How is albuminuria measured?

Measurement of Albuminuria

Albuminuria is most accurately measured using the urinary albumin-to-creatinine ratio (UACR) in a random spot urine sample, with positive screening results confirmed by quantitative analysis in an accredited laboratory. 1

Primary Methods for Measuring Albuminuria

Preferred Method: Urinary Albumin-to-Creatinine Ratio (UACR)

• First morning void midstream sample is preferred for initial testing 1
• Measured in a random spot urine collection
• Expressed as mg albumin/g creatinine
• Most convenient and accurate method for clinical practice
• No need for timed collections 1

Classification of Albuminuria by UACR

Category	UACR (mg/g creatinine)
Normal to mildly increased	<30
Moderately increased (formerly microalbuminuria)	30-299
Severely increased (macroalbuminuria)	≥300

Alternative Methods (in descending order of preference)

1. Quantitative laboratory measurement:

   • Immunoturbidimetric laboratory assay (gold standard)

   • 95% sensitivity and specificity for detecting moderately increased albuminuria 1

2. Semiquantitative/Qualitative screening tests:

   • Reagent strip urinalysis for albumin with automated reading
   • Sensitivity: ~76% (95% CI, 63-86%)
   • Specificity: ~93% (95% CI, 84-97%) 1
   • Important: Positive results must be confirmed by quantitative laboratory measurement 1

3. 24-hour urine collection:

   • More burdensome and frequently inaccurate
   • Does not provide more accurate estimates than spot UACR 1
   • Only recommended in special clinical circumstances

Important Considerations for Accurate Measurement

Confirmation of Results

• Due to high biological variability of albuminuria, two of three specimens collected within a 3-6 month period should be abnormal before confirming diagnosis 1
• Positive screening results by semiquantitative tests must be confirmed by quantitative analysis in an accredited laboratory 1

Factors Affecting Measurement Accuracy

• False elevations may occur with:

  • Exercise within 24 hours
  • Urinary tract infections
  • Fever
  • Congestive heart failure
  • Marked hyperglycemia
  • Marked hypertension
  • Hematuria
  • Menstruation 1

• Variability issues:

  • Day-to-day variability increases in absolute terms but decreases in relative terms with increasing baseline ACR 2
  • For patients with normoalbuminuria, changes greater than ±467% are required to indicate significant change
  • For macroalbuminuria, changes of ±83% are required to represent significant change 2

Common Pitfalls to Avoid

1. Using standard dipsticks for total protein instead of albumin-specific tests

   • Standard protein dipsticks are not sensitive enough to detect low albumin concentrations (20-50 mg/L) 1

2. Not following manufacturer's instructions for semiquantitative tests

   • Clinical operators often have lower sensitivity but better specificity than laboratory technologists
   • Waiting the full time (usually 60 seconds) between dipping and scanning is critical 1

3. Relying on a single measurement

   • Due to high biological variability, multiple measurements are recommended
   • Performing 2-3 tests at different times improves sensitivity and specificity 1

4. Not correcting for urine concentration

   • Measurement of albumin alone without creatinine is susceptible to false results due to hydration status 1, 3

Practical Implementation

1. For initial screening, collect a random spot urine sample (preferably first morning void)
2. Measure UACR using quantitative laboratory methods
3. If using semiquantitative tests, confirm positive results with laboratory measurement
4. Repeat testing within 3-6 months to confirm persistent albuminuria
5. For monitoring, check UACR every 3-6 months to assess disease progression and treatment response 4

By following these guidelines for measuring albuminuria, clinicians can accurately diagnose and monitor kidney disease, allowing for timely intervention to prevent progression and reduce associated cardiovascular risk.