Renal Function Tests Flashcards
what 3 things does renal fxn testing provide information of?
- renal blood flow
- GFR
- 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?
clearance: -BUN -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
- pregnancy
- hydration levels
- liver disease
- drugs
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)?
causes: -low BV: shock, burns, dehydration -CHF, MI -GI bleed -too much protein -high protein catabolism: starvation -sepsis 55% of ARFs
renal causes of increased BUN? %age?
causes: renal dz
-glomerulonephritis
-pyelonephritis
-tubular necrosis
-nephrotoxic drugs
40% of ARFs
postrenal causes of increased BUN? %age?
causes:
-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?
- vegetarian
- pregnant
- malnourished
- > 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
- drugs
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
- nausea/vomiting
- edema
- low back pain
- decreased urine output
- fatigue
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?
prerenal
decreased renal blood flow triggers renin-angiotension pathway= kidneys conserve Na+
tubular fxn test of FENa >2% is what kind of ARF?
renal
damaged tubules can’t effectively conserve Na
what is the most important indicator of renal dz?
PROTEINURIA
normal range is <150 mg/24 hrs
proteinuria 150-500mg/24 hrs indicates what? >2000 mg/24 hrs? >3500 mg/24 hrs?
150-500 mg/24 hrs can be fxnal if asx
>2000 mg/24 hrs indicates glomerular cause
>3500 mg/24 hrs indicates nephrotic syndrome
what can cause proteinuria in absence of structural abnormality in kidneys?
CHF! can also occur in pre-eclampsia, eclampsia, HTN, heavy metal toxicity, solvents
what is more accurate than 24 hr urine protein? what urine sample would you prefer? normal ratio?
urine protein to creatinine ratio is more accurate
use 1st morning void
ratio: <0.2 g protein/g creatinine
what population is microalbuniuria used with?
pts w/DM and/or HTN for detection of kidney dz
what is urine albumin to creatinine ratio used for?
detect microalbuniuria
diagnose & monitor kidney damage in pts w/type 1 DM for 5 yrs or more or type 2 DM
what are kidney stones known as?
nephrolithiasis
urolithiasis
renal calculi
what # of ppl get kidney stones in their life? who is it rare in?
1 in 20 will get them at some pt
rare in children, when present often associated w/specific metabolic disorder or anatomic abnormalities
what 3 renal pathologies are common to occur around puberty or in mid-teens?
cystinuria
idiopathic calcium oxalate urolithiasis
primary hyperparathyroidism
when do kidney stones form?
high level of minerals: calcium, oxalate or uric acid in urine or when there is a lack of citrate in the urine or insufficient water
what normally prevents formation of crystals?
citrate, magnesium & pyrophosphate
what 4 things might be done to evaluate why someone is forming stones?
- chemistry panel (calcium & bicarb levels)
- PTH if elevated serum Ca2+
- UA including pH and urine culture
- urinalysis (pH and sediment exam)
pH >7.5 is compatible for what kind of stones?
struvite or calcium phosphate
<5.5 is compatible for what kind of stones?
uric acid or cystine stones
what stones are not pH dependent?
calcium oxalate
what factors contribute to calcium oxalate stones?
- low urine volume
- high concentrations of calcium & oxalate
what factors contribute to calcium phosphate stones?
- alkaline urine
- high urine calcium concentrations
what can increase 24 hr urine calcium levels?
- primary hyperparathyroidism
- vitamin D excess
- corticosteroid tx
- PTH producing tumor
- cushing’s syndrome
- sarcoidosis, TB
- osteoporosis
- metastatic tumors in bone
- renal tubular acidosis
what can decrease 24 hr urine calcium levels?
- hypoparathyroidism
- vit D deficiency
- calcium malabsorption
- renal failure
pts w/tendency to calcium oxalate nephrolithiasis appear to ____ & ___ a high proportion of dietary ____ in the urine
absorb & excrete
oxalate
indications of high oxalate in urine? 3
- pts w/surgical loss of distal ileum (Crohn’s dz)
- IBS (1-3% of IBS pts form)
- jejunal bypass
normal oxalate values?
males: 7-44 mg/24 hrs
females: 4-31 mg/24 hrs
children: 13-38 mg/24 hrs
what other factors can increase 24 hr urine oxalate?
