measurement of kidney function Flashcards
1
Q
why do we measure renal function? 2
A
- Identification of renal impairment in your patient
- Modification of dosages of drugs which are cleared by the kidneys
2
Q
what patients are at risk of developing renal failure? 7
A
- Extremes of age
- Polypharmacy (regimens involving many drugs- risk of adverse reactions)
- Specific disease states (hypertension, diabetes, chronic heart failure, rheumatoid arthritis, renal disease, recurrent UTI)
- Patients receiving long-term analgesia
- Transplant patients
- Patients on drug therapy
- Patients undergoing imaging procedures
3
Q
how do we monitor a patients renal function? 3
A
- Patients clinical condition (clinical assessment, use of bedside clinical data)
- Modern imaging techniques (macroscopic views of renal blood, filtration and excretory function)
- Biochemical data (measurement of renal clearance of various substances, allows evaluation of the ability of the kidneys to handle water and solutes)
4
Q
how do we perform a clinical assessment of someone with suspected renal failure? 5
A
Look at the patient, and listen to what they tell you about their symptoms- can often give you clues about their renal function
Use of bedside data:
- Weight charts
- Fluid balance charts
- Degree of oedema
- Results of urine dipstick testing (urinalysis for protein, blood, glucose)
5
Q
what do modern imaging techniques show?
A
macroscopic views of renal blood flow, filtration and excretory function
6
Q
what is renography? 3
A
- Gamma camera planar scintigraphy
- Positron emission tomography (PET)
- Single photon emission computerised tomography (SPECT
7
Q
describe the use of biochemical data when diagnosing someone with renal failure? 7
A
- Useful for identifying renal impairment
- Evaluation of the ability of the kidneys to handle water and solutes
- Modifying dosages of drugs which are cleared by the kidneys
- Blood (plasma or serum) markers of renal function:
- Plasma or serum creatinine (sCr)
- Plasma or serum urea or blood urea nitrogen (BUN)
- Plasma= serum + clotting proteins
8
Q
what is creatinine? 4
A
- Breakdown product of creatine phosphate in muscle
- Generally produced at a constant rate
- Filtered at the glomerulus with some secretion into the proximal tubule
- Normal range in plasma= 40-120
9
Q
what can plasma creatinine be increased by? 5
decreased by? 5
A
- Increased by:
- Large muscle mass, dietary intake
- Drugs which interfere with analysis
- Drugs which inhibit tubular secretion
- Ketoacidosis
- Ethnicity (higher creatine kinase activity in black population)
- .
- Decreased by:
- Reduced muscle mass
- Cachexia/ starvation
- Immobility
- Pregnancy (due to increased plasma volume in the mother)
- Severe liver disease (as liver is also a source of creatinine)
10
Q
what is urea? 4
A
- Liver produces urea in the urea cycle as a waste product of protein digestion
- Filtered at the glomerulus, secreted and reabsorbed in the tubule
- Plasma urea described as BUN: blood urea nitrogen: normal range: 2.5-7.5
- > 20 indicates moderate to severe renal failure
11
Q
describe what can increase BUN? 7
decrease? 4
A
- Increased by:
- High protein diet
- Hypercatabolic conditions
- Gastrointestinal bleeding
- Muscle injury
- Drugs
- Tetracycline
- Hypovolaemia
- .
- Decreased by:
- Malnutrition
- Liver disease
- Sickle cells anaemia due to increased GFR
- SIADH (syndrome of inappropriate ADH)
12
Q
what would an ideal marker of kidney function be? 4
A
- A naturally occurring molecule
- Not metabolised
- Only excreted by the kidney
- Filtered but not secreted or reabsorbed by the kidney
13
Q
explain how different substances can be excreted or cleared by the kidney? 4
A
- Some are filtered by the glomerulus and are NOT reabsorbed (excretion rate= rate it was filtered (INULIN))
- Some are filtered and some of the filtered portion is reabsorbed (excretion rate= filtration rate- reabsorbed (typical of electrolytes))
- Some are filtered and completely reabsorbed (no excretion (glucose and amino acids)
- Some are secreted into the tubule (substance is therefore rapidly and effectively cleared (PAH))
14
Q
describe renal clearance? 6
A
- Clearance= the volume of plasma completely cleared of a given substance in a unit of time
- Compares rate at which glomeruli filter a substance with the rate at which the kidneys excrete it via the urine
- Measurement of difference in amount filtered and excreted allows estimation of the net amount reabsorbed or secreted by the renal tubules
- The clearance of a solute is the virtual volume of blood that would be totally cleared of a solute in a given time
- Solutes come from blood perfusing kidneys
- Rate at which kidneys excrete solute into the urine= rate at which solute disappears from blood plasma
15
Q
what can renal clearance provide information about? 3
A
- Glomerular filtration (F)
- Tubular reabsorption (R)
- Tubular secretion (S)
16
Q
describe the equation for clearance rate?
