Renal Function Tests Flashcards
5 kidney functions
- Excretion of waste products - creatinine, urea
- Maintenance of water balance
- Maintenance of blood pressure - water & sodium excretion/retention
- Maintenance of cardiac function - potassium excretion
- Maintenance of pH - excretion of hydrogen ions
2 functions of adrenal glands
- Bone function via synthesis of vit D (required for calcium absorption)
- Regulates stress response via synthesis of cortisol & adrenaline
What does the kidney filter out
- Sodium
- Chloride
- Sugar
- Amino acids
What do the kidneys reabsorb
- All glucose
- All amino acids
- Most sodium
- Chloride
What is the glomerulus responsible for
Filtering the blood
How are large mulecules kept out of nephron
Diameter of afferent arteriole much greater than diameter of efferent arteriole.
This induces a pressure within capillaries, forces molecules with a molecular weight of less than 68,000 g/mol out.
What is one of the first indicators of glomerular damage and why
- Albumin has a molecular weight of 68,000 g/mol
- The measurement of low concs of albumin in urine is one of the first indicators of glomerular damage
Glomerular function tests
Purpose of Clearance Test
Measures how effectively the kidneys filter & excrete substances from blood
Glomerular function tests
What does it mean if the clearance test is high
Higher the renal clearance, the more plasma that is cleared of the substance
Glomerular function tests
Purpose of serum creatinine test
Assesses waste product clearance
Glomerular function tests
What does it indicate if the serum creatinine is high
High in kidney dysfunction
Glomerular function tests
Purpose of serum urea test
Assesses waste product clearance
Glomerular function tests
What does it indicate if the serum urea test is high
High in kidney dysfunction
Purpose of GFR test
Estimates filtration rate of kidneys
Glomerular function tests
What does the GFR test being less than 60 suggest
Kidney disease
Glomerular function tests
Purpose of urinalysis test
Detects blood (haematuria), protein leakage (proteinuria, often albuminuria)
Glomerular function tests
What does the presence of protein/blood in the urinalysis indicate
Glomerular disease
What are the 2 commonly used clearance tests
- Creatinine clearance (CrCl) test
- Inulin clearance test
Clearance definition
The number of ml of blood cleared of a substance X per unit time, usually espressed in ml/min
Clearance test formula
(U x V) / P
- U = conc of substance in urine mmol/L
- P = conc of substance in plasma mmol/L
- V = volume of urine per unit time ml/min
Patient preparation for clearance test (5)
- Emphasise NB of accurate urine collection over 24 hours
- Avoid high protein meals - protein metabolism can influence creatinine levels
- Stop certain meds (e.g: NSAIDS) - can affect creatinine secretion
- Ensure hydration
- Maintain normal fluid intake, avoid excessive fluids - may alter kidney filtration rates
Why should the first morning urine be discarded
It contains waste accumulated overnight
Steps for24hr urine collection
- First collection in morning
- Discard
- Collect all urine for 24hrs
- Each time patient urinates it must be collected in provided container
- Kept in fridge or cool place
- Final urine sample exactly 24hrs after starting
What is needed after the 24 hr urine to calculate the clearance
Blood sample
Allows for the comparison of creatinine in blood vs urine to calculate clearance
What does the GFR measure
The flow rate of filtered fluid through the kidney
Con of the Cockcroft-Gault formula
Is known to overestimate by 10-40%
Which eGFR formula is more accurate and why
The Variable Modification in Renal Disease (vMDRD)
It takes into account more factors (age, race, gender)
What is the reference range for males for serum creatinine
62 - 115 umol/L
What is the reference range for females for serum creatinine
53 - 97 umol/L
When does serum creatinine increase in the blood
If kidney filtration is insufficient
How is creatinine formed
- Creatine kinase phosphorylates creatine
