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Labs Medicine > Lab Evaluation of Renal Function > Flashcards

Lab Evaluation of Renal Function Flashcards

1
Q

What are the lab markers of renal function?

A

urea (BUN)
creatinine
albuminuria
cystatin C (less widely available)

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2
Q

Describe renal handling of urea?

A

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

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3
Q

What are some factors that will increase serum urea?

A

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

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4
Q

What are cause factors that will decrease serum urea?

A 
decreased protein intake
over-hydration
pregnancy (bc higher GFR)
severe liver disease (don't make it)
anabolic effects from androgens or GH
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5
Q

What is a normal BUN/Cr ratio?

A

12-16:1

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6
Q

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

A

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

postrenal will have an elevated creatinine as well

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7
Q

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

A

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

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8
Q

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

A

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

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9
Q

What is creatinine a byproduct of?

A

muscle creatine

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10
Q

Describe renal handling of creatinine?

A

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

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11
Q

What are the limitations of serum creatinine?

A

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

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12
Q

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

A

cimetidine, trimethoprim, dapsone, pyrimethamine

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13
Q

What’s the best index of renal function?

A

the GFR

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14
Q

What is the GFR actually?

A

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

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15
Q

What are some analytes used to measure GFR?

A

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

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16
Q

What is the creatinine clearance equation?

A

CLcr = UcrV / Pcr

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17
Q

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

A
  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
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18
Q

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

A 
>90 stage 1
60-90 stage 2
30-59 stage 3
15-29 stage 4 (start dialysis)
<15 stabe 5 (kidney failure - get transplant)
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19
Q

What is cystatin C?

A

it’s a cystine protease inhibitor

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20
Q

What will levels of cystatin C do in kidney disease?

A

go up

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21
Q

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

A

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

unfortunately less available than creatinine

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22
Q

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

A

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

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23
Q

What is microalbuminuria defined as?

A

30-300 mg/day

24
Q

What is the ideal route for measuring proteinuria?

A

24 hr collection, but this is unreliable

25
Q

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

A

albumin:creatinine ratio

26
Q

What is the definition of chronic kidney disease?

A

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)

27
Q

Who are the folks at risk for CKD?

A

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

28
Q

What is the prevalence of CKD?

A

16.8% of adults over 20 yoa

more common in persons over 60 yoa

29
Q

Who should be screened for CKD and how often?

A

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

30
Q

WHen should a diabetic be treated for CKD?

A

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

31
Q

When should non-diabetics be treated for CKD?

A

with albumin/cr ratio over 300 mg albumin/g cr

or with eGFR < 60

32
Q

What is the GFR criteria for AKI?

A

increased Cr x2 or GFR decrease over 50%

33
Q

What is the urine output criteria for AKI?

A

UO less than 0.5 ml/kg/hr for 12 hours

34
Q

What are some clinical features of AKI?

A

Non-specific like n/v, malaise, abd pain, ileus, AMS, asterixis

HTN from altered fluid homeostasis

pericardial fluid with friction rub/tamponade

arrthmia (hyperK)

lung rales (hypervolemia)

platelet dysfunction with clotting abnormalities

35
Q

What are some lab features of AKI?

A 
increased BUN and Cr obviously
HyperK
Hyperphos (with subsquent hypocalcemia)
Anemia (decreased EPO)
Platelet abnormalities
36
Q

What are the three classifications of AKI?

A

prerenal (most common)
renal
postrenal

37
Q

What are the two general causes of prerenal AKI?

A

decrease in intravascular volume or a decrease in vascular resistance

38
Q

What lab can be used to distinguish axotemia due to prerenal causes from other forms of AKI?

A

FeNa

39
Q

A FeNa < __% suggests prerenal causes.

A

1% because when the GFR falls, the kidney will normally retain salt and water so the FeNa will fall

40
Q

What is the formula for FeNa?

A

(NaU x CrPl) / (CrU x NaPl) x 100%

41
Q

What are the three general categories of AKI that falls under the intrinsic renal category?

A

glomerular
tubuloinsterstitial
vascular

42
Q

What causes 85% of acute intrinsic renal injury?

A

acute tubular necrois

43
Q

What are the two general causes of acute tubular necrosis?

A

toxins or ischemia

44
Q

What are some drugs/toxins that can lead to ATN?

A 
aminoglycosides
amphotericin B
Vancomycin
Acyclovir
Cephalosporins
Radiographic contrast
Cyclosporin
Cisplatin
Heavy metals
45
Q

WHat are some endogenous nephrotoxins that can lead to ATN?

A

myoglobin
hemoglobin
uric acid crystals
bence-jones proteins

46
Q

What will be the lab findings in ATN?

A

hyperkalemia
hyperphosphatemia
BUN;Ct < 20:1 (because the tubules can’t reabsorb the BUN)
pigmented granular “muddy” casts on Urinalysis
FeNa > 3%

47
Q

Most cases of interstitial nephritis are due to what?

A

drugs

48
Q

WHat drugs can commonly lead to interstitial nephritis?

A 
penicillins
cephalosporins
sulfonamides
NSAIDs
rifampin
phenytoin
allopurinol
PPIs
49
Q

WHat are some infections that can lead to interstitial nephritis?

A 
strep
leptospirosis
CMV
histo
RMSF
50
Q

What are some autoimmune diseases that can lead to insterstitial nephritis?

A

SLE
SS
sarcoid
cyroglobulins

51
Q

Describe the clinical presentation of interstitial nephritis?

A

Most will have fever, about half will have a rash

arthralgias common

52
Q

What will you see on urinalysis in interstitial nephritis?

A

eosinophiluria (and eosinophilia) in addition to red cells, white cells, white cll casts and proteinuria (especially in NSAID-induced)

53
Q

What is the most common form of acute glomerulonephritis?

A

IgA nephropathy

54
Q

Direct immunofluorescence shows staining of the GBM for IgG. What’s the disease?

A

Anti-GBM glomerulonephritis; Goodpasteur’s syndrome if there is also lung involvement

55
Q

What are the clinical features of glomerulonephritis?

A

hypertension and edema!

56
Q

Describe the UA seen in glomerulonpheritis?

A 
hematuria
proteinuria less than 3 g/dL
red cell casts! specific!
red cells
white cells
low FeNa
57
Q

What additional labs should you order if you suspect glomerulonephritis?

A

C3 C4 CH50
ANCA, anti-GBM, ASO, ANA, cryoglobulins, hepatitis serologies, blood cultures, renal ultrasound, and maybe a renal biopsy

Labs Medicine (27 decks)

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