Renal function testing

Question 1

Renal disease vs renal failure

Answer 1

In renal disease, the kidney tissue is damaged.

Renal failure is a functional deficit in which 75% of the nephrons are not filtering.

Question 2

What are two classes of renal function tests? Which is predominant?

Answer 2

Tests to evaluate clearance/assess glomerulus (predominant)

Tests to evaluate tubular function

Question 3

Describe the BUN test.

Answer 3

Blood urea nitrogen.
Measures amount of urea in blood.
Urea is produced in protein catabolism, filtered by kidneys.
Higher BUN means lower kidney function.

Question 4

What factors interfere with BUN?

Answer 4

40% of urea is reabsorbed by tubules
Need a healthy liver to produce urea.
Protein intake
Muscle mass
Pregnancy
Hydration

Question 5

What is serum normal for BUN?

Answer 5

10-20 mg/dL

Critical value: >100 mg/dL

Question 6

What is the term for elevated blood urea?

Answer 6

Azotemia

Question 7

Prerenal causes of azotemia

Answer 7

55% of acute renal failures
Shock, low blood volume, dehydration
CHF, MI
High protein catabolism (high intake, starvation)

Question 8

Renal causes of azotemia

Answer 8

40% of acute renal failure
glomerulonephritis
pyelonephritis
tubular necrosis (nephrotoxic drugs)

Question 9

Postrenal causes of azotemia

Answer 9

5% of acute renal failure

Obstruction of ureters or bladder outlet

Question 10

Describe the creatinine test

Answer 10

x

Question 11

What are advantages of creatinine over BUN?

Answer 11

x

Question 12

What factors influence creatinine test

Answer 12

x

Question 13

What is the reference range for creatinine?

Answer 13

0.5-1.0 mg/dL

Question 14

What can you tell from the BUN/Creatinine ratio?

Answer 14

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

Question 15

What two factors tell you whether ARF has a prerenal or renal cause?

Answer 15

1. Urea & Creatinine: if they rise together, renal; if BUN is faster, pre-renal
2. Proteinuria: uncommon in pre-renal, present in renal.

Question 16

What is eGFR?

Answer 16

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.

Question 17

What is creatinine clearance and what does it tell you?

Answer 17

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

Question 18

What are normal values of creatinine clearance?

Answer 18

Male: 90-139 mL/min
Female: 80-125 mL/min
Decline with age, 6.5 per decade past 20.

Question 19

What is the most important indicator of renal disease?

Answer 19

Proteinuria

Question 20

Which tests can indicate whether pathology is pre-renal or renal?

Answer 20

BUN/Creatinine ratio

Fractional excretion of sodium

Question 21

Who needs a microalbuminuria test?

Answer 21

Type 1 DM for 5+ years
Type 2 DM
HTN