Assessment of kidney function and disease (7.5) Flashcards
Assessment of kidney function: Serum creatinine
Advantages and disadvantages
A metabolite of muscle (creatine phosphate)
Advantages:
- Produced at a constant rate
- Simple and cheap
- Best measurement for AKI
Disadvantages:
- Not an accurate measurement of GFR
- There is also some secretion of creatinine through the tubules (although predominantly excreted by glomerular filtration)
- Can be affected by diet (meat)
- Serum creatinine is proportional to muscle mass - must consider age and gender
- Can be affected by medication
- Non-linear relationship to GFR: Means big difference in GFR correlate with small changes in creatinine (GFR changes may be missed)
Assessment of kidney function: Creatinine clearance
‘Mechanism’, advantages and disadvantages
Combine urinary clearance and plasma levels of creatinine. Collect 24 hour urine sample.
CrCl = U x V / P
([urinary] x urinary volume / [plasma]
Advantages:
- More accurate than serum creatinine
- Can perform other tests using the urine samples
Disadvantages:
- Very inconvenient for the patient
- Costly
- Limited due to tubule secretion of creatinine
Assessment of kidney function: Cystatin C
Advantages and disadvantages
Advantages:
- Not affected by muscle mass or gender
- May be combined with creatinine to generate a more accurate GFR
Disadvantages:
- Affected by obesity, thryroid function, inflammation and smoking
- More expensive
- Only available in very few labs
Assessment of kidney function: Inulin
Advantages and disadvantages
x
Assessment of kidney function: Iohexol
Advantages and disadvantages
x
State the exogenous and endogenous renal filtration markers
State the key features of a filtration marker
Filtration marker: Ideally the marker must be cleared from the blood solely by glomerular filtration only and not secreted by renal tubules.
Exogenous: Usually only for research purposes or when critical (e.g. chemotherapy)
- Inulin (gold standard but has high costs), iohexol, EDTA
Endogenous: Creatinine, cystatin C
Assessment of kidney function: Renal biopsy
Indications, disadvantages
Indications:
- Required for definite diagnosis in many cases
- In AKI
- Nephrotic syndrome
- CKD, associated with proteinuria
Disadvantages: Invasive → morbidity
Assessment of kidney function: MDRD (modification of diet in renal disease study)
Parameters, advantages, disadvantages
Parameters: Serum creatinine, gender, age, correction factor (for ethnicity
Advantages:
- Corrected for body surface area (BSA)
Disadvantages:
- Underestimates GFR for values > 60 ml /min
- Creatinine assays vary between labs
- Not validated for ethnic group, elderly and extremes of ‘build’
CKD-EPI provides an improved method for calculation.
Assessment of kidney function: CKD-EPI (chronic kidney disease epidemiology equation)
Parameters, advantages, disadvantages
Parameters: Age, gender, serum creatinine and correction factor for ethnicity
Advantages:
- Less bias
- More accurate than MDRD
Disadvantages:
*NICE guidance states CKD-EPI should be used for GFRcreatinine estimates.
Assessment of kidney function: Cystatin C
Advantages and disadvantages
Small protein produced by all nucleated cells.
Advantages:
- Filtered by the glomerulus - no tubular secretion!
- Completely absorbed by the tubule, so no urinary excretion
- Not affected by muscle mass, diet or gender
- Can be combined with creatinine to produce more accurate eGFR
Disadvantages:
- Affected by obesity, thyroid status, diabetes, corticosteroids, inflammation and smoking
- Much more expensive
- Only available in 3 UK labs
Assessment of kidney function: Urinanalysis
- Can indicate whether the problem is occuring pre-, post- or renal. Dependent on whether blood and/or protein is present
- Proteinuria: Urinary dipstick can be misleading
Assessment of kidney function: State other relevant blood tests
- Urea
- Serum albumin
- Haemoglobin
- Ca2+/phosphate/PTH
- Immunology
Assessment of kidney function: Outline the 2012 KDIGO classification for the prognosis of CKD (using GFR and albuminuria)
Uses GFR and albuminuria to calculate the prognosis of CKD
Green = low risk, yellow = moderately increased risk, orange = high risk, red = very high risk
Assessment of kidney function: Imaging
Can allow structure and function to be examined
- Ultrasound: Most commonly used. Good for visualising obstruction.
- Intravenous urography: Inohexol administered and XRs taken to monitor excretion of the substance.
- CT/MR
- Isotopic studies: Can be used to assess how the kidneys are functioning in relation to each other
- Renal angiography
