Published on 01/03/2015 by admin
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Last modified 01/03/2015
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Proteinuria refers to abnormal urinary excretion of protein. Detection of proteinuria is important. It is associated with renal and cardiovascular disease; it identifies diabetic patients at risk of nephropathy and other microvascular complications; and it predicts end-organ damage in hypertensive patients. Although proteinuria may arise through various mechanisms (see below), it is most often an indication of abnormal glomerular function. It can be measured and expressed in various ways.
The mechanisms of proteinuria are shown in Figure 17.1.
Fig 17.1 Mechanism of proteinuria.
The glomerular basement membrane through which blood is filtered does not usually allow passage of albumin and large proteins, and proteinuria is most often due to abnormally ‘leaky’ glomeruli. The extent of this ‘leakiness’ varies enormously. At its most extreme, the glomerulus allows large quantities of protein to escape. When this happens, the ability of the body to replace the lost protein is exceeded, and the protein concentration in the patient’s blood falls. Protein is measured in blood either as total protein or albumin. When patients become hypoproteinaemic and hypoalbuminaemic due to excessive proteinuria, the normal balance of osmotic and hydrostatic forces at capillary level is disturbed, leading to loss of fluid into the interstitial space (oedema). This is known as the nephrotic syndrome (defined in terms of protein excretion – more than 3 g daily).
Some proteins are so small that, unlike albumin and other larger proteins, they pass through the glomerulus freely. The best-known examples are beta-2-microglobulin and alpha-1-microglobulin. Others include retinol-binding protein and N-acetyl-glucosaminidase. If these proteins are detected in excess in the urine, this reflects tubular rather than glomerular dysfunction, i.e. an inability of the renal tubules to reabsorb them. However, tubular function is normally investigated in other ways, and the measurement of these proteins in urine is normally confined to the screening and detection of chronic asymptomatic tubular dysfunction, or a small number of specific clinical scenarios, e.g. toxicity due to aminoglycosides, lithium, or mercury.
Clinical Biochemistry An Illustrated Colour Text-5E
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