A
Cx= (Ux x V)/ Px
- Ux= concentration of x in urine
- V= volume of urine formed in given time
- Px= concentration of x in systemic blood plasma or serum
- ml/min
17
Q
what are the drawbacks of renal clearance? 2
A
- Measuring clearance means measurement of overall nephron function
- This gives the sum of all transport processes occurring along nephrons but no information about precise tubular sites or mechanisms of transport
18
Q
describe glomerular filtration rate? 4
A
- This is the rate at which filtrate is produced in the kidneys
- GFR can be measured clinically and used as an indicator of renal function
- GFR can be estimated by measurement of the clearance of CREATININE, but creatinine is filtered and secreted into the tubule
- A more accurate estimation if provided by the measurement of INULIN clearance as it is filtered but not secreted into the tubule
19
Q
describe inulin clearance for the measurement of GFR? 5
A
- Inulin is a plant polysaccharide
- It is freely filtered but is not secreted and is not reabsorbed
- The rate of excretion in the urine equals the rate of filtration by the kidneys
- Insulin clearance= GFR
if a substance has clearance greater than inulin, then is must also be being secreted - If a substance has a clearance less then inulin, then is must be being reabsorbed or not filtered freely at the glomerulus
20
Q
what are the drawbacks of inulin clearance to measure GFR? 5
A
- Most reliable method of measuring GFR but not clinically useful
- Inulin must be administered by IV to get relatively constant plasma or serum levels
- Chemical analysis of inulin is technically demanding
- Could use radiolabelled compounds instead but these mat bind to protein and distort results
- Problems of IV infusion of GFR markers are avoided by using an endogenous substance with inulin like properties- creatinine
21
Q
describe creatinine clearance? 4
A
- Creatinine is filtered at the glomerulus but some of it is also secreted at the proximal tubule
- Therefore using the equation would overestimate GFR by about 20% in humans
- The colorimetry methods used to measure creatinine underestimates creatinine concentrations by about 20%
- These 2 errors cancel each other out and therefore calculate creatinine clearance is around the same as inulin clearance
22
Q
why do we use creatinine clearance for GFR? 5
A
- Cheap, easy, reliable, used clinically
- Avoids IV infusion, just requires venous blood and urine samples
- Creatinine usually produced by creatinine phosphate metabolism in muscle
- Must remember to take into account if the person has muscle disease/ damage or has had large quantities of meat to eat
- Usually measured over a 24-hour period to get reliable results and take samples before breakfast
23
Q
why do we estimate GFR using plasma creatinine only?
A
- Allows estimation of GFR without having to collect urine samples
24
Q
describe how we use PAH clearance to measure renal blood flow? 6
A
- If substance is completely cleared from the plasma, its clearance rate will be equal to the renal plasma flow (RPF)
- Clearance of PAH (para-aminohippuric acid) can be used to estimate this
- PAH is not normally present in the blood
- When given, almost all is cleared from the kidney in one passage, some is filtered in the glomerulus and some secreted by proximal tubules
- 10% passes tubule and travels from efferent arterioles into peritubular capillaries and then venous renal blood and is not secreted
- Uncorrected value for PAH clearance of often used and is known as ‘effective renal plasma flow’
25
Q
describe the biomarkers of renal disease? 9
A
- Indicators of renal function such as plasma creatinine or BUN increase only after there is a significant loss of renal function (60% loss)
- Urinary albumin/protein excretion can also be used as an indicator of chronic kidney disease
- Currently a lot of research and interest in identifying blood and urinary markers which increase in the early stages of renal failure and can be measured
- These are mostly proteins released into the plasma and/or urine:
- Kidney injury molecule- 1 (urine)
- Interleukin- 18 (urine)
- Fatty-acid binding proteins (urine)
- Neutrophil gelatinase-associated lipocalin (plasma and urine)
- Cystatin C (plasma)
