Assessment of kidney function using blood tests Flashcards
What kidney tests can be done in the lab?
U&E request gives serum concentrations of:
- Creatinine – Reference Interval higher in males than females
- Urea
- Sodium
- Potassium
- e-GFR calculated using serum creatinine concentration
Urine
- Albumin:Creatinine ratio (ACR)
- Protein:Creatinine ratio (PCR)
- Plus a range of supporting assays for Acute Kidney Injury (AKI) or Chronic Kidney Disease (CKD)
What is the rate of kidney blood flow?
1500mL/min
What is the equation for GFR?
GFR = (U x V)/S = ml/min
U = Urine concentration mmol/l
V = Urine flow rate ml/min
S = Serum concentration mmol/l
What does measurement of glomerular filtration require?
•A substance that is filtered and then neither absorbed or secreted in the renal tubules
24h urine collection and a blood sample
Why use creatinine for GFR?
- Derived from creatine in muscle
- Serum concentrations relatively stable
- 24 h urine output is also stable day to day
- No absorption and very little secretion in renal tubules
- Simple to measure
What causes variations in creatinine?
But variation in serum concentrations and 24h urine output by:
- Age
- Sex – male > female
- Lean body mass – ethnic origin
- Effect of Diet
How are creatinine variations overcome?
GFR related to body surface area – so standardise to a given body surface area
This allows:
- Comparison against a general standard
- Staging of renal disease
- Monitoring of change in renal filtration – individual or cohort
- Chosen standard = 1.73m2
What is the equation for eGFR?
MDRD equation as calculated in the laboratory computer:
eGFR = 175 x (serum creatinine)-1.154 x (age)-0.203 x (0.742 if female) x (1.21 if Afro-Caribbean)
This allows for variation by age, sex and ethnic origin
Age – decreasing GFR with age but largely offset by reducing muscle mass with age
MDRD = Modification of Diet in Renal Disease
- Very high plasma glucose – causes an osmotic diuresis leading to loss of water and sodium
- Vomiting – adding to his fluid loss
- Loss of salt and water – causes volume depletion leading to impaired renal function and also dehydration
- Failure of glucose metabolism leads to keto-acidosis – 3-OH butyrate
- Acidosis – plasma 3-OH butyrate and bicarbonate – lead to increased serum concentrations of potassium and phosphate
- Renal impairment – leads to potassium and phosphate accumulation and enhances the acidosis
What tests indicate dehydration?
Dehydration is very likely if the serum urea:creatinine ratio is greater than 100
NB – both measurements need to be in mmol/l
Some authors have used 80 as a cut-off
The mechanism is the passive re-absorption of urea in the nephron especially at low flow rates
Serum protein concentrations may also be elevated
What tests indicate AKI?
- a rise in serum creatinine of 26 µmol/L or greater within 48h,
- a 50% or greater rise in serum creatinine known or presumed to have occurred within the past 7 days,
- a fall in urine output to less than 0.5 mL/kg/hr for more than 6 hours for adults and more than 8 hours in children or young people,
- a 25% or greater fall in eGFR in children and young people < 18y
What is the AKI alert system?
These Alerts are calculated by the lab computer:
- AKI stage 1 is a rise of >1.5x baseline level, or of >26μmol/L within 48h
- AKI stage 2 is a rise of>2x baseline
- AKI stage 3 is a rise of>3x baseline or a rise of >1.5 baseline to >354μmol/L
What are the causes of AKI?
- Very elevated serum creatinine
- AKI Alert level 3
- Very raised serum CRP concentration
- Marked proteinuria – not just albumin – indicates marked glomerular damage
- Anaemia with a raised WBC
- Negative Anti-streptolysin O titre – not post-streptococcal
- Raised Anti-GBM antibody – GBM = glomerular basement membrane
This is Goodpasture’s syndrome – an important diagnosis as it requires treatment by plasma exchange to remove the antibody
This man was a body-builder
- High muscle mass
- High protein intake
- Strenuous muscle activity
- Pressing heavy weights
•
Note – mild hypokalaemia is not uncommon in body-builders
Write the equation for the origins of creatinine
Note – AKI although both serum creatinine results are within the reference interval
Also he is dehydrated and has evidence of a septic condition – raised serum CRP
What are the metabolic features of stage 2 and 3 CKD?
What are the metabolic features of stage 4 and 5 CKD?
What are the interventions in CKD?
What are the interventions in CKD?
What would you do next?
What is the likely diagnosis and treatment?
Early morning urine pH. This will be greater than 5.5 in Type 1 (distal) Renal Tubular Acidosis with hypokalaemia
Treatment to reduce the risk of nephrocalcinosis/renal stone formation:
potassium citrate, sodium bicarbonate, thiazide diuretics
24 h urine collection showed elevated concentrations of amino-acids, calcium and phosphate
Fractional excretion of phosphate:
(Urine phosphate x Serum creatinine)/(Serum phosphate x Urine creatinine)
(24.3 x 0.065)/(0.72 x 7.2) = 30.5%
>20% indicates excessive loss
This is Fanconi Syndrome – the condition may be either congenital or acquired
Diabetes insipidus
What condition causes each result?
