urinalysis and body fluids

©2008 F. A. Davis
CHAPTER 2 • Renal Function 27
29. Calculate the free water clearance from the following
results:
urine volume in 6 hours: 720 mL; urine osmolarity:
225 mOsm; plasma osmolarity: 300 mOsm
30. To provide an accurate measure of renal blood flow,
a test substance should be completely:
A. Filtered by the glomerulus
B. Reabsorbed by the tubules
C. Secreted when it reaches the distal convoluted
tubule
D. Cleared on each contact with functional renal tissue
31. Given the following data, calculate the effective renal
plasma flow:
urine volume in 2 hours: 240 mL; urine PAH:
150 mg/dL; plasma PAH: 0.5 mg/dL
32. Renal tubular acidosis can be caused by the:
A. Production of excessively acidic urine due to
increased filtration of hydrogen ions
B. Production of excessively acidic urine due to
increased secretion of hydrogen ions
C. Inability to produce an acidic urine due to
impaired production of ammonia
D. Inability to produce an acidic urine due to
increased production of ammonia
33. Tests performed to detect renal tubular acidosis after
administering an ammonium chloride load include
all of the following except:
A. Urine ammonia
B. Arterial pH
C. Urine pH
D. Titratable acicity
Case Studies and Clinical Situations
1. A 44-year-old man diagnosed with acute tubular
necrosis has a blood urea nitrogen of 60 mg/dL and a
blood glucose level of 100 mg/dL. A 2 urine glucose
is also reported.
a. State the renal threshold for glucose.
b. What is the significance of the positive urine glucose
and normal blood glucose?
2. A patient develops a sudden drop in blood pressure.
a. Diagram the reactions that take place to ensure
adequate blood pressure within the nephrons.
b. How do these reactions increase blood volume?
c. When blood pressure returns to normal, how does
the kidney respond?
3. A physician would like to prescribe a nephrotoxic
antibiotic for a 60-year-old man weighing 80 kg. The
patient has a serum creatinine level of 1.0 mg/dL.
a. How can the physician determine whether it is
safe to prescribe this medication before the patient
leaves the office?
b. Can the medication be prescribed to this patient
with a reasonable assurance of safety?
c. A creatinine clearance was also run on the patient
with the following results: serum creatinine,
0.9 mg/dL; urine creatinine, 190 mg/dL; 24-hour
urine volume, 720 mL. Should the patient
continue to take the medication? Justify your
answer.
4. A laboratory is obtaining erratic serum osmolarity
results on a patient who is being monitored at
6 a.m., 12 p.m., 6 p.m., and 12 a.m. Osmolarities are
not performed on the night shift; therefore, the midnight
specimen is run at the same time as the 6 a.m.
specimen.
a. What two reasons could account for these discrepancies?
b. If the laboratory is using a freezing point osmometer,
would these discrepancies still occur? Why or
why not?
c. If a friend was secretly bringing the patient a pint
of whiskey every night, would this affect the
results? Explain your answer.
5. Following overnight (6 p.m. to 8 a.m.) fluid deprivation,
the urine-to-serum osmolarity ratio in a patient
who is exhibiting polyuria and polydipsia is 1:1. The
ratio remains the same when a second specimen is
tested at 10 a.m. Vasopressin is then administered subcutaneously
to the patient, and the fluid deprivation is
continued until 2 p.m., when another specimen is
tested.
a. What disorder do these symptoms and initial laboratory
results indicate?
b. If the urine-to-serum osmolarity ratio on the 2 p.m.
specimen is 3:1, what is the underlying cause of
the patient’s disorder?
c. If the urine-to-serum osmolarity ratio on the 2 p.m.
specimen remains 1:1, what is the underlying cause
of the patient’s disorder?