Urine Albumin-to-Creatinine Ratio (ACR)
The recommended urine test to screen for chronic kidney disease is the urine albumin-to-creatinine ratio (ACR) on a first morning void midstream sample, or alternatively a random untimed spot urine sample if a morning sample is not feasible. 1
Preferred Testing Approach
The most recent KDIGO 2024 guidelines establish a clear hierarchy for initial CKD screening 1:
First choice (in descending order of preference):
- Urine ACR on first morning void midstream sample
- Reagent strip urinalysis for albumin with ACR and automated reading
If measuring total protein instead:
- Urine protein-to-creatinine ratio (PCR)
- Reagent strip urinalysis for total protein with automated reading
- Reagent strip urinalysis for total protein with manual reading
Why ACR is Superior
The ACR has replaced 24-hour urine collections as the preferred method because it corrects for variations in urinary protein concentration due to hydration and is far more convenient 2. A cutoff value of ACR ≥30 mg/g (≥3 mg/mmol) indicates abnormal albuminuria and requires confirmation 1, 2.
Confirmation Strategy
A critical pitfall is acting on a single positive result. The guidelines are explicit about confirmation:
- If initial ACR is ≥30 mg/g on a random sample, confirm with a subsequent first morning void sample 1
- Two or more positive results temporally spaced over 3 months establish persistent albuminuria and confirm CKD 2
- If using reagent strips initially, confirm any positive result with quantitative laboratory ACR measurement 1
This confirmation step is essential because intra-individual variability of urinary albumin is high (coefficients of variance 30-50%), and factors like exercise, fever, menstruation, hematuria, and urinary infection can cause false elevations 3, 4.
Practical Considerations
Specimen collection: First morning void is preferred in both adults and children because it minimizes the impact of exercise, posture, and hydration status 1. However, a random specimen is acceptable for initial screening 2.
For children specifically: Obtain both urine PCR and ACR together as the preferred approach, or use reagent strip urinalysis for both total protein and albumin 1, 4.
Point-of-care testing: Semi-quantitative strip tests can be used for screening where laboratory access is limited, but they show only moderate specificity (60-93%) despite high sensitivity (78-90%) 5, 6. Positive strip test results must be confirmed by quantitative laboratory ACR measurement 1.
Complete CKD Screening
While the question asks specifically about urine testing, comprehensive CKD screening requires both ACR and estimated GFR (eGFR) from serum creatinine 7, 8, 9. CKD is defined by persistent ACR ≥30 mg/g and/or eGFR <60 mL/min/1.73 m² for at least 3 months 9.
Common Pitfalls to Avoid
- Don't rely on dipstick alone without quantitative confirmation - this leads to excessive false positives
- Don't use 24-hour urine collections - they are inconvenient, frequently inaccurate, and provide no advantage over spot ACR 2
- Don't diagnose CKD on a single abnormal test - persistence over 3 months is required
- Don't ignore factors affecting interpretation - review medication list, recent exercise, menstrual status, and symptoms of urinary infection before confirming abnormal results 4