Week 3 Flashcards
Renal function tests that evaluate clearance include:
Renal function tests that evaluate Tubular function include:
CLEARANCE TESTS: Blood Urea Nitrogen (BUN) Serum Creatinine Creatinine clearance Albumin Creatinine Ratio
TUBULAR FUNCTION TESTS:
Fractional Excretion of Na+
The ideal substance to measure GFR would be…
100% filtered at the glomeruli easily measured not bound to plasma not metabolized non-toxic stable in blood Be excreted only by the kidneys Be neither reabsorbed nor secreted by the renal tubules
Blood Urea Nitrogen (BUN)
Derived from:
How much absorbed? Clearance?
Derived from protein catabolism via urea cycle in liver
AMino acids -> NH3 -> Urea -> circulation
Filtered by glomerulus -> 40% reabsorbed (b/c water follows it)
Urea clearance is about !60% of true GFR! under usual conditions
BUN can also be affected by:
The liver can cause abnormal Urea levels, so BUN isn’t specific for the kidneys filtration ability
BUN ; why is it considered a “rough indicator” of GFR and renal blood flow?
Serum/plasma testing- usually part of Comprehensive Metabolic Panel (CMP) or Basic Metabolic Panel (BMP)
Evaluates liver function
Indirect measurement of renal function
Rough indicator of GFR and renal blood flow
BUN
interfering factors:
Protein intake (diet) affects BUN Muscle mass pregnancy can decrease levels b/c hemodilution and incr. GFR Hydration levels liver dz decr. production drugs
BUN
Normal serum levels: (Adult)
Critical Value:
Causes for decreased values:
Normal serum levels
-Adult: 10-20 mg/dl (elderly may be slightly higher)
Critical Value: >100 mg/dl indicates serious impairment of renal function
Causes for decreased values:
- Fluid overload
- Malnutrition
- Severe liver disease
Azotemia
Azotemia = incr. concentration of non-protein nitrogenous waste products (ex: urea, creatinine) in the blood
- most renal dzs cause inadequate excretion of urea, so blood levels of urea increase
- Other conditions can cause incr. BUN, so you must distinguish between pre-renal azotemia, renal azotemia, and post-renal azotemia
!Patients with increased BUN have azotemia!
Prerenal causes of incr. BUN accounts for ___% of acute renal failures and includes the following conditions:
55% of acute renal failures
- Low blood volume, shock, burns, dehydration
- CHF, MI
- GI bleed
- Too much protein
- High protein metabolism, starvation
- sepsis
Sudden drop in blood pressure (shock) or interruption of blood flow to the kidneys from severe injury or illness
Renal causes of increased BUN account for ___% of acute renal failure and includes the following conditions:
40%
Renal disease, glomerulonephritis, pyelonephritis, acute tubular necrosis, nephrotoxic drugs
Direct damage to the kidneys by inflammation, toxins, drugs, infx, or reduced blood supply
Postrenal causes of incr. BUN account for ___% of acute renal failure and includes the following conditions:
5% of acute renal failure
- Obstruction of ureters (Stones, tumors, congenital)
- Bladder outlet obstruction (prostatic hypertrophy (BPH), Cancer, congenital)
Sudden obstruction of urine flow due to enlarged prostate, kidney stones, bladder tumor, or injury
Serum Creatinine Where does it come from? Levels depend on... Better or worse than BUN? \_\_% filtered by the kidneys
Superior to BUN!
- A catabolic product of creatinine phosphate from skeletal muscle
- Blood creatinine values depend on muscle mass, which fluctuates very little unless some muscle-wasting pathology exists or starvation
- More stable marker than BUN
!Creatinine is almost completely filtered by the kidneys! but is also secreted by the proximal tubule
Creatinine
increases before or after BUN?
Creatinine elevations suggest:
What populations generally have lower levels? Why?
increases later than BUN
Creatinine elevations suggest chronic dz process and parallel BUN increases
Elderly and young children generally have lower levels due to decreased muscle mass
Normal ranges for serum creatinine
Is it affected by liver function?
Male: 0.6-1.2 mg/dl
Female: 0.5-1.1 mg/dl
Used to diagnose impaired renal function
Minimally affected by liver function (unlike BUN)
Creatinine and BUN are not good screening tests for overall renal function! Why?
What are they good for then?
