Measurement of Kidney Function Flashcards
What is kidney function measured in terms of
Kidney function is measured in term of GFR
What is the noraml GFR
90-120 ml/min/1.73m2
Why might premature babies and LBW infants have a lower GFR than normal
Premature babies and LBW infants often have lower nephron numbers as the nephron develpoment finishes by the 35-36th week of foetal development
This causes a lower GFR
Also few nephrons develop after birth
How is GFR affected with advancing age
GFR decreases with age as the kidney shrinks with age
Renal cortex decreases in size -> less nephrons -> decreased GFR
How is GFR affected by pregancy
Pregnancy causes there to be an increased fluid volume resulting in increased GFR and increased kidney size
Returns to normal post-partum
What is compensatory hypertrophy
Where decreased nephorn number leads to the remaining nephrons working harder -> increase in kidney size
Common in children - especially if child only has one functioning kidney in utero
Compensatory hypertrophy leads to greater risk of kidneys wearing out as nephrons are working harder
Why might GFR decline
Decline in nephron number
Decline of GFR within individual nephrons
What is the equation used for renal clearance
What factors should a substance have to be used to measure kidney clearance
Be produced at a constant rate
Be freely filtered across the glomerulus
Not be reabsorbed in the nephron
Not be secreted into the nephron
Name some substances that are used to measure kidney clearance
Inulin
Radioactive labelled marker - 51 Cr-EDTA
Creatinine
Why is inulin not used as a marker for kidney clearance
It is not continuously produced in the body
It requires catheter insertion and timed urine collection
How is 51 Cr-EDTA used as a marker of kidney clearance
It is given in timed injections and then blood samples are taken after specific times
Can then work out how quickly the marker has been filtered out of the blood to give the GFR
Has some reabsorption
Why does creatinine give an overestimate of the GFR
Some creatinine is secreted directly into the nephron
How is serum creatinine used to calculate the GFR
Serum creatinine is compared to the normal values for that patient to see if their serum creatinine is normal or has changed from the previous one
Changes in serum creatinine away from the normal value/patients normal value suggests the GFR has changed
What factors affect the serum creatinine level
Intake of protein/diet
Metabolism of the patient
Muscle bulk of the patient
Gender, age and ethnicity (blacks have more)
Renal excretion and secretion
Changes in muscle use - increased muscle use will increase amount of creatinine in the body
Extra-renal excretion - normally very low
Certain drugs affect creatinine levels
Why is serum creatinine lower than the true GFR
Because some creatinine is secreted into the nephron
What models are used to estimate the GFR from serum creatinine levels
MDRD eGFR - based on only a few patients with CKD -> inaccurate. Caucasians only
CKD-EPI - based on more patients than MDRD eGFR from more diverse backgrounds
MDRD eGFR model is inaccurate in which patients
People without kidney disease - transplant donors
Children
Pregnancy
Old age - labelled as having kidney disease
Other ethnicities
Amputees/those with reduced muscle mass
Patients with high level of kidney function
Patients whose GFR changes quickly - AKI
Why is the eGFR less accurate in those with mild kidney disease
Reduced GFR
Reduced nephron number leads to nephron hypertrophy so no change in GFR
Reduced filtration of creatinine due to reduced GFR - increases serum creatinine -> increased secretion into nephron so will maintian relatively stable state of serum creatinine - look at patients history to see if creatinine or eGFR has changed
What are urodynamics used for
Urodynamics are used to determine the aetiology of voiding dysfunction and lower urinary tract symptoms
What is a urodynamic study split into
Voiding phase - voiding pressure flow study. Allows diagnosis of obstruction as detrusor pressure and urinary flow rate can be measured and outlet resistance calculated
Filling and storage phase - gives information on bladder contractility
What types of studies are done in a urodynamic study
Flow rate - volume of urine through urethra per unti time
Peak flow rate - maximum measured flow rate
Voided volume - total volume expelled through the urethra
Flow time - total time over which measurable flow occurs
Average flow rate - voided volume/flow time
Time to maximum flow
What are the types of urinalysis
Basic urinalysis - gross examination or urine and dipstick analysis
Microscopic urinalsys - detects cellular elements, casts and crystals
When is the best time to do a urinalysis
First voided morning urine as it is the most concentrated
