4 - Lab Tests for Kidney

Question 1

How do you calculate filtered rate? How do you calculate excreted rate? How do these values compare to one another?

Answer 1

Filtered rate of X (mg/min) = plasma concentration of X (mg/ml) x GFR (ml/min)

Excreted rate = urine (mg/ml) x urine flow (ml/min)

These are usually the same, therefore plasma x GFR = urine x urine flow.

X must be freely filtered - not secreted not reabsorbed.

Question 2

What is the equation for calculating clearance of a substance?

Answer 2

If we know that filtration rate = excretion rate (plasma x GFR = urine x urine flow)

Then we can rearrange to solve for GFR so that GFR = urine concentration x urine flow / plasma concentration

GFR can be measured as how much creatinine we clear from the body:

Clearance = Urine concentration of X x V (flow) / Plasma concentration of X

Question 3

What are requirements for a substance to be able to be used in the clearance equation? (ie what can “X” be?) Give examples of exogenous and endogenous markers that can be used?

Answer 3

• Freely filtered
• Non-toxic
• Not secreted or reabsorbed
• Not changed during excretion.

Can be exogenous markers such as inulin, iothalamate, and iohexol. Creatinine can be used as an endogenous marker.

Question 4

What is creatinine and what makes it a good indicator for measuring GFR?

Answer 4

Produced from metabolism of muscle creatine and from dietary meat intake.

• Released into circulation on a constant basis.
• Freely filtered
• Non-toxic
• Secreted - small amt in proximal tubule
• Not reabsorbed
• Not changed during excretion

Question 5

Assuming that GFR is 120 ml/min and serum creatinine is 1 mg/dl, what is the difference in GFR between a serum creatinine of 4mg/dl and a serum creatinine of 5mg/dl

Answer 5

When a serum creatinine is 4, you divide 120/4 to get a GFR of 30.

When serum creatinine is 5, you divide 120/5 to get a GFR of 24.

The difference between these two GFR values is 6 ml/min.

Question 6

What is the relationship between GFR and serum creatinine?

Answer 6

It’s an inverse relationship.

Normal creatinine is 1 and GFR is 20.

If creatinine doubles, the GFR halves.

If creatinine triples, the GFR goes down by a factor of 3.

Question 7

How do creatinine levels vary by person?

Answer 7

Increased in men (0.9 to 1.3)

In women (0.8 to 1)

In children (0.5 to 1)

Since creatinine production is largely from muscle creatine, greater muscle mass leads to greater creatinine production. Blacks, men, and younger adults have most muscle mass. As children get older, their serum creatinin levels increse.

Question 8

What is the problem with using creatinine clearance to estimate GFR?

Answer 8

Creatinine clearance overestimates GFR due to a small amount of secretion in the tubules.

Measures clearance requires 24 hours of urine collection.

Creatinine production is dependent on muscle mass.

Question 9

What variables does the cockcroft-gault formula for creatinine clearance take into account?

Answer 9

• Serum creatinine
• Age
• Weight
• Gender

Question 10

If the cockcrost-gault equation were used in someone who was obese or edematous, it would _____ the creatinine clearance.

Answer 10

Overestimate.

That’s the problem with this equation, it was made when obesity was less common.

Question 11

What does the MDRD equation take into account? What is the problem with this?

Answer 11

• Serum creatinine
• Age
• Gender
• Race

It underestimates GFR if the actual GFR is greater than 60 ml/min/1.73 m2

Question 12

What variables for the CKD-EPI equation take into account? When is this equation beneficial?

Answer 12

• Serum creatinine
• Age
• Gender
• Race

More accurate with an estimated GFR greater than 60 ml/min/1.73 m2

Question 13

What is a new way that may be used in the future to estimate GFR?

Answer 13

Cystatin C:

• protein produced by all nucleated cells
• freely filtered
• production seems less affected by age and gender than creatinine

Question 14

How is chronic kidney disease classified?

Answer 14

As a GFR <60 ml/min/1.73m2 for >3 months

Or evidence of kidney damage (such as protein in the urine)

There’s give stages based on GFR that we don’t need to know.

Need dialysis when GFR is <10.

Question 15

What is the time span in which acute kidney injury (AKI) occurs?

Answer 15

Over hours to days.

Remember that you can’t use the equations we just described because those are only valid for steady state conditions; new steady state is needed and creatinine rises slowly.

Early sign can be decreased urine output.

Question 16

Assuming you have a creatinine of 1 with a GFR of 120 and both kidneys magically disappear. What is your creatinine IMMEDIATELY afterwards? What is your estimated GFR, based on the creatinine? What is your actual GFR?

Answer 16

1 mg/dl creatinine

GFR = 120 mg/ml

But GFR is actually zero becasue you have no kidneys. When you see a blood test you don’t know what happens after the blood test. This means that blood tests won’t show acute kidney disease.

Question 17

What is the definition of acute renal failure?

Answer 17

Acute and sustained reduction in renal function.

Biochemically: a 0.5 mg/dl or 25% increase in serum creatinine.

Question 18

What criteria is used to stratify acute kidney injury?

Answer 18

R: risk

I: injury

F: failure

L: loss of function

E: end-stage renal disease

Question 19

How do the RIFLE criteria and AKIN criteria for defining acute kidney injury compare?

Answer 19

They both classify it as occuring within a short period of time (not more than 7 days in RIFLE, within 48 hours for AKIN).

Question 20

Why is creatinine not a good indicator as far as time is concerned?

Answer 20

It takes time for creatinine to ride, typically ~48 hours from time of injury.

Even if GFR decreases to 0, creatinine typically only rises 1-2 mg/dl/day

Question 21

Creatinine is a functional marker of _________. What will probably be used in the future?

Answer 21

Organ damage.

Biomarkers will probably be used in the future.

Question 22

How is chronic kidney disease classified when you have a normal GFR?

Answer 22

Based on how much protein is in the urine.

Question 23

What is a normal amount of urinary protein? What happens when there’s more than that in the urine?

Answer 23

Total urinary protein should be less than 150 mg/day

Urine protein of greater than 3.5g/24 hours is nephrotic

Nephrotic syndrome: has low serum albumin and edema

Question 24

How do we detect proteinuria?

Answer 24

Urine dipstick: shows a negative and positive reading for protein

• Primarily detects albumin
• Dependent on urine concentration
• Semi-quantitative measure becasue it can be +, ++, +++, or ++++

Question 25

What is microalbuminuria?

Answer 25

A little bit extra albumin in the urine (they are normal sized albumin molecules)

Defined as 30-300 mgs of albumin in 24 hours or 30 to 300 mg albumin/gram of creatinine

This is usually below distick threshold, especially with dilute urine.

Question 26

What are two quantitative tests that can be used to measure protein in the urine?

Answer 26

24 hour urine.

“spot” urine sample

Question 27

How does albumin get into the urine? How do other proteins get into the urine?

Answer 27

If you have a lot of other proteins in your urine (not albumin) it’s an overflow problem.

If you have a lot of albumin in your urine, it has to be a glomerular problem allowing increased permeability so the large albumin molecules could get through.

Question 28

What qualitative tests can be done on serum or urine to look for protein?

Answer 28

Protein electrophoresis.

Serum protein electrophoresis would show a large spike from albumin and smaller bumps from other proteins in smaller concentrations.

If you have something like cancer, you may be making a lot of a second protein type that is detected with protein electrophoresis.