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YR5 Chemical Pathology > Assessment of Renal Function 1 > Flashcards

Assessment of Renal Function 1 Flashcards

1
Q

What is the best measure of kidney function? What is the average? How does this change with age?

A

Glomerular filtration rate

120ml/min normal (7.2L/hour)

Age related decline ~ 1ml/min per year

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2
Q

What is clearance? and what is the association between clearance and GFR?

A

Volume of plasma that can be completely cleared of a marker substance in unit time.

Can use clearance to calculate GFR.

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3
Q

What 3 criteria are necessary for a marker to be used to measure GFR via clearance?

A

Marker not bound to serum proteins

Freely filtered at the glomerulus

Not secreted/ reabsorbed by tubular cells

C = GFR.

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4
Q

What is the equation of clearance?

A

C = (U x V)/P

U: Urinary conc. mg/ml

P: Plasma conc. mg/ml

V: Urine flow rate in ml/min

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5
Q

What is the gold standard measurement for GFR? Give 5 features of this?

A

Inulin clearance

5.2kD fructose polymer

Neutral charge

Freely filtered

Not processed by tubular cells

Not endogenous- steady state infused required

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6
Q

When can you use endogenous markers to measure GFR?

A

Only at steady state (when renal function not changing rapidly)

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7
Q

What was the first endogenous marker of GFR?

A

Blood urea

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8
Q

Give 5 features of urea as a marker of GFR?

A

By-product of protein metabolism synthesised in liver

Freely filtered at glomerulus

Variable (30-60%) reabsorption by tubular cells

Dependent on nutritional state, hepatic function, GI bleeding

Very limited clinical value

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9
Q

Why is urea a bad marker of GFR when urine flow is low?

A

Tubular reabsorption increases at low rates of urine flow

So in fluid depletion, plasma urea conc increases even though renal function is normal

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10
Q

Why is serum creatinine useful as a marker of GFR?

A

Derived from muscle cells (small amount from intestinal absorption)

Freely filtered

Actively secreted into urine by tubular cells

Daily production is relatively stable

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11
Q

Describe the relationship between creatinine and GFR

A

Non-linear

GFR can decrease by 50% before plasma creatinine rises above ref range

Normal plasma creatinine: not necessarily normal renal function

Raised plasma creatinine: impaired renal function

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12
Q

What is the rate of generation of creatinine affected by?

A

Not equivalent in different individuals

Muscularity: proportional to mass

Age: muscle decreases with age, but so do does GFR- so creatinine conc fairly stable

Sex (M>F)

Ethnicity (higher in Afro-Caribbean)

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13
Q

Changes in muscle mass can change plasma creatinine conc, give examples of what may cause an increase or decrease?

A

Increase: refeeding, ingestion of meat rich diet

Decrease: Starvation, wasting diseases, post-surgery

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14
Q

What is the equation for creatinine clearance?

A

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

Adjust by 0.85 if female

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15
Q

What are the limitations for the equation for creatinine clearance?

A

Estimates creatinine clearance (not GFR)

May overestimate GFR, esp. when <30ml/min

Not widely used or validated in screening for CKD

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16
Q

What is eGFR?

A

Complex equation derived from cohort studies

Requires age, sex, serum creatinine

17
Q

What is the equation for eGFR?

A

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

Adjust by 0.742 if F

18
Q

What are the limitations for eGFR?

A

May underestimate GFR if above average weight + young

Derived from studies on patients with CKD, may be less applicable in those with normal function/ pregnant/ children

Less accurate at high GFRs, tends to underestimate

19
Q

What is a clinical alternative for serum creatinine? Give 6 features

A

Cystatin C

Low molecular mass peptide

Cysteine protease inhibitor

Constitutively produced by all nucleated cells at constant rate

Freely filtered

Serum conc. correlates inversely with GFR

Almost completely reabsorbed + catabolised by tubular cells

20
Q

After centrifuging the urine, what should you look for in microscopy?

A

Crystals

Red blood cells

White blood cells

Casts

Bacteria

21
Q

What can be performed instead of a 24hr urinary collection to quantify proteinuria?

A

Spot urine measurement

22
Q

What clinical test can reliably exclude haematuria?

A

Urine dipstick -ve for blood

23
Q

How is proteinuria used as a marker of renal damage?

A

Glomerular BM does NOT usually allow passage of albumin + large proteins

Impairment of integrity allows these large molecules through

Significant quantity of protein = significant damage to BM

24
Q

How is proteinuria measured to correct for urine concentration?

A

Measure creatine

Protein : Creatinine Ratio (PCR)

25
Q

What are the limitations of 24 hour urine collection? What is used instead to estimate PCR?

A

Cumbersome + messy

Highly inaccurate without specific patient education

Estimation of proteinuria superceded by urinary PCR

26
Q

What is indicated by high specific gravity on urine dip?

A

More concentrated urine

27
Q

What is the protein measurement sensitive to on urine dip? What may this indicate?

A

Sensitive to ALBUMIN (not BJPs)

Abnormal secretion of proteins by kidney either due to glomeruli damage (leaky) or inability to reabsorb protein normally

28
Q

What is the blood marker sensitive to on urine dip?

A

RBCs

Haemaglobin

Myoglobin

29
Q

What does a negative leucocyte esterase indicate on urine dip?

A

Reliably excludes bacteria

30
Q

What does the Nitrite measurement on urine dip indicate?

A

+ve indicates bacterial infection, esp. G-ve : bacteria convert nitrates from diet to nitrite

-ve: can’t reliably exclude bacteria

31
Q

What is indicated by urinary ketones?

A

Body is metabolising fat:

DKA

Alcoholic ketoacidosis

Prolonged fasting state

Vomiting

32
Q

What does glucose on urine dip indicate?

A

Filtering load of glucose exceeds what can be reabsorbed by renal tubules, usually indicates hyperglycaemia

Prompts further Ix

Not always DM: urine threshold for glucose lower in certain states e.g. pregnancy

33
Q

What imaging is used for suspected renal stones?

A
  1. CT KUB
  2. US KUB (1st line if child/ pregnant)

YR5 Chemical Pathology (16 decks)

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