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a path 3 > CHEMPATH: Assessment of renal function > Flashcards

CHEMPATH: Assessment of renal function Flashcards

1
Q

What, by consensus, is the best measure of kidney function?
A. Serum creatinine itself
B. Serum creatinine and urea
C. Urine protein:creatinine ratio
D. Glomerular filtration rate
E. Cystatin C

A

D. Glomerular filtration rate

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2
Q

What is the normal GFR?

A

120ml/min normal (7.2L/hour)

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3
Q

How does the GFR change with age?

A

Decreases - greater decrease in males than females

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4
Q

Define clearance. What 3 characteristics must a marker have for its clearance to be used to estimate GFR?

A

Clearance = the volume of plasma that can be completely cleared of a marker substance in unit time

The marker used for clearance must be:

  1. not bound to serum proteins
  2. freely filtered at glomerulus
  3. not secreted/reabsorbed by tubular cells

Then a marker’s clearance = GFR

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5
Q

What is the equation for clearance?

A

C = (U x V)/P

  • C = clearance of X
  • U = urinary concentration of X
  • V = urinary flow rate
  • P = plasma concentration of X
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6
Q

What is the ‘perfect marker’ for clearance for GFR measurement? Why is it not used clinically?

A

Gold-standard = inulin (a fructose polymer, neutral charge, freely filtered and not processed by tubules)

BUT

  • Steady state infusion is required
  • Measurement is not simple
  • So research tool only
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7
Q

Name 3 single-injection substances which can be used clinically to assess clearance.

A
  • 51 Cr EDTA
  • 99 Tc DTPA
  • Iohexol
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8
Q

What is direct vs indirect clearance?

A

Direct - when clearance is calculated from urine collection

Indirect - when clearance is calculated from plasma regression curve

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9
Q

What are the pros and cons of urea being measured as a marker of GFR?

A

Urea was the first endogenous marker of EGFR - a by product of protein metabolism

Pros:

  • Freely filtered

Cons:

  • ~30-60% reabsorption by tubular cells
  • depends on nutritional state, hepatic function, GI bleeding

Overall, limited clinical value.

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10
Q

Which factor(s) limit(s) the use of serum creatinine as a marker of GFR?

A. It is influenced by intake of fat
B. It is lower in the black population
C. It is related to muscle mass
D. It is reabsorbed by the renal tubules
E. All of the above

A

C. It is related to muscle mass

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11
Q

Where is urea vs creatinine derived from ?

A

Urea - protein metabolism

Creatinine - muscle cells (small amount from intestinal absorption)

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12
Q

What 4 factors is generation of creatinine affected by?

A
  • Muscularity
  • Age
  • Sex
  • Ethnicity
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13
Q

Does creatinine fulfil the criteria for a perfect marker to be used for GFR?

A

It is freely filtered BUT secreted into urine by tubular cells

Plasma creatinine concentration is inversely related to the GFR. A fall in GFR will produce a rise the the plasma creatinine concentration but GFR can decrease by a half before plasma creatinine concentration rises beyond reference range.

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14
Q

What is the Cockcroft Gault equation used to estimate?

A

Creatinine clearance (not GFR)

eCCr = (1.23 x (140 age) x weight) / serum creatinine

Adjust by 0.85 if female

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15
Q

Is the Cockcroft Gault equation likely to under- or over-estimate GFR?

A

May overestimate GFR, especially when <30ml/min

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16
Q

Which equation is likely to understimate GFR?

A

Estimated GFR equation

Complex equation derived from cohort studies (MDRD) Requires age, sex, serum creatinine and ethnicity

eGFR = 186 x ( Creat x 0.0113) -1.154 x Age -0.203

Adjust by 0.742 if female

17
Q

Which equation is currently used to calculate the GFR? How is this an improvement on other equations?

