Urine Albumin-to-Creatinine Ratio (UACR): Definition and Performance
UACR is a ratio that divides urinary albumin concentration by urinary creatinine concentration, typically expressed as mg albumin per g creatinine, and should be measured from a random spot urine collection—preferably first morning void—rather than a timed or 24-hour collection. 1, 2
What UACR Measures and Why It Matters
- UACR corrects for variations in urinary protein concentration caused by hydration status, making it far more reliable than measuring albumin alone 1, 2
- Measuring albumin concentration without simultaneous creatinine measurement is susceptible to false-negative and false-positive determinations due to urine concentration variability from hydration 1, 2
- Normal UACR is defined as <30 mg/g creatinine, with values between 30-299 mg/g previously termed "microalbuminuria" and values ≥300 mg/g previously termed "macroalbuminuria" 1, 2
- Even UACR values within the normal range (<30 mg/g) carry prognostic significance for cardiovascular and renal outcomes, with risk increasing continuously across the spectrum 2, 3
How to Perform UACR Testing
Specimen Collection
- Obtain a random spot urine sample, preferably from the first morning void after awakening 1, 2
- A random specimen is acceptable if first morning void is not feasible 1
- Avoid collection within 24 hours of exercise, during menstruation, or in the presence of fever, infection, congestive heart failure, marked hyperglycemia, or marked hypertension, as these cause transient elevations 2
- No food intake for 2 hours prior to collection is recommended to minimize variability 4
Laboratory Analysis
- The specimen is analyzed using spectrophotometric methods: tetrabromophenol blue dye-binding assay for albumin (measured at 625 nm wavelength) and colorimetric Jaffe assay for creatinine (measured at 515 nm wavelength) 5
- Urine samples are typically diluted with distilled water (1:80 ratio) and mixed separately with albumin and creatinine reaction mixtures 5
- The creatinine reaction requires incubation at room temperature (25°C) for 30 minutes before analysis 5
Interpretation and Confirmation
- Two of three specimens collected within 3-6 months should be abnormal before confirming persistent albuminuria due to high biological variability 1, 2, 6
- UACR demonstrates substantial within-individual variability (coefficient of variation ~49%), meaning a repeated UACR can be as high as 3.78 times or as low as 0.26 times the first measurement 6
- If a single UACR increases from one value to another, the probability that it truly increased by at least 30% is only 50% with single collections, rising to 97% when 2 collections are obtained at each time point 6
Common Pitfalls and How to Avoid Them
- Do not use 24-hour or timed urine collections—they are more burdensome and add little to prediction or accuracy compared to spot UACR 1
- Do not rely on albumin-specific dipstick alone without measuring creatinine, as this is less expensive but susceptible to false results from hydration variations 1
- Be aware that extremely dilute urine may yield unmeasurable UACR results; in such cases, repeat testing with optimized collection technique (first morning void, consistent timing) is warranted 4
- Consider that lower gender-specific cutoffs may be more appropriate in certain populations (such as renal transplant recipients), though current guidelines use 30 mg/g for both sexes 1, 7