Renal Pathophysiology

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RENAL PATHOPHYSIOLOGY: THE ESSENTIALS

Contrast media glucose

Normal range

1.040

1.030

1.020

Specific gravity

1.010

200 400 600

800 1,0001,200 1,400

Osmolality (mOsm/kg)

As described in Chapter 2, the urine osmolality can vary from a low of 50 to 100 mOsm/kg (specific gravity 1.002 to 1.003) after a marked water load and sub sequent suppression of antidiuretic hormone (ADH) release to a high of 1,000 to 1,400 mOsm/kg (specific gravity 1.030 to 1.040) with dehydration and maximum ADH effect. Thus, a random value is of little meaning unless correlated with the plasma osmolality or volume status. In the clinical setting, measurement of urine osmolality is primarily used in the differential diagnosis of hyponatremia, hypernatremia, or polyuria (see Chapter 3). It may also be helpful in distinguish ing between prerenal disease (decreased renal perfusion) and acute tubular ne crosis as the cause of acute kidney injury (AKI) (see Chapter 11). Examination of Urine Sediment The sediment should first be inspected under a low-power objective (10 × ) with reduced light. The high, dry objective (40 × ) can then be used to identify the casts and cells that might be present. Casts Casts represent precipitated proteins and cells that form within the tubular lumen. As a result, they have a cylindrical shape and regular margins to con form to the shape of the tubular lumen. These characteristic findings distin guish casts from irregular clumps of cells or debris. All casts have an organic matrix composed primarily of THMP (or uromod ulin). The chemical characteristics of this protein determine the conditions „ FIGURE 8.2. Relationship between the specific gravity and osmolality in urine from normal subjects. Normal urine contains little glucose or protein ( shaded area ). For comparison, the relationship between specific gravity and osmolality for a pure glucose solution is included. (Modified from Miles BE, Paton A, de Wardener HE. Maximum urine concentration. Br Med J. 1954;2[4893]:901-905.)

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