- ethylene glycol ingestion
- genetic disorder
- pancreatic dz
- liver cirrhosis
- pyridoxine deficiency
- sarcoidosis
- celiac dz
what other factors can decrease 24 hr urine oxalate?
- renal failure
- high urinary Ca2+
what is hypocitraturia?
urine citrate excretion <434 mg in men
when will you see hypocitrturia?
- IBD
- intestinal malabsortion
- renal tubular acidosis
what is citrates role?
plays a role in inhibiting Ca2+ formation and prevents stones
what does a high protein diet cause?
increase acidosis= release Ca2+ from bones into urine= decrease release of citrate in urine to buffer acidosis= increase stone formation
normal levels of uric acid?
250-750 mg/24 hrs
where does uric acid come from? how much is excreted by kidneys? where does the rest go? elevated levels can cause what?
breakdown product of purine metabolism in the liver
75% excreted, remained via GI tract & out
elevated levels can cause gout
uric acid stones form with what pH? what body state? what can it be linked to?
- unusually low pH & hyperuricosuria
- increased risk in obese, metabolic syndrome
- may be linked to insulin resistance
when do you see increased uric acid?
- gout
- metastatic cancer
- multiple myeloma
- leukemia
- cancer chemotherapy
- high purine diet
- lead toxicity
when do you see decreased levels of uric acid?
- renal disease
- eclampsia
- chronic alcoholism
what are triple phosphate crystals? what kind of urine are they present in? what can precipitate these? what kind of stones do they form?
- magnesium ammonium phosphate
- alkaline urine
- commonly seen in pts w/UTI
- struvite stones
what 6 things do you do when a pt presents with recurrent nephrolithiasis?
- stone analysis
- serum & blood tests (Ca, PTH, vit D, electrolytes)
- urine dip & microscopic exam (pH, sediment)
- culture
- 24 urine collection (volume, creatinine, Ca oxalate, Na, citrate, uric acid, cystine)
- helical CT w/o contrast
where do homovanilic acid and vanillylmandelic acid come from?
end products of catecholamine metabolism
what can vanillylmandelic acid and homovanilic acid be used to diagnose?
-pheochromocytoma, neuroblastoma, ganglioneuroma, ganglioneuroblastoma & rare adrenal tumors
what is a collage which provides tensile strength to bone matrix of bone? when is it released? what void catches the most?
cross-link of type I
collagen
released during bone resorption & excreted unchanged into urine
2nd morning void sample used
what will increase deoxypyridinoline?
- osteoporosis
- osteolytic metastatic cancers
- hyperthyroidism
- see in children
- osteomalacia
- hyperparathyroidism
- multiple myeloma
- paget’s dz
- long-term steroid use
- cushing’s
what is the significance of N-telopeptides?
response can be determined w/in 3-6 mo rather than 1-2 yrs you have to wait btw DEXAs so can see bone loss earlier
when do you see HCG (dx and timing)? where does the hormone come from?
pregnancy, about 10 d after conception
comes from placental trophoblast
some cancers can also produce this hormone
when will you see increased HCG?
- normal pregnancy
- ectopic pregnancy
- germ cell tumors of ovaries or testes
- hydatidiform mole- pregnancy that turns into cancer
- choriocarcinoma
when are drug tests used?
- employers may use before hiring
- to test appropriateness of prescribing controlled substance for pt
- before increasing dose of analgesic medications
- before referring to a pain or addiction specialist
what are the 2 drug screening tests?
immunoassay: used as screening, rapid, inexpsensitive, Abs to detect drugs, good for detecting marijuana and cocaine
gas chromatography/mass spec: confirmatory test, expensive, time consuming, more accurate
what can cause an immunoassay to have false negatives?
- dilute urine
- time
- labs established limits
- sample tampering
- negative result may not be correct
- many other things can also cause false positive
