What is a normal urine microalbumin level?

Normal Urine Microalbumin Levels

A normal urine microalbumin level is 30 mg albumin/g creatinine or less, as defined by the National Kidney Foundation and the National Institute of Diabetes and Digestive and Kidney Diseases. 1

Definition and Measurement Methods

• Normal urine microalbumin is defined as urinary albumin excretion of less than 30 mg/24h, less than 30 mg/g creatinine on a random spot urine sample, or less than 20 μg/min on a timed collection 1
• Laboratories should report albuminuria as milligrams of albumin per gram of creatinine, with a reference range of 30 mg albumin/g creatinine or less 1
• Spot urine collection is preferred over timed collections due to ease of collection and standardization 1
• First-morning spot collections are recommended, especially for children and adolescents, to avoid confounding effects of orthostatic proteinuria 1, 2

Classification of Albuminuria

• Normal to mildly increased: <30 mg/g creatinine 1
• Moderately increased albuminuria (formerly microalbuminuria): 30-299 mg/g creatinine 1
• Severely increased albuminuria (macroalbuminuria): ≥300 mg/g creatinine 1

Clinical Significance of Microalbumin Levels

• Microalbuminuria (30-299 mg/g creatinine) is the earliest clinical manifestation of diabetic nephropathy 1
• Even high-normal urine albumin levels (>10 mg/g but <30 mg/g) may predict increased risk of chronic kidney disease progression in patients with type 2 diabetes 3
• Microalbuminuria is a marker of increased cardiovascular morbidity and mortality in both diabetic and non-diabetic patients 2, 4

Testing Recommendations

• Populations at increased risk for chronic kidney disease (those with diabetes, hypertension, or family history of CKD) should be screened for microalbuminuria at least annually 1
• For patients with type 1 diabetes, testing is recommended 5 years after diagnosis, although a baseline reading at diagnosis may be appropriate 1
• For patients with type 2 diabetes, annual testing should begin at the time of diagnosis due to difficulty in precisely dating disease onset 1

Important Considerations for Sample Collection

• Patients should refrain from vigorous exercise for 24 hours before sample collection 1
• Refrigerate urine samples for assay the same or next day 1
• One freeze is acceptable if necessary, but avoid repeated freeze-thaw cycles 1
• To confirm persistent albuminuria, 2 of 3 measurements should be above the reference range (>30 mg albumin/g creatinine) within a 3-6 month period 1

Factors That Can Cause Transient Elevations in Microalbumin

• Exercise within 24 hours prior to collection 1, 5
• Acute infections and fever 2, 5
• Congestive heart failure 2, 5
• Marked hyperglycemia 1, 5
• Marked hypertension 1, 5
• Urinary tract infections 2, 5
• Hematuria and pyuria 2

Common Pitfalls in Microalbumin Testing

• Standard dipstick tests for proteinuria do not detect small increases in urine albumin excretion; specific assays for microalbumin are required 1, 2
• Single measurements can be misleading due to significant day-to-day variability in urinary albumin excretion 1, 2
• Failure to adjust for creatinine can lead to errors from variations in urine concentration 2
• Not accounting for sex differences in creatinine excretion can affect interpretation (some guidelines suggest multiplying the concentration in men by 0.68 to provide a sex-independent reference range) 1

By understanding normal microalbumin levels and proper testing methods, clinicians can better identify early kidney disease and implement interventions to prevent progression to more severe kidney dysfunction.