Renal Pathophysiology



 The general correlation between the different patterns of urinary findings and certain disease states  The meaning of the urine sodium concentration and the fractional excretion of sodium (FENa) and how they are used to distinguish between prerenal disease and acute tubular necrosis as the cause of acute renal failure Introduction Patients with renal disease can present to the physician in a number of different ways. Some have symptoms that are directly related either to the urinary tract (such as flank pain or gross bleeding that turns the urine red) or to associated extrarenal findings induced by the renal disease (such as edema or hyperten sion). However, many patients are asymptomatic, and the presence of under lying renal disease is incidentally discovered when routine laboratory tests reveal an elevated plasma creatinine concentration or an abnormal urinalysis. The major types of renal disease are grouped according to the following commonly used functional classification: „ Prerenal disease, in which reduced renal perfusion is the primary abnormality „ Postrenal disease, in which obstruction at some site in the urinary tract partially or completely blocks the flow of urine „ Intrinsic renal disease, which can be caused by glomerular, vascular, or tu bulointerstitial disorders The major causes of renal disease, most of which is discussed in the fol lowing chapters, are listed in Table 8.1. Once the presence of renal disease has been documented, the primary goals are to establish the correct diagnosis and to assess the severity of the renal dysfunction. The initial approach to diagnosis begins with the history, physical examination, and careful evaluation of the urine. As will be seen, some urinary findings are virtually pathognomonic for a particular type of disease. Even a rel atively normal urinalysis is a positive finding because it can help to narrow the differential diagnosis. The severity of renal dysfunction is primarily assessed by estimating the glomerular filtration rate (GFR) via measurement and serial mon itoring of the plasma creatinine concentration and calculation of the estimated GFR (eGFR) or the measured creatinine clearance (see Chapter 1). The urinalysis is of variable importance in evaluating the severity and activity of the renal injury. In glomerular diseases, for example, the presence of heavy proteinuria and an active urine sediment with many red cells and casts generally reflect more severe disease than mild proteinuria or a few cells and casts. However, this relationship between the urinary findings and disease severity does not always apply. When acute inflammation in the glomeruli (called glomerulonephritis ) resolves, there may be a transition to chronic dis ease with marked scarring. At this time, the urinalysis typically becomes less abnormal (due to diminished inflammation) despite progressive nephron loss and eventually a decline in GFR.

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