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Q: What is the significance of microalbuminuria in the natural history of diabetic nephropathy?
A: When measured under the right conditions (see
above) and confirmed by a second measurement, a spot urine microalbumin level of >30 to 300 µg/mg creatinine indicates incipient nephropathy. Levels >300 µg microalbumin/mg creatinine are consistent with gross proteinuria; precise quantitation of proteinuria in a 24-hour urine collection is indicated for proper management of such patients.
Sequential phases in the natural history of diabetic nephropathy involve initial glomerular hyperfiltration, followed by microalbuminuria, persistent gross proteinuria (>300 µg/mg creatinine or >300 mg/24 h), nephrotic syndrome, and end-stage renal failure. Histologic lesions consistent with diabetic kidney disease are seen in ~100% of patients with type 1 diabetes with proteinuria and ~85% of patients with type 2 diabetes with proteinuria and concomitant retinopathy. Microalbuminuria, a potentially reversible stage of incipient nephropathy, precedes gross proteinuria by several years in both type 1 and type 2 diabetes. After gross proteinuria has developed, progression to nephrotic syndrome, impaired glomerular filtration, and end-stage renal failure occurs over several years at a variable, but inexorable, pace. The mean duration from diagnosis of type I diabetes to development of overt proteinuria is 17 years, and the time from the occurrence of proteinuria to end-stage renal failure averages 5 years. The natural history of nephropathy is less well understood in type 2 than in type 1 diabetes because of the uncertainty of the duration of diabetes and presence of confounding risk factors such as hypertension and dyslipidemia.2-4
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