Flashcards in Renal Function Tests Deck (76)
what 3 things does renal fxn testing provide information of?
-renal blood flow
why is it imperfect?
-various factors other than damage to renal parenchyma can influence results
what are the 2 definitions of renal disease?
-presence of histological lesions but does not specify any degree of renal dysfxn
-75% of nephrons destroyed/lost but doesn't imply underlying histological lesions
what 4 tests are used to test clearance? what 1 for tubular fxn?
-albumin creatinine ratio
-fractional excretion of Na+
the ideal substance to measure GFR would be what 8 things?
-freely filtered at glomeruli
-not bound to plasma
-not be metabolized
-be excreted only by kidneys
-neither reabsorbed nor secreted
-stable in blood & urine
where does BUN come from? how much is filtered by glomerulus? how much is reabsorbed?
-protein catabolism via urea cycle in liver
-40% reabsorbed so urea clearance is about 60% of true GFR
what substance do you test for BUN levels? what does it evaluate and indicate?
-serum/plasma testing (CMP or BMP)
-evaluates liver fxn
-rough indicator of GFR & renal blood flow
what factors can interfere with BUN?
what are the normal levels of BUN for an adult? critical value? causes for decreased values?
adult value: 10-20 mg/dl
critical value: >100 mg/dl (serious impairment of renal fxn)
decreased values due to: fluid overload, malnutrition, severe liver disease
what is azotemia? pts with azotemia will have what?
azotemia: increased concentration of non-protein nitrogenous waste products
pts w/azotemia will have increased BUN
prerenal causes of increased BUN? what percentage of acute renal failures (ARF)?
-low BV: shock, burns, dehydration
-too much protein
-high protein catabolism: starvation
55% of ARFs
renal causes of increased BUN? %age?
causes: renal dz
40% of ARFs
postrenal causes of increased BUN? %age?
-obstruction of ureters: stones, tumors, congenital
-bladder outlet obstruction: prostatic hypertrophy, cancer, congenital
5% of ARFs
what is serum creatinine? what do the levels depend on? so only when do you see this? more or less stable than BUN?
-catabolic product of creatine phosphate from skeletal muscle
-levels depend on muscle mass; only see fluctuation in muscle mass with muscle-wasting pathology
-more stable than BUN
what happens to creatinine in the kidneys in terms of filtration?
-almost completely filtered by kidneys but also secreted by PCT
when does serum creatinine rise in relation to BUN? what does elevation suggest? who normally has lower levels?
-creatinine levels raise later than BUN
-elevation= chronic disease process
-young and elderly due to lower muscle mass
what are the normal levels? what is creatinine used to diagnose? are its levels affected by liver fxn?
used to diagnose impaired renal fxn minimally affected by liver fxn unlike BUN
what is the rectangular hyperbola?
-large change in GFR early in renal dz cause sm changes in BUN or creatinine
-small changes in GFR late in renal dz cause big changes in BUN or creatinine
what BUN/creatinine ratio is indicative of prerenal azotemia? renal azotemia?
prerenal ratio: >20:1
renal ratio: 10-20: 1
in prerenal dz what will rise first urea or creatinine? in renal dz? will you see proteinuria in prerenal or renal dz?
prerenal: disproportionate rise in urea
renal: risk together
proteinuria in renal dz
as GFR goes down what goes up?
eGFR is inaccurate when?
->70 or <18
to test creatinine clearance what test(s) do you need?
24 hr urine & blood draw during urine collection time
when is CC invalid?
GFR decreased to <30% of normal
normal CC values? how much does it decrease by each decade after 20 yo?
male: 90-139 ml/min
female: 80-125 ml/min
values decrease 6.5 ml/min each decade after 20
what are 4 factors which affect CC?
-incomplete collection gives false low value
what is great about cystatin C? normal range?
-filtered by glomerular membrane & metabolized by PTs
-estimates GRF independent of gender, age, race, muscle mass, cirrhosis, no need to correct for height and weight
SUPERIOR TO CREATININE
-adults: 0.54-1.55 mg/L
Ssxs of kidney failure?
-loss of appetite
-low back pain
-decreased urine output
how does ARF present in terms of hx, kidney size, blood disorders and casts? CRF?
ARF: over hours to days, hx of normal renal fxn, kidney size normal, no anemia or casts
CRF: deterioration of renal fxn over time, hx of increased BUN & creatinine, kidney usu sm, anemia usu & casts in urine