Renal Pathophysiology

CHAPTER 8 Urinalysis and Approach to the Patient With Renal Dysfunction

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pathognomonic of this condition. Conversely, the absence of eosinophiluria does not exclude an acute allergic interstitial nephritis as other types of white blood cells may predominate (neutrophils, lymphocytes). Eosinophiluria can be detected by the use of special stains (such as Hansel stain) on the urine sediment but is not routinely done. Epithelial Cells and Lipiduria Renal tubular epithelial cells are 1.5 to 3 times the size of a white cell with a round, large nucleus. Although epithelial cells from the lower urinary tract tend to be much larger with a small nucleus, the only way to be certain of their renal origin is if the cells are contained within a cast. Occasional renal epithelial cells are excreted in the urine, a probable re flection of a normal cell turnover. Increased numbers of epithelial cells may be shed into the urine in a variety of renal diseases, including tubulointersti tial disorders and glomerular diseases associated with proteinuria. In the lat ter setting, the tubular cells may undergo fatty degeneration with fat droplets appearing in the cytosol; these fat-laden cells are called oval fat bodies . The fat droplets may also be free in the urine, where they are of the same size as or smaller than the red cells. They can be identified by viewing the urine under polarized light. Fat is doubly refractile and shows a characteristic “Maltese cross” appearance (Plate 8.1). The fat within the epithelial cells is probably derived from the filtration and subsequent cellular uptake of lipoprotein-bound cholesterol. This se quence will occur only when glomerular disease leads to the filtration of nor mally nonfiltered macromolecules. Thus, lipiduria is essentially diagnostic of glomerular disease and nephrotic syndrome. In addition to intracellular droplets, both free fat droplets and fatty casts may be seen. Crystals A variety of crystals can be seen in the urine sediment depending on the urine composition, concentration, and pH (Plate 8.3). For example, uric acid tends to precipitate in an acidic urine (pH < 5.5), whereas phosphate salts precip itate in an alkaline urine (pH > 7.0). In comparison, the solubility of calcium oxalate is pH independent. Urinary crystals can be seen in normal subjects and are generally of no diagnostic importance. One major exception is the presence of cystine crys tals with their characteristic hexagonal shape. These crystals are essentially seen only in patients with cystinuria, a hereditary disorder characterized by mutations in two genes that encode a protein responsible for cystine and dibasic acid transport or an amino acid transporter. Mutations lead to im paired proximal cystine reabsorption, increased cystine excretion, and the formation of cystine stones.

CHAPTER 8 Urinalysis and Approach to the Patient With Renal Dysfunction

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