Renal function testing Flashcards
(21 cards)
Renal disease vs renal failure
In renal disease, the kidney tissue is damaged.
Renal failure is a functional deficit in which 75% of the nephrons are not filtering.
What are two classes of renal function tests? Which is predominant?
Tests to evaluate clearance/assess glomerulus (predominant)
Tests to evaluate tubular function
Describe the BUN test.
Blood urea nitrogen.
Measures amount of urea in blood.
Urea is produced in protein catabolism, filtered by kidneys.
Higher BUN means lower kidney function.
What factors interfere with BUN?
40% of urea is reabsorbed by tubules Need a healthy liver to produce urea. Protein intake Muscle mass Pregnancy Hydration
What is serum normal for BUN?
10-20 mg/dL
Critical value: >100 mg/dL
What is the term for elevated blood urea?
Azotemia
Prerenal causes of azotemia
55% of acute renal failures
Shock, low blood volume, dehydration
CHF, MI
High protein catabolism (high intake, starvation)
Renal causes of azotemia
40% of acute renal failure
glomerulonephritis
pyelonephritis
tubular necrosis (nephrotoxic drugs)
Postrenal causes of azotemia
5% of acute renal failure
Obstruction of ureters or bladder outlet
Describe the creatinine test
x
What are advantages of creatinine over BUN?
x
What factors influence creatinine test
x
What is the reference range for creatinine?
0.5-1.0 mg/dL
What can you tell from the BUN/Creatinine ratio?
Pre-renal azotemia: >20:1 ratio, BUN is rising faster than creatinine.
Renal azotemia: 10-20:1 ratio, BUN and creatinine rise approx same rate
What two factors tell you whether ARF has a prerenal or renal cause?
- Urea & Creatinine: if they rise together, renal; if BUN is faster, pre-renal
- Proteinuria: uncommon in pre-renal, present in renal.
What is eGFR?
Estimated glomerular filtration rate.
Calculated from serum creatinine, indicates ability of kidneys to filter blood.
Skewed by a variety of factors, including vegetarian diet, pregnancy, and age >70 or <18.
What is creatinine clearance and what does it tell you?
Better estimate of glomerular filtration than eGFR.
Requires 24-hr urine collection + blood draw.
Measures rate at which creatinine is removed from blood.
Overestimates by 10%. Invalid when GFR < 30% of normal
What are normal values of creatinine clearance?
Male: 90-139 mL/min
Female: 80-125 mL/min
Decline with age, 6.5 per decade past 20.
What is the most important indicator of renal disease?
Proteinuria
Which tests can indicate whether pathology is pre-renal or renal?
BUN/Creatinine ratio
Fractional excretion of sodium
Who needs a microalbuminuria test?
Type 1 DM for 5+ years
Type 2 DM
HTN
