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Flashcards in Lab Evaluation of Renal Function Deck (57)
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What are the lab markers of renal function?

urea (BUN)
cystatin C (less widely available)


Describe renal handling of urea?

it's freely filtered in the glomerulus and then 40-80% is reabsorbed in the tubule (depending on GFR, hydration, etc.)


What are some factors that will increase serum urea?

high protein in GI (diet or GI bleed)
decreased renal perfusion
decreased renal function


What are cause factors that will decrease serum urea?

decreased protein intake
pregnancy (bc higher GFR)
severe liver disease (don't make it)
anabolic effects from androgens or GH


What is a normal BUN/Cr ratio?



An elevated BUN/Cr ration with a normal creatinine means they have a prerenal or postrenal azotemia?

prerenal azotemia (because the tubule responds to the dehyration by reabsorbing more urea)

postrenal will have an elevated creatinine as well


What are some causes of a low BUN/Cr ration?

low protein intake
severe liver disease
acute tubular necrosis (can't reabsorb the urea)


What percentage of nephrons must be destroyed before the BUN levels rise?

about 80%, so it's not particularly sensitive for renal disease


What is creatinine a byproduct of?

muscle creatine


Describe renal handling of creatinine?

freely filtered at the glomerulus; as levels rise, active secretion increases so the more you have, the more you secrete

there's generally no tubular reabosorption (unlike urea) except in severe CHF and diabetes


What are the limitations of serum creatinine?

it's affected by dietary cooked meat, age, muscle mass

also secreted by the tubule but this is saturable

blocked by some drugs

proportional tubular secretion increases as renal function falls

extra-renal degradation of creatinine increases with renal failure, so levels will start to go down


What are some of the drugs that will block the secretion of creatinine in the tubule?

cimetidine, trimethoprim, dapsone, pyrimethamine


What's the best index of renal function?

the GFR


What is the GFR actually?

the volume of fluid filtered from the glomerular capillaries into Bowman's space per unit time (mL/min)


What are some analytes used to measure GFR?

inulin (polyfructose)
contrast dyes like iohexol and iothalamate
creatinine (endogenous)


What is the creatinine clearance equation?

CLcr = UcrV / Pcr


What are the two ways to get a diagnosis of CKD?

1. kidney damage for >3months as defined by structural or fucntional abnormalitie sof the kidney, with or without decreased GFR manifested by either pathologic abnormalities or markers of kidney damage (blood or urine tests) or imaging abnormalities

2. GFR less than 60 for over 3 months with or without kidney damage


What are the stafes of chronic kidney disease based on GFR?

>90 stage 1
60-90 stage 2
30-59 stage 3
15-29 stage 4 (start dialysis)
<15 stabe 5 (kidney failure - get transplant)


What is cystatin C?

it's a cystine protease inhibitor


What will levels of cystatin C do in kidney disease?

go up


Why is cystatin C a helpful alternative to creatinine for detecting CKD?

it's less influenced by age, gender and race and it can detect subtle declines in renal function

unfortunately less available than creatinine


WHat is a normal amount of daily protein loss in urine?

< 150 mg/day with less than 30 mg/day of albumin


What is microalbuminuria defined as?

30-300 mg/day


What is the ideal route for measuring proteinuria?

24 hr collection, but this is unreliable


What is the screening lab procedure of choice for patients at risk of kidney disease (HTN, DM, fam hx, etc.)

albumin:creatinine ratio


What is the definition of chronic kidney disease?

persistent and usually progressive reduction in glomerular filtration rate (less than 60) and/or albuminuria (more than 30 mg of urinary albumin per gram of urinary creatinine)


Who are the folks at risk for CKD?

patients with diabetes, HTN, fam hx of CKD (polycycstic kidney disease), chronic use of OTC painkillers and hx of trauma to the kidneys


What is the prevalence of CKD?

16.8% of adults over 20 yoa

more common in persons over 60 yoa


Who should be screened for CKD and how often?

diabetics annually
HTN every 3 yrs
FHx of CKD every 3 yrs


WHen should a diabetic be treated for CKD?

when albumin/cr ration is over 30 mg alb/ 1g cr