Proteinuria Classification in SI Units
Normal, microalbuminuria, and nephrotic range proteinuria are defined as: normal <30 mg/g creatinine, microalbuminuria 30-300 mg/g creatinine, and macroalbuminuria (overt proteinuria) >300 mg/g creatinine, with nephrotic range proteinuria specifically defined as >3500 mg/g creatinine. 1, 2
Standard Definitions Using SI Units
| Category | Spot Collection (mg/g creatinine) | 24-Hour Collection (mg/24h) | Timed Collection (μg/min) |
|---|---|---|---|
| Normal/Normoalbuminuria | <30 | <30 | <20 |
| Microalbuminuria | 30-300 | 30-300 | 20-200 |
| Macroalbuminuria/Overt proteinuria | >300 | >300 | >200 |
| Nephrotic range proteinuria | >3500 | >3500 | >2400 |
Important Considerations for Measurement
Confirmation requirement: Due to variability in urinary albumin excretion, at least 2 of 3 specimens collected within a 3-6 month period should be abnormal before confirming a diagnosis 1
Preferred collection method: First-morning spot urine samples are recommended, with results reported as albumin-to-creatinine ratio (ACR) in mg/g creatinine 1
Factors that can temporarily increase albumin excretion: 1
- Exercise within 24 hours
- Urinary tract infection
- Fever
- Congestive heart failure
- Marked hyperglycemia
- Pregnancy
- Marked hypertension
- Hematuria
Gender considerations: Women normally have lower urinary creatinine concentrations than men, resulting in higher ACR values for the same level of albumin excretion 1. Some guidelines suggest multiplying men's creatinine values by 0.68 to provide a sex-independent reference range 1
Clinical Significance
Microalbuminuria: Early marker of kidney damage, particularly in diabetes, and an independent risk factor for cardiovascular disease 2, 3
Macroalbuminuria: Associated with progressive decline in GFR and increased risk of progression to end-stage renal disease 1, 2
Nephrotic range proteinuria: Defined as proteinuria >3.5 g/24h (>3500 mg/g creatinine) and typically accompanied by hypoalbuminemia, edema, and hyperlipidemia 2
Screening Recommendations
Screen for microalbuminuria in type 1 diabetes 5 years after diagnosis and at diagnosis for type 2 diabetes 1, 2
Annual screening is recommended for all diabetic patients using spot urine ACR 2
Consider screening patients with hypertension and family history of kidney disease 4
By understanding these classifications and following proper measurement techniques, clinicians can effectively identify and monitor kidney disease progression, allowing for timely intervention to improve outcomes related to morbidity and mortality.