how to investigate disorders of renal function W2

Question 1

what is renal clearance?

Answer 1

theoretical concept which aims to quantify the rate at which a substance is removed from the bloodstream.
“volume of plasma completely cleared of a substance per unit time”

Question 2

what is glomerular filtration rate? (GFR)

Answer 2

volume filtered by glomeruli, per unit time
(related to renal clearance but more clinically meaningful)

Question 3

GFR equation?

Answer 3

GFR = concentration of urine X urine output / concentration of plasma
(for a substance that is completely lost from the plasma to the urine)

Question 4

what substances does the GFR equation apply to

Answer 4

• must not alter GFR
• freely filtered at glomerulus
• not reabsorbed/actively secreted in nephron
• not metabolised/produced by the kidney

Question 5

why measure GFR?

Answer 5

• assessment of global renal function
• GFR loss is generally correlated with loss of kidney function
• guides management in CKD
• rate of decline predicts need for renal replacement therapy
• guides dosing of potentially toxic drugs

Question 6

methods for measuring GFR - exogenous substances?

Answer 6

insulin (rarely used outside research)
radioisotope tracers (rarely used outside research)
iohexol (sometimes used in paediatrics)

Question 7

methods for measuring GFR - endogenous substances?

Answer 7

creatinine
cystatin C

Question 8

creatinine features? (for GFR measurement)

Answer 8

small molecule
produced at relatively constant rate from muscle metabolism
some active tubular secretion
long established role in GFR measurement

Question 9

cystatin C features? (for GFR measurement)

Answer 9

small protein
inhibitor of proteases
produced by all nucleated cells
no significant tubular secretion/absorption
10-20x more expensive to measure than creatinine
relatively new method of measuring GFR

Question 10

creatinine clearance?

Answer 10

plasma creatinine measurement
24 hour urine collection
inconvenient, inaccurate and imprecise

Question 11

creatinine clearance accuracy at very low GFR (advanced renal failure)?

Answer 11

further inaccuracy
less creatinine filtered
amount of creatinine secreted becomes proportionally much larger

Question 12

method for GFR measurement that doesn’t require urine?

Answer 12

plasma creatinine alone (very large inter-individual differences so inaccurate)

Cockcroft-Gault equation (plasma creatinine adjusted for weight, age, sex)

estimated GFR (eGFR) (similar to last but doesn’t need weight. commonly used for routine assessment)
-MDRD (inaccurate for patients without renal disease)
-EPI (up to date)

Question 13

what do MDND and EPI lose applicability in?

Answer 13

children, pregnancy, very elderly
muscle mass extremes
rapidly-changing renal function
very low GFR

Question 14

what is used to classify CKD?

Answer 14

eGFR (MDND or EPI) and urinary albumin:creatinine ratio (ACR)

Question 15

diagnosis of CKD requires what?

Answer 15

eGFR consistently <60mL/min/1.73m^2
can involve eGFR >60 if:
-persistent proteinuria/microalbuminuria
-haematuria
-renal anatomical/genetic abnormality

Question 16

acute kidney injury (AKI) definition?

Answer 16

abrupt loss of renal function commonly characterised by acute oliguria and increases in plasma urea and creatinine. often accompanied by loss in ability to regulate water, electrolyte and acid balance.

Question 17

AKI clinical criteria for detection?

Answer 17

rely on creatinine and UO.
any of the following:

-increased plasma creatinine of
≥26µmol/L within 48 hr or
≥50% in last 7 days

-UO<0.5mL/kg/hr for >6 hr in adults (>8hr in children)

Question 18

ways to assess renal tubular function?

Answer 18

ammonium chloride loading test
electrolyte homeostasis
fluid balance and sodium
urine concentrating ability
suspected DI and water deprivation test

Question 19

i give up

Answer 19

very boring lecture brain has stopped working