You can lose 50% of both kidneys function and still have minimal changes in creatinine
-Large changes in “early” renal disease (50% loss of function) will cause very small changes in BUN and Creatinine
BUN and Creatinine are good for following the later stages of renal disease because: Small changes in GFR late in renal disease cause big changes in BUN or serum creatinine
Typical ratio btwn BUN:Creatinine in Prerenal Azotemia is:
Prerenal Axotemia = BUN:Creatinine of >20:1
- Decr. blood volume or renal circulation (shock, dehydration, MI, CHF)
- Incr. protein intake or catabolism (High protein tube feedings, GI hermorrhage, Starvation)
Renal Azotemia = BUN: Creatinine of 10-20:1
- Chronic diffuse bilateral kidney dz or damage
- Acute tubular necrosis
- Severe acute glomerular damage
How can you tell if your patient has Pre-renal dz or intrinsic Acute Renal Failure? (2 ways to tell)
Pre-renal => disproportionate rise in Urea (ratio = >20:1), protein in urine uncommon
Renal => tend to rise together (ratio = 10-20:1), protein in urine present on dipstick testing
Renal Function Tests
Provides info regarding:
renal blood flow, glomerular filtration rate (GFR), and tubular function
- Requires analysis of plasma/serum samples and/or urine samples
- Imperfect: other things other than damage to renal parenchyma can influence results; localized and general damage; temporary and permanent malfunction
Estimated Glomerular Filtration Rate (eGFR)
The ability of the kidneys to filter blood
As GFR goes down, serum creatinine goes up
A calculation using serum creatinine, the patient’s sex and age using the MDRD equation
Included whenever serum creatinine values are requested
eGFR is inaccurate when:
Vegetarian Pregnant Malnourished >70 or 60ml/min/1.73m^3 muscle disease When GFR by MDRD equation is >60 ml/min/1.73m^3
Creatinine Clearance; Blood and urine
Requires 24 hour urine collection and blood draw
Both samples are analyzed for creatinine
Provides a quantitative measure of the rate at which creatinine is removed from the blood, expressed in ml/min
Values are corrected for body surface are (BSA); must obtain patient height and weight
How to instruct a patient for a 24 hour urine sample:
On the 1st day of collection, first morning void goes into the toilet. Then all other subsequent urine is collected up to the next morning’s first morning void. After 24 hrs, stop collection.
Creatinine clearance norms:
Values decrease ___ ml/min for each decade of life after ___ yo due to…..
Males: 90-139 ml/min
Females: 80-125 ml/min
Values decrease 6.5 ml/min for each decade of life after 20 years due to decline in GFR
Cystatin C
normal range for adults:
gives a better estimate of GFR w/ one blood test; not the best for early detection
- filtered by the glomerular membrane and metabolized by proximal tubules
- estimates GFR independent of gender, age, race, muscle mass and cirrhosis, does not need to be corrected for height and weight.
- !Superior to serum creatinine!
- Normal range for adults: 0.54-1.55 mg/L
(this is a blood test! not a urine test)
Stages of Renal Failure
1: Kidney damage (protein in urine) w/ normal or elevated GFR [GFR = 90 or mo]
2: Kidney damage w/ [GFR = 60-89]
3: Kidney damage w/ [GFR = 30-59]
4: Kidney damage w/ [GFR = 15-29]
5: Kidney failure: end-stage renal dz (ESRD). Patients require dialysis or transplantation. [GFR
Acute vs. Chronic Renal Failure
ARF: Occurs over hours or days, pt has Hx of normal renal function, kidney size usually normal, anemia usually absent!, no broad casts in urine sediment
CRF: Fradual deterioration of renal function over time, pt. has Hx of incr. BUN and Cratinine, kidney size small, ANEMIC! often, broad casts present in urine sediment
pg. 37
Flow chart to look at
Pre-renal vs. Renal azotemia
How do your FENa levels differ in each case?
Pre-renal azotemia: FENa is 1%; decr. renal blood flow triggers renin-angiotensin pathway -> kidneys repson by conserving Na
Renal azotemia: FENa is >2% ; damaged tubules cannot effectively conserve Na
Pg. 40
Proteinuria is the !most important indicator of renal dz!
test is indicated if there is more than trace protein consistently found on routine UA
Normal adult range is less than 150 mg/24hr
Incr protein caused by either glomerular damage or diminished tubular catabolism of small molecular weight proteins
24 hr. urine protein
Functional range if asymptomatic:
if proteinuria ___________ mg/24hrs in adult (or ____ mg/kg/24hrs in child) indicates _________
If proteinuria ________ indicates __________
proteinuria 150-500 mg/24hrs can be functional if asymptomatic
if protenuira >2000 mg/24hrs in adult (or 40 mg/kg/24hrs in child) indicates !glomerular cause!
If proteinuria > 3500 mg/24hrs indicates !nephrotic Syndrome!
________ may cause proteinuria in the absence of structural abnormality.
Proteinuria can occur in non-renal diseases like:
CHF (Congestive Heart Failure)
Non-renal causes of proteinuria:
- CHF
- High serum protein (malignant etiology)
- Pre-eclampsia or eclampsia
- Hypertension
- Toxicity from heavy metals, solvents
Urine protein to Creatinine ratio
Used to monitor:
Which is more accurate (24 hr urine protein OR Urine protein to creatine ratio?)
Normal adult ratio:
Used to monitor persistent Proteinuria More accurate than 24 hr urine protein Use first morning void Normal adult ratio: 3.5 g protein/ 1g creatinine -Correlates with 3.5 g protein/day
