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Flashcards in Renal Function Tests Deck (76)
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what 3 things does renal fxn testing provide information of?

-renal blood flow
-tubular fxn


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?

-serum creatinine
-creatinine clearance
-albumin creatinine ratio
tubular fxn
-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 non-toxic
-be excreted only by kidneys
-neither reabsorbed nor secreted
-stable in blood & urine
-easily measured


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?

-protein intake
-muscle mass
-hydration levels
-liver disease


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
-GI bleed
-too much protein
-high protein catabolism: starvation
55% of ARFs


renal causes of increased BUN? %age?

causes: renal dz
-tubular necrosis
-nephrotoxic drugs
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?

male: 0.6-1.2
female: 0.5-1.1
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?

serum creatinine


eGFR is inaccurate when?

->70 or <18
-muscle dz


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?

-exercise increases
-pregnancy increases
-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
-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


tubular fxn test of FENa <1% is what kind of ARF?

decreased renal blood flow triggers renin-angiotension pathway= kidneys conserve Na+