Determination of Renal Function and Basics of TDM Flashcards
(14 cards)
Describe excretion and how to calculate it.
Things that are eliminated from the kidney into the urine.
Rate of elimination = rate of filtration + (rate of secretion - rate of absorption)
Describe filtration.
Mass movement of water and solutes from plasma to the renal tubule that occurs in the renal corpuscle
* driven by hydraulic pressure (blood pressure) in the capillaries of the glomerulus
* majority of proteins are too large or too charged to be filtered through the kidney
* Molecules bound to protein cannot cross
Describe secretion.
After filtration, the tubules continue to secrete additional substances into the tubular fluid
* via active transport
* enhances the kidney’s ability to eliminate certain wastes and toxins
* occurs in the proximal tubule
Describe reabsorption.
Movement of water and solutes from the tubule back into the plasma
* distal tubule and collecting duct
* non-ionizing drugs are reabsorbed
* ionizing drugs are excreted
Explain glomerular filtration rate (GFR)
Marks renal function
The volume of plasma filtered across the glomerulus per unit time
* Represents the total sum of filtration rates of all nephrons in both kidneys
* best index of overall renal function
* cannot be measured directly –> must use creatinine clearance
* normal GFR = 100-140 ml/min
Describe creatine.
- Derived from meat or supplements
- produced in the liver, pancreas, and kidney
- transported to muscle tissue
Describe creatinine.
Waste product of creatine from muscle metabolism.
* released into serum at a constant rate proportional to muscle mass
* more muscle = more creatinine eliminated
Explain serum creatinine.
Scr - amount of creatinine in the blood
* normal range in younger people - 0.5-1.1 mg/dL
What are some things to consider when estimating GFR in patients?
- Creatinine clearance (CrCl) is roughly proportional to GFR
- however, creatinine undergoes active renal tubular secretion
- 10-40% (lower end with good kidneys, higher end with bad kidneys)
- total % creatinine secreted (rather than filtered by glomeruli)
How do you calculate CrCl?
CrCl = Crfilt + crsec
* GFR is the same as CrCl - Crsec but Crsec is very small –> therefore, GFR is equated to CrCl
* equation is less accurate for those with worse kidneys
What is the relationship between src and GFR?
Large Scr changes = less significant decline in GFR over lower GFR ranges
Smaller Scr changes = more significant decline in GFR over higher GFR ranges
A fall in GFR decreases creatinine filtration and produces a proportionate rise in the serum creatinine concentration
Name factors that can impact production and secretion of Src.
Production:
* diet
* muscle mass
* acute muscle damage
secretion:
* secretion % can cause Scr to appear “normal” even with significant real GFR declines
* secretion of creatinine gets close to saturated once Scr 1.5-2.0 mg/dL is reached
Describe Src caveats.
- Scr doesn’t immediately reflect acute changes in kidney function (lag)
- type of lab assay used may impact result
- some medications directly impact creatinine secretion (transport proteins)
Name the types of drugs that can inhibit tubular secretion of creatinine.
- diuretics - increase kidney water excretion
- antibiotics/anti-infectives
- cimetidine - potent inhibitor of creatinine
- anti-HIV medications
- pharmacokinetic boosters - pair with other drugs to increase therapeutic effect of other drugs
- anticancer drugs