- This forms creatine phosphate (energy source for muscles)
- Creatinine is a breakdown product of creatine phosphate
What is the Jaffe assay used to measure
Creatinine
In the Jaffe rxn, what is creatinine reacted with
Alkaline pictrate
What colour is the alkaline pictrate complex formed during the Jaffe rxn
RED
What will the presence of noncreatinine chromagens produce in the Jaffe assay
Falsely elevated creatinine values
Steps of the modified Jaffe procedure
- Absorbance of rxn mixture is recorded, acetic acid is then added to rxn vessel
- Acetic acid eliminates the colour produced by creatinine
- The colour from interfering substances (e.g: proteins) is not eliminated
- Absorbance due to creatinine calculated by subtracting the absorbance of the acidified tube from the initial absorbance
Causes of elevated creatinine (4)
- Not always representative of a true reduction in GFR
1. Increased intake of cooked meat
2. Excessive intake of protein/ creatine supplements
3. Intense excercise - released from the muscle during damage
4. Dehydration
What is the Urea:Creatinine ratio meant to be in between
10:1 and 20:1
Causes of increased urea:creatinine ratio (4)
- Low muscle mass
- GIT bleeds
- Fever
- Burns
Causes of decreased urea:creatinine ratio (2)
- Severe liver disease
- Decreased protein intake
What is the ref range for serum urea
1.7 to 8.3 mmol/L
What is urea
The nitrogenous end product of protein & amino acid catabolism
How is urea formed
- Amino acids or proteins deaminated in the liver
- Ammonia is liberated
- Ammonia does not enter blood in normal conditions
- It’s converted into urea by rxns that require enzymes found only in the liver
Urease assay equation
CH4N2O (urea) + 2H2O + H+ ——> 2NH4+ + CO2
(In the presence of urease)
Urease assay - Nesseler’s Rxn: what is ammonia reacted with and how is is read
- Ammonia formed in the urease rxn is reacted with Nesselers reagent (iodine salt of mercury & potassium)
- Forms a brown coloured end product
- Can be measure spectrophotometrically
Urease assay - Berthelot Rxn: what is ammonia reacted with and how is is read
- Ammonia formed in the urease rxn reacts with phenol and sodium hypochlorite
- Forms a blue indophenol
- Can be measured spectrophotometrically
What serves as a catalyst in the Berthelot rxn for the urease assay
Sodium nitroprusside
Pre renal causes of uraemia (elevated urea) (4)
- Nitrogen narcosis (the bends) - change in consciousness & neuromuscular function caused by breathing compressed inert gas
- Low blood pressure - shock, haemorrhage, dehydration
- Decreased blood flow to kidneys - heart failure, atherosclerosis
- Increased dietary protein / protein catabolism
2 renal causes of uraemia
- Kidney disease (nephropathy) - decreased glomerular filtration #
- Kidney failure (CKD)
Post renal causes of uraemia
- Urinary output obstruction - by calculi, tumours of bladder
Decreased urea - pre renal causes (4)
- Liver disease - impaired urea synthsis in urea cycle
- High fluid intake - dilution effect
- Malnutrition/decreased protein intake
- Anabolic states (pregnancy, growth, post surgery) - increased protein synthesis, decreased urea formation
Moderately increased albuminuria is a sensitive indicator of what
Glomerular damage
What is moderately increased albuminuria defined as in 24 hr collection
Excretion of 30-300 mg/24hr
What does tubular reabsorption ensure
Important constituents are not lost unnecessarily from the blood (water, glucose, sodium, AAs)
What in the urine is indicative of kidney tubular dysfunction
Beta 2 microglobulin
Is filtered by kidneys & reabsorbed by tubules
What kind of techniques have been developed to measur conc of beta 2 microglobulins in urine samples
Immunological techniques
What is acute renal disease identified by
Rising serum urea & creatinine
Tx of chronic renal failure
- Dialysis
- Transplant
Clinical significance of chronic liver disease
- Hyperkalaemia
- Abnormal calcium - will affect bone density
- Abnormal phosphate metabolism
- Anaemia
