Renal Pathophysiology

CHAPTER 8 Urinalysis and Approach to the Patient With Renal Dysfunction

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Total protein-to-creatinine ratio

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8 10 12 14 Protein excretion (g/day/1.73 m 2 )

measurement of small amounts of albumin excretion (µg, microalbumin uria). Like the urine protein/creatinine ratio, the microalbumin/creatinine ratio is a valid estimate of microalbumin excretion rates. The normal rate of albumin excretion is < 20 mg/day (15 µg/min); persistent albumin excretion between 30 and 300 mg/day (20 to 200 µg/min) is called microalbuminuria and, in patients with diabetes, is usually indicative of diabetic nephropathy. Specific Gravity and Osmolality The concentration of the unspun urine can be estimated with a urometer, which measures the specific gravity of the urine. The specific gravity is de fined as the weight of the solution compared with the weight of an equal volume of distilled water. Plasma, for example, is 0.8% to 1.0% heavier than water and therefore has a specific gravity of 1.008 to 1.010. The specific gravity is proportional both to the number of solute particles present and to the weight of the solute particles present. It is, therefore, different from the more accurate measurement of urine osmolality because osmolality is determined only by the number of solute particles present. The relationship be tween these parameters is relatively predictable in normal subjects in whom the urine primarily contains urea and sodium, potassium, and ammonium salts; for example, a urine osmolality of 300 mOsm/kg—similar to that of the plasma— is equivalent to a specific gravity of 1.008 to 1.010 (Fig. 8.2). However, there is a disproportionate increase in the specific gravity when larger solutes are present, such as glucose (molecular weight 180) and radiocontrast media (molecular weight ∼ 550). In these settings, the urine specific gravity can exceed from 1.030 to 1.040 even though the urine osmolality may be only 300 mOsm/kg. „ FIGURE 8.1. Protein-to-creatinine ratio to estimate protein excretion. The relationship between estimates of protein excretion on random urine determina tions of protein and creatinine with 24-hour measurements of total protein excre tion. (Modified from www.uptodate.com.)

CHAPTER 8 Urinalysis and Approach to the Patient With Renal Dysfunction

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