 6
- A single elevated UACR should never be used alone for diagnosis 3, 6
- First morning void samples are preferred to minimize variability 1, 3
- If first morning samples are difficult to obtain, collect all samples at the same time of day with the patient well-hydrated and having not eaten for 2 hours prior 1
Screening Recommendations
Type 1 Diabetes: Begin annual UACR screening 5 years after diagnosis 1, 3
Type 2 Diabetes: Begin annual UACR screening at the time of diagnosis 1, 3
Patients with eGFR <60 mL/min/1.73 m² or albuminuria >30 mg/g: Repeat UACR every 6 months 1, 7