Optimal Timing for Urine ACR Testing
First morning void urine samples should be used for measuring urine albumin-to-creatinine ratio (ACR), as this timing provides the lowest coefficient of variation (31%) and best correlates with 24-hour protein excretion. 1
Why First Morning Void is Preferred
The first morning urine sample is the gold standard for ACR measurement because:
- It minimizes intra-individual variability compared to random samples, providing more reliable and reproducible results 1
- It correlates well with 24-hour protein excretion, eliminating the need for cumbersome timed collections 1
- It avoids confounding factors such as exercise, upright posture (orthostatic proteinuria), and variations in hydration status that affect random samples throughout the day 1
Practical Implementation When First Morning Sample is Not Feasible
If obtaining a first morning void is impractical in your clinical setting, all urine collections should be standardized to the same time of day to minimize variability 1. When using non-morning samples:
- The patient should be well hydrated 1
- No food should be ingested within the preceding 2 hours 1
- The patient should not have exercised recently 1
Clinical Context for Diabetes and Hypertension
For patients with diabetes and hypertension specifically:
- Annual screening with morning spot urine ACR is recommended starting at diagnosis for type 2 diabetes and 5 years after diagnosis for type 1 diabetes 1
- If ACR is ≥30 mg/g creatinine or eGFR <60 mL/min/1.73 m², repeat testing every 6 months using the same timing protocol 1
Important Caveats to Avoid Misinterpretation
Even with optimal timing, be aware that ACR demonstrates high biological variability (coefficient of variation ~49% in recent studies) 2. Therefore:
- Confirm any elevated ACR (≥30 mg/g) with 2 additional first morning samples over 3-6 months before diagnosing persistent albuminuria 1, 3
- Avoid testing during conditions that transiently elevate ACR: active urinary tract infection, menstruation, marked hyperglycemia, uncontrolled hypertension, congestive heart failure exacerbation, or within 24 hours of vigorous exercise 1
Bottom Line for Clinical Practice
Always request first morning void samples for ACR testing in patients with diabetes and hypertension. 1 This single practice point will provide the most accurate assessment of albuminuria status and minimize the need for repeat testing due to spurious results. If logistical constraints prevent first morning collection, ensure consistency in collection timing and pre-collection conditions to maintain interpretability of serial measurements. 1