A multipart question:
- What is the normal urinary albumin to creatinine ratio?
- What is the normal urinary protein to creatinine ratio?
- Why are these different?
- What are the units?
- What is the definition of microalbuminuria?
Normally, small amounts of albumin and other proteins are filtered at the glomerulus. These proteins are almost completely reabsorbed at the proximal tubule. Since not all protein is completely reabsorbed, it is normal to have a small amount of urinary protein.
In a 24 hour urine collection, it would be considered normal to have up to 150 mg per day of protein, of which approximately 10 mg is albumin. 24 hour urine collections are often impractical and it is difficult to ensure a complete collection. Therefore, spot urine samples can be assessed for the ratio of protein to creatinine and albumin to creatinine. One can then estimate by extrapolation what the 24 hour excretion rate would be.
- Normal urine albumin to creatinine ratio is <17 mg albumin/g creatinine for men, <25 mg albumin/g creatinine for women.
- Normal urine protein to creatinine ratio is <200 mg protein/gram creatinine.
- Urinary protein can be comprised of both albumin and other proteins. Therefore, the urine albumin would not account for all protein excretion.
- See above. Note that the values are different depending on whether youmeasure urine creatinine in mmol/L or mg/dL.
- Microalbuminuria refers to albumin excretion of 30-300 mg/day, which correlates to a urine albumin:creatinine ratio of 17-250 mg/g for men and 25-355 mg/g for women.
K/DOQI Clinical Practice Guidelines for Chronic Kidney Disease: Evaluation, Classification, and Stratification PART 5. EVALUATION OF LABORATORY MEASUREMENTS FOR CLINICAL ASSESSMENT OF KIDNEY DISEASE
Levey AS et al. Ann Intern Med. National Kidney Foundation practice guidelines for chronic kidney disease: evaluation, classification, and stratification. 2003 Oct 7. 139(7):605.