Assessing Kidney Function Flashcards
1
Q
What are the functions of the kidneys?
A
- Excretion of products of metabolism.
- Water regulation.
- Electrolyte regulation.
- Maintaining Hb.
- Maintaining calcium homeostasis.
2
Q
What is glomerular filtration rate and how can it be used to assess kidney function?
A
- GFR is proportional to the number of functioning glomeruli present - so reflects ‘function’ of the kidney.
- Mainly looks at clearance: urinary output of ‘x’ / minute relative to its concentration in the blood.
- GFR is the volume of plasma filtered / unit time = 120mL/min.
- ‘X’ needs to be freely filtered, not reabsorbed, not secreted, not modified / metabolised after filtration and not variably produced.
3
Q
What are the molecules which can be infused into a patient to assess kidney function?
What is the problem with these?
A
- Inulin and polyfructosan: plant extracts, non-toxic.
- Requires infusion, multiple sampling of urine and plasma concentrations.
- The problem is they need to be infused into patients to produce a steady state and this is difficult because it can be a laborious process and not practical for patients coming into an outpatient clinic.
- BUT, there are biologic molecules present in the blood which can fulfil this criteria.
4
Q
Describe the use of creatinine as a measure of kidney function.
A
- Metabolic product of creatinine and phosphocreatinine.
- Exogenously acquired from meat MW113 Da.
- Does not bind plasma proteins.
- Freely filtered.
- Almost never reasborbed.
BUT
- Secreted by tubules - over estimated by up to 10%.
- If you have a higher creatinine level, more is excreted in the tubules. So, when there is increased filtration there is an overestimated creatinine level.
- Increased error at lower GFR.
- Creatinine related to muscle mass.
- Severe malnutrition / elderly / no meat diet.
- Drugs trimethoprim / cimetidine compete for secretion.
5
Q
Describe the use of urea as a measure of kidney function.
A
- Metabolic product of amino acids.
- Exogenously acquired from protein intake.
- MW60 Da.
- Freely filtered
BUT
- 50% is reabsorbed by PCT: depends on water / Na reabsorption.
- Liver disease reduces urea plasma levels.
- Protein degradation in intestine increases urea.
6
Q
What are the indications for a clearance measurement when estimates based on serum creatinine may be inaccurate?
A
- Extremes of age and body size.
- Severe malnutrition or obesity.
- Disease of skeletal muscle.
- Paraplegia or quadriplegia.
- Vegetarian diet
- Rapidly changing kidney function.
- Pregnancy.
7
Q
What happens to GFR with age?
A
- GFR declines with age - you would not say that someone has reduced kidney function without validating this against age - they may be older and have a physiological reduction in GFR in health.
8
Q
What is cystatin C useful for?
A
- Rises very sharply if there is any reduction in GFR - good to assess acute kidney injury.
- It is a cysteine protease inhibitor that is produced in nearly all nucleated human cells.
- Independent of body mass, age, sex, inflammation or malignancy.
- MW 13Da.
- Freely filtered.
- Reabsorbed and metabolised by proximal tubule cells.
- Serum levels correlate with GFR.
- Better for elderly populations.
- BUT not yet vaildated.
9
Q
What is eGFR?
A
- Formulae based on sigle serum creatinine alone.
- Weight surrogate for muscle mass.
10
Q
What is modification of diet in renal disease (MDRD)?
A
- Computerised.
- Useful if stable serum creatinine.
- Less accurate if near normal GFR (under-estimate if >60 mL/min/1.73m2).
11
Q
Describe renal blood flow.
A
- Affects GFR due to the hydraulic pressure in the glomerulus based on afferent and efferent arterioles dilation / constriction.
- Kidneys take 20% of cardiac output - 1L/minute.
- Blood flow within the kidney is not homogenous (there are variations between the cortex and medulla) but for clinical practice total renal blood flow influences understanding of GFR.
- Para-aminohippurate clearance: completely extracted from plasma during a single pass through the kidney and eliminated in urine unchanged.
12
Q
What happens when ACE-I inhibit AT-II production, or ARBs block AT-II at the receptor?
A
- ACEI and ARB block cause false creatinine levels.
- Start by checking patient’s bloods before starting the drugs then assess 1-2 weeks later to see if there is any change.
- If the patient has 2 functioning kidneys there should be no change.
- Greater than 25% drop in GFR and more than 30% increase in creatinine makes you think kidney problem and you should stop the drug.
13
Q
Which types of imaging can be used to assess kidney function?
What are each of these images looking for?
A
- ERPF = effective renal plasma flow.
14
Q
Describe this measure of kidney function.
A
- Clearance closest to inulin.
- Both excreted by glomerular filtration.
- Low radiation dose.
- Smaller fraction of DTPA is bound to proteins than EDTA.
- Not useful if impaired renal function - GFR <30mL/min (20% extracted with each pass through the kidney; 90% within 4 hours).
15
Q
Describe this measure of kidney function.
A
- Concentrated in the renal cortex - cytoplasm of the proximal tubules.
- Binds to plasma proteins (retained for longer) - 6 hours.
- 2 hours after injection 50% retained in the cortex of kidneys.
- Relative kidney function - USS may show small kidney which might lead you to think that that kidney is not functioning properly but DMSA scan can then show that function is normal.
- Areas of scarring / non-functioning.
- Not actually that useful in assessing kidney function.
16
Q
Describe this measure of kidney function.
A
- Highly protein bound 70-90%.
- Cleared by proximal tubules 89%.
- Extraction fraction is 40-50% (better than DTPA).
- Independent indicator or ERPF and renal function.
18
Q
Describe the use of proteinuria as an assessment of kidney function.
A
- Proteinuria on dipstick is actually albuminuria.
- It assesses the integrity of the glomerular filter.
- Albumin is water-soluble
- Un-glycosylated protein
- MW 65,000 Da
- Negatively charged
- Presence is a sign of kidney damage.
- Urinary albumin varies dependent upon posture, exercise, acute diuresis. Dipstick correlates poorly with 24 hour proteinuria.
- False positives if menstruation / UTIs.
- Does not detect light chains in urine.
- Urine albumin: creatinine ratio (ACR) or protein:creatinine ratio (PCR).
- Urinary albumin measurement provides a more specific and sensitive measure of change in glomerular permeability than total protein.
- UACR correlates with early glomerulosclerosis in diabetics - before there is a drop in GFR.
- Guides management.
