Lab Evaluation of Renal Function Flashcards
What are the lab markers of renal function?
urea (BUN)
creatinine
albuminuria
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 over-hydration pregnancy (bc higher GFR) severe liver disease (don't make it) anabolic effects from androgens or GH
What is a normal BUN/Cr ratio?
12-16:1
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
starvation
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?
- 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
- 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
When should non-diabetics be treated for CKD?
with albumin/cr ratio over 300 mg albumin/g cr
or with eGFR < 60
What is the GFR criteria for AKI?
increased Cr x2 or GFR decrease over 50%
What is the urine output criteria for AKI?
UO less than 0.5 ml/kg/hr for 12 hours
What are some clinical features of AKI?
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
What are some lab features of AKI?
increased BUN and Cr obviously HyperK Hyperphos (with subsquent hypocalcemia) Anemia (decreased EPO) Platelet abnormalities
What are the three classifications of AKI?
prerenal (most common)
renal
postrenal
What are the two general causes of prerenal AKI?
decrease in intravascular volume or a decrease in vascular resistance
What lab can be used to distinguish axotemia due to prerenal causes from other forms of AKI?
FeNa
A FeNa < __% suggests prerenal causes.
1% because when the GFR falls, the kidney will normally retain salt and water so the FeNa will fall
What is the formula for FeNa?
(NaU x CrPl) / (CrU x NaPl) x 100%
What are the three general categories of AKI that falls under the intrinsic renal category?
glomerular
tubuloinsterstitial
vascular
What causes 85% of acute intrinsic renal injury?
acute tubular necrois
What are the two general causes of acute tubular necrosis?
toxins or ischemia
What are some drugs/toxins that can lead to ATN?
aminoglycosides amphotericin B Vancomycin Acyclovir Cephalosporins Radiographic contrast Cyclosporin Cisplatin Heavy metals
WHat are some endogenous nephrotoxins that can lead to ATN?
myoglobin
hemoglobin
uric acid crystals
bence-jones proteins
What will be the lab findings in ATN?
hyperkalemia
hyperphosphatemia
BUN;Ct < 20:1 (because the tubules can’t reabsorb the BUN)
pigmented granular “muddy” casts on Urinalysis
FeNa > 3%
Most cases of interstitial nephritis are due to what?
drugs
WHat drugs can commonly lead to interstitial nephritis?
penicillins cephalosporins sulfonamides NSAIDs rifampin phenytoin allopurinol PPIs
WHat are some infections that can lead to interstitial nephritis?
strep leptospirosis CMV histo RMSF
What are some autoimmune diseases that can lead to insterstitial nephritis?
SLE
SS
sarcoid
cyroglobulins
Describe the clinical presentation of interstitial nephritis?
Most will have fever, about half will have a rash
arthralgias common
What will you see on urinalysis in interstitial nephritis?
eosinophiluria (and eosinophilia) in addition to red cells, white cells, white cll casts and proteinuria (especially in NSAID-induced)
What is the most common form of acute glomerulonephritis?
IgA nephropathy
Direct immunofluorescence shows staining of the GBM for IgG. What’s the disease?
Anti-GBM glomerulonephritis; Goodpasteur’s syndrome if there is also lung involvement
What are the clinical features of glomerulonephritis?
hypertension and edema!
Describe the UA seen in glomerulonpheritis?
hematuria proteinuria less than 3 g/dL red cell casts! specific! red cells white cells low FeNa
What additional labs should you order if you suspect glomerulonephritis?
C3 C4 CH50
ANCA, anti-GBM, ASO, ANA, cryoglobulins, hepatitis serologies, blood cultures, renal ultrasound, and maybe a renal biopsy
