Renal Function and Automation Flashcards
Osmolality determinations are preferred, over S.G., for the evaluation of ___.
Renal concentrating ability.
Why is osmolality more accurate?
Because each solute particle contributes equally to the osmolality value.
How is osmolality tested?
Freezing point depression and vapor pressure osmometers.
What are the three most prevalent urine solutes?
Urea, sodium, and chloride.
When ADH production or secretion is defective, this indicates what?
Neurogenic diabetes insipidus.
What condition occurs when there is a lack of renal response to ADH?
Nephrogenic diabetes insipidus.
A fluid deprivation test evaluates what?
The ability of renal tubular cells to selectively absorb and secrete solutes. In other words, it assesses the renal concentrating ability of the kidneys.
During a fluid deprivation test, if the urine osmolality is greater than 800 mOsm/kg, what does this indicate?
A normal result.
What occurs if a fluid deprivation test is failed?
A patient is administered with ADH (vasopressin) and tested again a few hours later to determine:
(1) Urine osmolality is >800 mOsm/kg
(2) Urine osmolality to serum osmolality (U/S) is greater than 3.0.
What is indicative of a negative response to ADH?
Nephrogenic problem - the renal receptors for ADH are dysfunctional.
How would you calculate osmolar clearance?
The ratio of urine osmolality to serum osmolality multiplied by the timed urine volume.
=Uosm (mOsm/kg)/Sosm (mOsm/kg) X Vol (mL/min)
The total volume of urine excreted by the kidneys actually consists of what two separate volumes?
Osmolar clearance water and solute-free water.
What is the osmolar clearance water?
The volume of water required to eliminate the solutes from the plasma.
What is the solute-free water?
Additional water that exceeds bodily needs, is retained in the tubules, and is eliminated in the urine.
A negative free-water clearance is indicative of what?
The kidneys are reabsorbing water and are producing urine that is hyperosmotic or hypertonic.
For the kidneys to remove metabolic wastes and selectively reabsorb solutes and water, they require adequate ___ through the glomeruli.
renal plasma flow.
The renal plasma flow (RPF) determines what?
The amount of plasma ultrafiltrate processed by the nephrons to the kidneys.
Define renal clearance.
The volume of plasma in milliliters that is completely cleared of a substance per unit of time.
How do you calculate renal clearance?
C (mL/min) = U (mg/dL) x V (mL/min) / P (mg/dL)
C= renal clearance; U= urine concentration; P= plasma concentration; V=timed collection volume.
Substances that are removed exclusively by glomerular filtration (e.g., insulin) can be used to determine what?
Glomerular filtration rate (GFR).
Any substances that (1) ___, (2) ___, and (3) ___, can be used to determine GFR.
(1) Maintains constant plasma level.
(2) Excreted solely by glomerular filtration.
(3) Is not reabsorbed or secreted by the tubules.
What is the reference method for GFR determination?
Inulin Clearance.
Why is creatinine used, over urea, for clearance testing?
Creatinine is not reabsorbed by the renal tubules. Nor is it affected by the urine flow rate, and plasma levels are not altered by a normal diet.
How is creatinine formed?
As a byproduct of muscle metabolism; formed from creatine and phosphocreatine.
Why does the creatinine clearance test dependent on the gender of the patient?
Creatinine production depends directly on muscle mass; males and athletes produce more creatinine compared to women, children, and the elderly.
Because of dependence on individual muscle mass, creatinine clearance values are normalized to the external body surface area of an average individual by a factor of ___.
1.73 m^2
What is the equation for normalized creatinine clearance?
C (mL/min) = (UxV/P) x (1.73/SA)
C=creatinine clearance; U=urine concentration of creatinine; V=volume of urine excreted in mL/min; P=plasma concentration of creatinine; SA=patient’s body surface area.
How does lipemic specimens effect the osmolality?
False increase.
How does lactic acid effect osmolality?
False increase.
What is the ideal ratio of urine to serum osmolality after controlled fluid intake?
3:1.
Following an ADH injection, a patient DOES NOT have a urine:serum osmolality of 3:1. What does this indicate?
Lack of ADH receptors.
Following an ADH injection, a patient has a urine:serum osmolality of 3:1. What does this indicate?
Lack of ADH production.
What is the function of ADH?
Reabsorbs water from the tubules.
What is the specimen of choice for GFR testing?
24-hour urine collection.
What are some factors
