Clinical Estimation of GFR and Patterns of Clearance Flashcards
Which substances are ideal for measuring GFR?
inulin, sucrose, and mannitol and creatinine
What is considered the gold standard for measuring GFR?
inulin (no longer used in US) ;
Creatinine
What are characteristics that make a substance ideal for measuring GFR?
- stable plasma concentration that is easily measured
- freely filtered into Bowman’s space
- Not reabsorbed, secreted, synthesized, or metabolized by the kidney
How would a decrease in GFR affect creatinine?
increase plasma creatinine concentration
How would an increase in GFR affect creatinine plasma concentration?
It would decrease it
Name some substances from highest to lowest clearance in the kidney tubules.
PAH > creatinine > inulin > urea > sodium > glucose = albumin
What is free water clearance?
how much solute free water is being excreted
What is the calculation for free water clearance and how do you decipher it?
CH2O = V - (UosmV) / Posm
V = CH2O + Cosm
V = urine flow rate
Uosm = urine osmolarity
Posm = plama osmolarity
CH2O ( +) = hypotonic urine is formed (osmolality < 300 mOsm/ kg
CH2O (-) = hypertonic urine is formed (osmolality > 300 mOsm/kg
Postive-free water clearance tends to cause increased plama osmolality; negative free water clearance causes reduced plasma osmolality
What is FeNa+?
fractional excretion of Na+ it indicates the fraction (percentage) of the filtered Na+ that is excreted. It is useful in differentiating if issue is prerenal from intrarenal acute renal failure
Does aldosterone increase or decrease the FeNa+?
it decreases it because it increases the reabsorption of sodium
Does ANP increase of decrease FeNa+?
it increases it
What are the effects of ADH on urea?
ADH inc. reabsorption of urea in medullary collecting duct > increase in BUN > decrease in clearance > if the plasma concentration is increasing in the renal venous plasma, less is cleared from plasma
