Renal Pathophysiology

CHAPTER 8 Urinalysis and Approach to the Patient With Renal Dysfunction

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Quantification based on dipstick is dependent on how concentrated the urine is. Assessing the specific gravity of the sample can provide some in sight into whether the urine is concentrated or dilute; a specific gravity of 1.010 is considered isosthenuric or similar to normal plasma (280 mOsm/kg). An alkaline urine with pH >6.5 can lead to a false positive result. See below for more discussion of proteinuria assessment. Proteinuria The glomerular capillary wall allows the relatively free filtration of smaller, low-molecular-weight proteins (such as immunoglobulin light chains and amino acids) but restricts the filtration of larger macromolecules (such as albumin and immunoglobulin G [IgG]). The factors responsible for these permselective properties of the glomerular capillary wall are reviewed in Chapter 9. What is important for the purposes of this discussion is to be fa miliar with the three different types of proteinuria that may be seen: „ Glomerular proteinuria —Glomerular proteinuria refers to an increase in the permeability of the glomerular capillary wall that leads to the ab normal filtration and subsequent excretion of larger, normally nonfiltered proteins such as albumin. This problem can be seen with any form of glo merular disease. „ Tubular proteinuria —Low-molecular-weight proteins are normally fil tered and then largely reabsorbed in the proximal tubule. (The small amounts of albumin that are filtered are also mostly reabsorbed at this site.) Tubu lointerstitial diseases that impair tubular function can interfere with this re absorptive process, resulting in increased excretion of these smaller proteins. Tubular proteinuria is a marker of chronic kidney disease but usually has no clinical sequela unless accompanied by other defects in proximal function, potentially leading to problems such as metabolic acidosis (from bicarbon ate wasting) and hypophosphatemia and rickets (from phosphate wasting). „ Overflow proteinuria —In some conditions, increased production of smaller proteins leads to a rate of filtration that exceeds normal proximal reabsorptive capacity. This occurs most commonly with overproduction of monoclonal immunoglobulin light chains in multiple myeloma and other plasma cell dyscrasias. Limitations of the Dipstick The dipstick commonly used in the initial evaluation of the urine is impreg nated with a dye that changes color according to the quantity of proteins present, particularly albumin. Although the dipstick is reasonably accurate for the detection of glomerular proteinuria (see the following text), it will miss nonalbumin proteins such as immunoglobulin light chains. Similarly, periodic measurements of urinary microalbumin on random urine are the standard for monitoring patients for the development of diabetic nephropa thy. However, this assay will also miss nonalbumin proteins in the urine that

CHAPTER 8 Urinalysis and Approach to the Patient With Renal Dysfunction

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