A patient with Crohn’s disease is admitted after 4 days of diarrhea. Which of the following urine-specific gravity values do you expect to find in this patient?
A. 1.005
B. 1.011
C. 1.020
D. 1.030
Correct Answer: D. 1.030
The normal range of specific gravity of urine is 1.010 to 1.025; a value of 1.030 may be seen with dehydration. Specific gravity measures the kidney’s ability to concentrate or dilute urine in relation to plasma. Because urine is a solution of minerals, salts, and compounds dissolved in water, the specific gravity is greater than 1.000. The more concentrated the urine, the higher the urine-specific gravity.
Option A: In diabetes insipidus, there is an absence or decrease of antidiuretic hormone. Without antidiuretic hormone, the kidneys produce an excessive amount of urine, often up to 15 to 20 liters per day with low specific gravity.
Option B: The first and most common reason for an increase in urine-specific gravity is dehydration. The second reason for a high specific gravity is an increased secretion of antidiuretic hormone (ADH). ADH causes increased tubular water re-absorption and decreased urine volume. Trauma, stress reactions, surgery, and many drugs cause an increase in ADH secretion.
Option C: Glomerulonephritis and pyelonephritis cause a decreased urine volume and low specific gravity. In these diseases, damage to the kidney’s tubules affects the ability of the kidney to reabsorb water. As a result, the urine remains dilute.
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