How to Calculate UACR
The urine albumin-to-creatinine ratio (UACR) is calculated by dividing the urine albumin concentration (in mg) by the urine creatinine concentration (in g), expressed as mg/g. 1
Calculation Formula
UACR (mg/g) = Urine Albumin (mg/L) ÷ Urine Creatinine (g/L)
Or equivalently:
UACR (mg/g) = [Urine Albumin (mg/dL) ÷ Urine Creatinine (mg/dL)] × 1000
For international units:
UACR (mg/mmol) = UACR (mg/g) ÷ 10
Specimen Collection
Use a first morning void midstream urine sample for optimal accuracy in both adults and children. 2 This is the preferred specimen type as it minimizes biological variability and provides the most reproducible results.
- A random spot urine sample is acceptable for initial screening
- If ACR ≥30 mg/g on random sample, confirm with a first morning void 2
Interpretation Categories
The KDIGO and ADA classification system defines three albuminuria categories 1:
- A1 (Normal to Mildly Increased): UACR <30 mg/g (<3 mg/mmol)
- A2 (Moderately Increased): UACR 30-299 mg/g (3-29 mg/mmol)
- A3 (Severely Increased): UACR ≥300 mg/g (≥30 mg/mmol)
Important Clinical Considerations
Confirm abnormal results before making clinical decisions. If initial UACR is ≥30 mg/g, repeat testing on 2 of 3 samples collected within 3-6 months to document persistent albuminuria before diagnosing moderately or severely increased albuminuria. 1
Factors That Affect UACR Accuracy
Be aware of conditions that falsely elevate or decrease UACR 2:
Falsely Elevated:
- Hematuria or menstruation
- Vigorous exercise within 24 hours
- Urinary tract infection
- Symptomatic illness
Falsely Decreased:
- Dilute urine (low creatinine excretion)
- High body weight in obese patients (see below)
Body Weight Correction for Obesity
Recent evidence shows that for patients with BMI ≥28.5 kg/m² (or body weight ≥78 kg), UACR alone underestimates daily albuminuria and requires body weight correction. 3
For BMI <28.5 kg/m²:
- Daily albuminuria (g) = 0.033 + 0.999 × UACR (mg/mg)
For BMI ≥28.5 kg/m²:
- Daily albuminuria (g) = -3.885 + 1.538 × UACR (mg/mg) + 0.045 × body weight (kg)
Laboratory Method Considerations
Use quantitative laboratory measurement of albumin and creatinine rather than reagent strips alone for diagnostic purposes. 2 If reagent strips are positive, confirm with quantitative laboratory measurement and express as a ratio to urine creatinine.
The laboratory should use validated assays with a lower limit of quantitation for albumin of 3 mg/L to maximize diagnostic utility, particularly for detecting low-level albuminuria that still carries prognostic significance. 4