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Renal > Assessing Kidney Function > Flashcards

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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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9
Q

What is eGFR?

A
  • Formulae based on sigle serum creatinine alone.
  • Weight surrogate for muscle mass.
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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).
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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.
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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.
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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.
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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).
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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.
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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.
17
Q

Describe the use of urinalysis as a measure of kidney function.

A
  • Urine consists of 95% water and >3000 chemicals.
    • Metabolic breakdown products
    • Drugs
    • Anions / cations
    • Environmental chemicals
    • Bacterial breakdown products
  • Clinically relevant:
    • Blood - red cells / free haem
    • Bilirubin
    • Ketones - DKA / fasting
    • Glucose - diabetes
    • Protein - specific for albumin
    • Nitrites - bacterial product
    • Leucocytes - UTI / allergies
    • pH - not acurate on dipstick
    • Specific gravity
      *
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.
19
Q

Describe the role of the kidneys in maintaining Hb.

A
  • Erythropoietin increases reticulocyte production and release from the bone marrow.
  • 90% produced in the kidney - 10% in the liver.
  • Made in type 1 fibroblastoid cells in peritubular interstitium of the cortex and outer medulla.
  • Hypoxia stimulates erythropoietin mRNA synthesis.
  • Acidosis reduces O2 affinity of Hb so increases tissue oxygenation, reducing erythropoiesis.
  • CKD causes the fibroblastoid cells to become myofibroblastoid so less erythropoietin production.
20
Q

Describe the role of the kidneys in calcium homeostasis.

A
  • Calcium is regulated by bone turnover and gut absorption.
  • 25-30% calcium is absorbed by the gut (duodenum and jejunum). Absorption is increased by vitamin D.
  • Calcium is freely filtered and reabsorbed along the nephron, actively entering cells via PTH stimulation.
  • Transport across the cell is regulated by vitamin D and out of the cell by both PTH and vitamin D regulation.
  • 65% PO4 absorbed in the duodenum and jejunum, enhanced by vitamin D.
  • PO4 is freely filtered and reabsorbed. 80% reabsorbed in the PCT, down regulated by PTH.
  • Low calcium increases PTH.
  • PTH causes bone reabsorption and increases vitamin D synthesis in the kidney.
  • Net result should be correction of Ca/PO4 and PTH : these are issues in CKD.

Renal (10 decks)

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