Clinical Biochem 3 Flashcards
Why do we check renal function? What is the kidney responsible for?
- clearing waste material from blood
- Maintain salt/water balance
- Regulate blood pressure
- Stimulate bone marrow to make RBCs
- Control Ca/Phos absorption and secretion
Differentiate between hemodialysis and peritoneal dialysis
Hemodialysis
- blood is cleaned outside the body with a machine
Peritoneal dialysis
- blood is cleaned inside the body
- Dialysis fluid is added to abdominal cavity using catheter
- toxins and water are absorbed by the fluid, dirty fluid is replaced by clean fluid
What 2 renal function tests (natural vs non-natural) can be used to estimate GFR. give examples
Exogenous (not natural) substances administed
- eg. Inulin, iothalamte and radioactive substances
Endogenous (natural) substances are measured in the body and used as surrogate markers of GFR
- eg. Blood Urea Nitrogen (BUN), Serum Creatinine (SCr)
Describe Inulin
- It is not metabolized, secreted, reabsorbed or protein bound therefore = 100% is cleared from kidney = true measure of GFR
- Gold standard for measure of GFR but is invasive and requires specialized tools (research only)
- calculate inulin clearance by measuring plasma and urine inulin conc. and urine flow
What is BUN (blood urea nitrogen) used for? Reference range?
2.5-8 mmol/L
Non-specific screening/monitoring tool
- To assess hydration, renal function, protein tolerance, catabolism
Describe Iothalamte and Radioactive Substances
- Invasive: requires injectin of foreigh substances, frequent blood sampling, and timed urine collection
- research tool only
Why is BUN not as accurate as inulin or radioactive markers
Urea undergoes some tubular reabsoportion
What does a decreased BUN indicate? What does it not indicate?
- indicate malnourishment
- may be associated with liver disease
NOT CAUSED BY renal dysfunction
(no pathological consequence)
What are the 3 causes associated with elevated BUN
Pre-renal causes
Intra-renal causes
Post-renal causes
Explain Pre-renal causes that elevate BUN (2)
- Decreased renal blood flow –> decreased GFR by 10%
eg. Congestive heart failure, dehydration, hypotension - Increased protein breakdown –> increased urea production (no effect on GFR)
eg. GI bleed, burn, fever, too much protein
Explain Intrarenal causes that elevate BUN (2)
- Acute renal failure
- Nephrotoxic drugs
- severe hypertension
- Glomerular nephritis
- Tubular nerosis - Chronic renal failure
- diabetes
Explain post-renal causes that elevate BUN (2)
Obstruction of urine flow (post-kidney
- Ureter
- bladder neck
- Urethra
Reference range for Serum Creatinine SCr? Where does it come from? how is it eliminated? Does diet or urine flow affect it?
58-110 mmoles/L
- Marker of renal function
- Comes from breakdown of creatine phosphate
- eliminated through GF
- not affected by diet or urine flow
Explain renal and non-renal causes of elevated SCr
Renal: due to decreased GFR = less creatinine clearance
Non-renal (temp. increase): large meal of meat, vigorous exercise, increased muscle mass
Explain the causes of decreased SCr
Decreased muscle mass/activity
eg.
Coma
Taking neuromuscular blocking agents
Patients with spinal cord injuries
Elderly
What is the rule of thumb for SCr normal renal function
Stable SCr = normal renal function
- doubling value into reference range is more dangerous than consistency below range
What reflects the changes in GFR? how/ what is the trend?
SCr increases, there is a delay depending on renal function
- the worse the renal function the longer it’ll take to adjust
Reference range for creatinine clearance (CrCl)? what is it used to?
90-140 mL/min
Used to
- assess kidney function
- monitor effect of drugs on slowing progression of kidney disease
- monitor patients on nephrotoxic drugs
- determine dose adjustments for renally eliminated drugs
What is the relationship between SCr and CrCL and renal function? when does SCr rise?
SCr is inversely proportional to CrCL and renal function
CrCL function = renal function
Before SCr rises:
- decline in CrCL
- 50% of nephrons are non-functional
- because of this, SCr alone is not a good indicator of early decreased kidney function
What is the requirement for measuring CrCl?
Requires 24 hour urine collection
- time consuming, labour intensive
What are drawbacks of the Cockcroft-Gault Equation
- Based on small study only on men
- Based on patients with stable SCr value, so not useful for changing renal function/SCr
- Misleading for SCr affected by non-renal factors
- DOES NOT CONSIDER BODY SURFACE AREA
What are drawbacks of the Schwartz equation
- May overestimate CrCL
- only for paediatric patients 1 week- 18 years
- not for changing SCr/ renal function
What are the drawbacks of the Modification of Diet in Renal Disease (MDRD) equation?
- estimates ONLY GFR, NOT CrCL
- not for changing SCr/renal function
What happens if we use the equations for rapidly decreasing/increasing SCr value?
SCr decreasing –> underestimate of renal function (good function) = reduce dose unneccesarily
SCr increasing –> overestimate of renal function (bad function) = increase dose too high for kidneys to handle
What happens if you use the equations for patients on dialysis
Dialysis machine removes Cr –> underestimate of renal function (good function)
What consideration for CrCL measurement if renal dysfunction is temporary or chronic?
Temporary: IV rehydration
Chronic: use CG/MDRD equation -> SCr levels are stable
Consideration for drug response with urgent or non-immediate symptom cases
Urgent: start with normal dose and reasses renal function
Non-urgent: start low and tritrate up to response or toxicity
What consideration should you take with size of therapeutic window?
Careful with drug with narrow therapeutic window. Start low, go slow
When should CG equation be used?
- when determining dosing recommendations and dosing adjustments
- since it is a measure of actual CrCL and not relative renal function
When should MDRD equation be used?
MORE ACCURATE
- used to estimate GFR
