week 3: renal function tests Flashcards
• What info do you get from renal function testing? What is analyzed?
o Renal blood flow
o Glomerular filtration rate(GFR)
o Tubular function
o Plasma/serum or urine samples
• Why is renal function testing imperfect?
o various factors other than damage to renal parenchyma can influence results.
o Localized and generalized damage
o Temporary and permanent malfunction
• What is diff between renal dz and failure?
o Dz: presence of histologic lesions in the kidney but does not specify any degree of renal dysfunction
o Failure: 75% of the total nephron population has become non-functional but does not necessarily imply underlying histologic lesions
• What are the 2 types of renal function tests?
o Clearance: BUN, serum creatinine, creatinine clearance, albumin creatinine ratio
o Function: fractional excretion of Na+
• Why is measuring GRF important? Based on?
o It’s essential to renal function
o Most frequently performed test of renal function
o Based on clearance
• What is clearance?
o vol of plasma to clear a substance by glomerular filtration
• what characteristics of the ideal substance to measure GFR?
o Freely filtered at glomerulus o Not bound to plasma proteins o Not metabolized o Non-toxic o Excreted only by kidneys o Not reabsorbed nor secreted by renal tubules o Stable in blood and urine o Easily measured
• What is BUN?
o From protein catabolism in urea cycle in liver: amino acids -> NH3 -> Urea -> blood
o Filtered by glomerulus, 40% reabsorbed
o Urea clearance is 60% of true GFR
• Why/how is BUN used?
o Specimen types are serum, plasma: part of CMP or BMP
o Evaluates liver function; and indirect measure of renal function
o Rough indicator of GFR and renal blood flow
• What can interfere with BUN levels?
o Protein intake o Muscle mass o Pregnancy (increase) o Hydration o Liver dz (decrease) o Drugs…
• What are key BUN levels?
o Serum normal adults: 10-20 mg/dl (Elderly: may be slightly higher)
o Critical value: >100 mg/dl indicates serious impairment of renal function.
• What can cause decreased BUN?
o Fluid overload
o Malnutrition
o Severe liver dz
• What is increased BUN?
o Azotemia: Increased concentration of non-protein nitrogenous waste products (e.g. urea, creatinine) in the blood
• When does azotemia occur?
o Most renal dzs cause inadequate excretion of urea, so BUN rises
o Must distinguish b/w type: pre-renal, renal, post-renal
• What are pre-renal causes of azotemia?
o 55% of acute renal failures o Low blood volume, shock, burns, dehydration o CHF, MI o GI bleed o Too much protein intake o High protein catabolism, starvation o Sepsis
• What are renal causes of azotemia?
o 40% of acute renal failure
o Direct damage to kidneys by inflammation, infection, toxins, drugs, reduced blood supply
o Renal dz: GN, PN, acute tubular necrosis
o Nephrotoxic drugs
• What are post-renal causes of azotemia?
o 5% of acute renal failure
o Obstruction of ureters, e.g. stones, tumors, congenital
o Bladder outlet obstruction, e.g. prostatic hypertrophy (BPH), cancer, congenital
• What is serum creatinine?
o More stable marker than BUN
o catabolic product of creatine phosphate from skeletal muscle
o almost completely filtered by kidneys, but also secreted by proximal tubule
o values depend on muscle mass, which fluctuates very little unless some muscle-wasting pathology exists.
• When/what levels of creatinine would you see in dz?
o levels increase later than BUN
o elevations suggest chronic disease process & parallel BUN increases
o Elderly and young children normally have lower levels due to reduced muscle mass
• What is the effect of muscle mass on serum creatinine in normal and dzed kidneys?
o Increased muscle mass higher serum creatinine, but normal output
o Dzed kidney causes much lower output, and higher serum levels
• What are normal serum creatinine levels? Use?
o Male: 0.6-1.2 mg/dl
o Female 0.5-1.1 mg/dl
o diagnose impaired renal function
o Minimally affected by liver function (unlike BUN)
• What is relationship b/w BUN/Creatinine and % functional nephrons?
o Rectangular hyperbola
o Large changes in GFR “early” in renal disease cause small changes in BUN or creatinine
o Small changes in GFR late in renal disease cause big changes in BUN or serum creatinine
• What is BUN:Creatinine ratio?
o Prerenal azotemia: >20:1; disproportional rise in urea
o Renal: 10-20:1; tend to rise together; protein present on dipstick test
• What is estimated glomerular filtration rate (eGFR)?
o ability of the kidneys to filter blood
o GFR goes down, serum creatinine goes up
o A calculation using serum creatinine, the patient’s sex and age using the MDRD equation
o Included whenever serum creatinine values are requested
• When is eGFR inaccurate? Use what instead?
o Use creatinine clearance instead
o Vegetarian, pregnant, malnourished, elderly or infant, mm dz
o When GFR by MDRD eq is >60 mL/min/1.73m2
• How/why is creatinine clearance measured?
o Requires 24-hour urine collection & blood draw.
o quantitative measure of the rate at which creatinine is removed from the blood, expressed in ml/min.
o Values are corrected for body surface area (BSA); must obtain patient height and weight.
• What is eq for creatinine clearance?
o CC= urine creat/serum creat x 24 hrs/1440 min x m2/1.73m2 x vol (mL)
• Is creatinine clearance a good measure of GFR? When not?
o Provides relatively good estimate of GFR.
o BUT it tends to over-estimate it by about 10% due to tubular secretion of creatinine.
o When GFR decreases to < 30% of normal, CC is invalid because the secreted faction becomes a much larger proportion of the total urinary creatinine.
