Investigating Disorders of Renal Function Flashcards
what is renal clearance
theoretical concept
rate at which a substance in removed from the blood stream via the kidneys
what is sometimes used to calculate GFR in paediatrics
iohexol - non-radioactive contrast agent
injected and then serial blood tests
what endogenous substance can be used for GFR other than creatinine and what is its advanatge
cystatin C
no significant tubular secretion (creatinine has some)
GFR equation
GFR = (concentration in urine x volume of urine x time it’s collected over)/concentration in plasma
what type of urine sample is needed for creatinine clearance (GFR using creatinine)
24 hour urine collection
when is creatinine clearance innaccurate
in advanced renal failure where there is very low GFR
why is the Cockcroft-Gault equation useful
it is able to adjust a plasma creatinine level for age, weight and sex
what is the Cockcroft-Gault equation
eGFR for men = ((140-age) x weight)/creatinine concentration x 0.81
eGFR for women = ((140-age) x weight)/(creatinine concentration x 0.85)
what type of GFR is done most commonly in GPs
eGFR (only plasma creatinine measurement required) and MDRD equation
at what GFR is MDRD equation accurate
in those with low GFR
not very low or normal
what equation is better than the MDRD and why
EPI
it is more reliable in higher GFRs so may be able to spot kidney disease at an earlier stage
in what populations can the MDRD and EPI equations not be applied
possibly very elderly
children
pregnancy
muscle mass extremes (frail, amputee, heavily built)
rapidly changing renal function
very low GFR
what two things are used to classify CKD
eGFR (MDRD equation)
albumin:creatinine eqution
definition of CKD
progressive and irreversible loss of kidney function caused by irreversible damage to increasing numbers of nephrons
CKD diagnosis requires
eGFR<60
or eGFR >60 and one of:
- persistent proteinuria
- haematuria
- renal anatomical/genetic abnormality
what features are associated with AKI
oliguria (less peeing than normal)
increases in plasma urea and creatinine (as not peeing it out)
loss in ability to regulate water, electrolyte and acid base balance
what investigation is used to detect AKI
creatinine (increase from baseline)
and urine output (decrease)
(eGFR doesn’t change quickly enough to detect AKI)
what urine output would indicate AKI
<0.5ml/kg/hr for >6 hours in adults
>8 hours in children
what change in plasma creatinine indicates AKI
> 26 umol/l increase within 48 hours
50% increase in last 7 days
how does the pH of urine compare to plasma
urine is usually significantly more acidic than plasma
urine pH < 5.5
plasma pH 7.35-7.45
a urine pH>5.5 can be caused by
renal tubular acidosis type 1
a urine Na+ of >30mmol/l suggests
inappropriate Na+ loss due to
- tubular damage
- inadequate ADH
how do you measure urine concentrating ability
urine osmolarity
positive leukocytes in dipstick suggests
UTI
bilirubin postive
uroblinogen negative
on dipstick suggests
cholestatic jaundice