A

Equation by CKD-EPI 2009* which is similar to the MDRD eGFR equation (using same 4 variables) but reduces bias at GFRs >60ml/min (although still imprecise at higher GFRs)

*(CKD-epidemiology collaboration 2009)

18
Q

What are the 4 variables use in the MDRD eGFR equation?

A
  • Age
  • Sex
  • Ethnicity
  • Serum creatinine
19
Q

What are the clinical alternatives to serum creatinine meaurement?

A

Cystatin C - a cysteine protease inhibitor constitutively produced by all nucleated cells

20
Q

What are the pros and cons of Cystatin C use to measure GFR?

A

Pros:

  • Constant rate generation
  • Freely filtered

Cons:

  • Almost completely reabsorbed and catabolised by tubular cells
21
Q

True or false:

A spot urine measurement to quantify proteinuria can be done instead of a 24 hour urinary collection?

A

True - because it can be used to assess protein:creatinine ration which is a quantitative assessment of amount of proteinuria because the creatinine corrects for urinary concentration e.g. in dehydration

22
Q

Define protein:creatinine ratio.

A

Quantitative assessment of amount of proteinuria.

Measurement of creatinine corrects for urinary concentration.

23
Q

Why is PCR preferred over 24 hour urine collection?

A

24hr urine collection is:

  • Cumbersome and messy
  • Highly inaccurate without specific patient education
24
Q

List three single sample and three 24hour collection urine investigations.

A

Single sample

  • Dipstick testing
  • Microscopic examination
  • Proteinuria quantification
  • Electrolyte estimation

24hour collection

  • Creatinine clearance estimation
  • Stone forming elements
  • (Proteinuria quantification)
  • (Electrolyte estimation)
25
Q

Which of the following is true regarding urine dipstick testing?

A. If the dipstick is negative for blood it reliably excludes haematuria
B. Haematuria is the only cause of a positive dipstick test for blood.
C. You can reliably exclude bacteriuria if the urine dipstick is negative for nitrites
D. The urine dipstick detects Bence Jones proteins
E. Glycosuria detected by the dipstick means the
patient has diabetes.

A

C. You can reliably exclude bacteriuria if the urine dipstick is negative for nitrites

26
Q

List 5 measures on the urine dipstick.

A
  • pH - 4.5-8 is normal
  • Specific gravity - effectively the density of the urine
  • Protein -sensitive to albumin, not Bence-Jones proteins
  • Blood - myoglobin will also give a positive test
  • Leucocyte esterase
  • Nitrite - detects bacteria esp. gram negatives
27
Q

How is urine microscopy done? What is the sample examined for?

A

Centrifuge at 3000rpm for 5- 10 minutes

Examine sediment for:

  • Crystals
  • Red blood cells
  • White blood cells
  • Casts
  • Bacteria
28
Q

A 50 year old, known alcoholic, presents generally unwell, seemingly intoxicated, with acute kidney injury. Urine microscopy reveals calcium oxalate crystals, what diagnosis do you suspect?

A

Ethylene glycol poisoning

29
Q

What component of urine is shown?

A

Calcium Oxalate Crystals

30
Q

What component of urine is shown?

A

RBC

31
Q

What component of urine is shown?

A

White blood cells

32
Q

What component of urine is shown? What does it indicate?

A

Casts - indicated glomerular damage

33
Q

What component of urine is shown?

A

Bacteria

34
Q

You admit a 28 year old man who you suspect has a renal stone, what is your first choice of imaging?

A. Plain KUB
B. CT
C. Ultrasound KUB
D. IVU
E. MRI

A

B. CT - CT is first line when you suspect kidney stones

35
Q

Name 5 differrent types of renal imaging and invasive investigations.

A
  • Plain KUB films - radiolucent stones will not show
  • Intravenous urogram (IVU)
  • KUB ultrasound - operator dependent
  • Cross sectional CT and MRI
  • Functional imaging (static and dynamic renograms)

Invasive:

  • Renal biopsy - US or CT guided

a path 3 (89 decks)

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