clin path: assessment of renal function Flashcards
what is GFR
the volume of fluid diltered from the glomerular capillaries into the bowmans capsule per unit time
Low GFR means
nephron loss = renal damage
what is renal clearance
- an indication of the kidneys ability to remove compounds
- ## depends on GFR, tubular reabsorption and subular secretion
a substance must be …….. to be considered equal to GFR
- filtered
- not reabsorbed
- not secreted
- non-toxic
- not plasma bound protein
how is clearance calculated
what marker substances are useful for determining GFR by clearance
experimental:
- inulin
- exogenous creatinine infusion
- radioisotopes
clinical:
- endogenous creatinine
- iohexol
- exogenous creatiniine
what is the challenge with measuring only creatinine as a test for GFR
- needs both an accurate urine collection over time and a blood sample
- only ever an estimate as creatinine doesnt comply with the rule on a good clearance marker (there is some tubular secretion, some GI clearance at high concentrations)
clinically how do we assess GFR
- measure factors in blood that would normally be renally excreted
- plasma non-protein nitrogenous waste
- plasma/serum urea concentration (not really in large animals)
- plasma/serum creatinine concentration (mostly complies with GFR marker requirements
- however, high urea or creatinine implies 70-75% nephron loss
discuss urea as a marker for GFR
- less reliable than creatinine as a measure of GFR but standard measure included on blood profiles
- pre-renal effects include filtrate follow rates
- distal nephron resorption of urea for medullary concentration gradient
- more sensitive to pre-renal than creatinine
- post prandial increase after protein meal BUT also GI bleeding
- less reliable in horses and ruminants because of microbial NH3 production
- issue if have a species that doesnt metabolise urea (fish, reptiles, birds)
aside from creatinine and urea, what other measures in the plasma and markers for GFR
- phosphate in the plasma = reduced renal excretion
- potassium
- SDMA
urinalysis includes
- SG
- pH
- protein (pre renal, glomerular, tubular, or post renal)
- glucose
- blood
pre renal proteinuria is due to
overload like glucosuria in hyperglycemia
glomerular renal proteinuria is due to
damaged glomeruli
tubular renal proteinuria is due to
unable to resorb normal amounts of filtered protein
post renal proteinuria is due to
urinary tract inflammation
what is cystatin C
- measurement in the blood
- estimate of GFR
- not found to be superior to creatinine in our species
- should be completely resorbed in convoluted tubule
- if present in urine suggests tubular dysfunction/damage
what substances are absorbed in the proximal tubule
what is fractional clearance
- electrolytes are filtered, secreted and absorbed
- clearance changes in tubule disease
- ratio of electrolyte clearance to creatinin clearance
- how much are we losing - relating clearance of something with creatinine clearance and comparing the ratio
- low fractional clearance suggests net conservation (resorption)
low fractional clearance means (< 1)
net conservation (resorption)
using USG to infer concentrating ability relies on the assumption that:
the only things in urine are what are supposed to be in healthy urine
i.e.: no glucose, no protein
how do you interpret USG
how does USg hep us interpret azotemia/dehydration
why is hyposthenuria an ADH problem and not a renal problem
the only way to make dilute urine (hyposthenuria) is if you have enough ascending limbs of the LoH, therefore the nephrons are functional but ADH isnt doing its job to conserve water
outline the different types of casts seen in sediment analysis and what they mean
which clinical pathology prarameter will be the least affected at the least degree of nephron loss
USG
which clinical pathology parameter will be affected at the most severe renal failure
hyperkalaemia (less than 5% nephron availibility
renal or pre-renal disease
1: pre renal = high urea and creatinine but making hypersthenuric urine
2: ranel = high urea and high creatinine AND hyposthenuric
3: high urea but normal creatining and USG, could be post prandial OR have GI haemorrhage (digest extrea protein from blood)
4: is azotaemic with hyposthenurea = ADH issue. also have high Ca, hypercalcaemia can give us diabetes insipidus because it antagonises ADH
a dog has a high concentration of both urea and creatinine USG is 1.040. this best fits:
pre-renal. concentrating well (USG)
the relationship between serum creatinine and GFR is
inverse curvilinear
