[Microalbuminuria: theoretical bases and new applications]

The constant presence of albumin, as detected by common biochemical methods, in multiple urine samples of a patient, was first considered by Bright, in 1836, as a cardinal sign of renal disease ("clinical proteinuria"). Since then this view was widely adopted for studying the clinical evolution of the patients with diabetes mellitus, whose high risk to develop proteinuria and subsequently a progressive decline of renal function was well known. Thus the finding of "clinical proteinuria" by traditional, merely biochemical techniques, has been considered for more than one century as the opening event in the onset of diabetic nephropathy, and a distinctive sign of glomerulopathy in general. More recently, this view has been deeply criticized, mainly because it lies on the implicit assumption that the sensitivity limits of the biochemical tests for the detection of urinary protein concentrations (about 300 mg/dl), coincide with the ones that can distinguish non nephropathic from nephropathic patients (either diabetic or not). Indeed new techniques, that detect urinary proteins down to 1 microgram/ml, have shown that the upper limit of protein excretion in healthy people is well below the minimum concentration detectable by all the traditional tests. Therefore a new clinical entity, named "microproteinuria" has been defined, meaning the urinary excretion rate ranging between the "physiological" and the "clinical" proteinuria; its pathophysiologic, diagnostic and prognostic significance has been extensively evaluated in the last 20 years. Microproteinuria has been shown to represent a crucial event in the natural history of the diabetic nephropathy; in diabetic patients it is strictly related to the risk of future (months to years) development of overt nephropathy and chronic renal failure, and it may predict the risk of macroangiopathic complications. More recently new settings have been proposed for the study of microproteinuria, as an early and sensitive marker of cardiovascular diseases in hypertensive non diabetic patients and even in non hypertensive non diabetic elderly people. The role of microproteinuria in the diagnosis and follow-up of many non-diabetic glomerulopathies is a very interesting though still unexplored field.