RENAL FUNCTION TESTS Flashcards
• About______ of blood (_____plasma) passes through the kidneys every minute.
• About_______ is filtered per minute by the kidneys & this is referred to as glomerular filtration rate (GFR).
1200 ml; 650 ml
120-125 ml
GFR
120-125 ml/min
(gold standard) measures the rate by which the kidneys remove a filterable substance from the blood
Clearance tests
Clearance tests
Sample used
24-hour urine
• Sample used is 24-hour urine
Ideal sample
• The substance must be neither reabsorbed nor secreted by the tubules
• The substance must be stable during the 24-hour collection
• Substance’s availability in the body
• Consistency of plasma level
• Availability of the test in the lab
- earliest glomerular filtration test
Urea
Advantages:
• 1. Present in all urine specimens
• 2. Available lab methods
• 3. Endogenous
Urea
• Disadvantage:
• 50% of the filtered_____ is reabsorbed by the tubules = hydration needs to be done
urea
- a polymer of fructose that is a prebiotic fiber; considered as the gold standard in measuring GFR
Inulin
Gold standard for measuring gfr
Inulin
Advantages:
• 1. Highly stable
• 2. Neither reabsorbed or secreted
Disadvantage:
• Exogenous-requires infusion at a constant rate (3-4 hours) because it is not a normal body constituent
• Impractical
Inulin
• Enables visualization of the filtration in the kidneys
• More labor intensive and costly
Radionuclides
- tested by their disappearance from the plasma, thereby eliminating the need for urine collection.
Radionuclides
• Examples:
• 125I-iothalamate and 99mTc-DTPA (diethylene-triamine -pentaacetate)
• 51Cr-EDTA
Radionuclides
• degrades in acidic environment
Beta-2-microglobulin
• Dissociates at a constant rate from WBCs and is rapidly removed from the plasma by the kidneys
• However, test is not reliable in patients with immunologic disease or malignancy
Beta-2-microglobulin
- forms part of the class I MHC present in leukocytes (11,800 kda)
Beta-2-microglobulin
• A low-molecular weight protein isolated in the CSF
• Freely filtered and reabsorbed by the PCT
Beta trace protein
• Filtered freely and not reabsorbed
• Strong linear correlation with inulin clearance
• Measured ONLY using HPLC (expensive and time-consuming)
Tryptophan glycoconjugate
Tryptophan glycoconjugate
Measured only using
HPLC
• A small protein produced by all nucleated cells
Cystatin C
• Readily filtered, reabsorbed, and broken down by renal tubules
• Plasma level is inversely proportional to GFR
• Changes in serum concentration are used as indirect estimate of GER
Cystatin C
• Potential marker for long-term monitoring of renal function
Cystatin C
• Advantages:
• Constant in serum levels
• Independent of age, gender, and muscle mass
• More sensitive to GFR changes than serum creatinine
• Disadvantage: Higher analysis cost
Cystatin C
- most widely used endogenous procedure
Creatinine
• Disadvantages:
• Secreted by tubules and secretion increases as blood level increases
• Chromogens in the plasma can react in the chemical analysis for creatinine
• Bacteria will break down urinary creatinine if specimen is kept at room temp for extended period
Creatinine
• Disadvantages:
• A diet heavy in meat consumed during collection of a 24hr urine will influence the creatinine level
• Not reliable in patients with muscle wasting diseases
• Interference by medication (salicylate, trimethoprim, cimetidine)
Creatinine
COMPUTE FOR GLOMERULAR FILTRATION RATE
• Reported in______; Normal is______
• Males________
• Females________
ml/min; 120ml/min
107-139ml/min
87-107ml/min
Computation for GFR
• One must first know the following
• 1. Urine creatinine in mg/dL
• 2. Plasma creatinine in mg/dL
• 3. Urine volume
Urine volume
• Calculated as number of mL of urine divided by the minutes used to collect the specimen
• Can be used to estimate kidney function for CKD staging or whether to adjust or discontinue medications based on kidney function
COCKCROFT-GAULT FORMULA
• Measure serum creatinine via isotope dilution mass spectrometry
MDRD (Modification of Diet in Renal Disease)-IDMS formula
WHEN INTERPRETING GFR, ONE MUST CONSIDER…
• It is determined not only by the number of nephrons but also by their______
• Example: If one half of the nephrons are nonfunctional, GFR still remains normal if the remaining nephrons double their capacity.
functional capacity
GFR
• Conclusion: It cannot detect_____. It can only evaluate the extent of nephron damage. It can also be used to determine if a person can be started on a medication.
early renal disease
TUBULAR REABSORPTION
TEST(CONCENTRATION TEST)
•_________ is the first sign of renal disease
Loss of tubular reabsorption capacity
• As previously mentioned, the ultrafiltrate that enters the tubules has a specific gravity of______ and it is expected that the final urine is more concentrated or diluted depending on hydration
1.010
•________ evaluate the ability of renal tubular cells to selectively absorb and secrete solutes.
• Measures the renal concentrating ability of the kidneys
Fluid deprivation tests
• Urine is collected after 12 hours (8am)
• IF the urine osmolarity is above 800mOsm or higher - NORMAL
• IF the urine osmolarity is below 800mOsm, fluid restriction is continued for two more hours
FLUID DEPRIVATION TEST
• Overnight water/fluid deprivation test for 12 hours (8pm to 8am)
FLUID DEPRIVATION TEST
FLUID DEPRIVATION TEST
• IF the urine osmolarity is above_____ or higher - NORMAL
• IF the urine osmolarity is below_____, fluid restriction is continued for____ more hours
• After that, (2) are collected and osmolarity is tested
• Normal: if urine osmolarity is or above 800mOsm or if the urine to serum osmolarity ratio is or greater than 3:1
• (Note: normal evening meal but no water or any fluids after)
800mOsm
800mOsm; two
urine and plasma
FLUID DEPRIVATION TEST
• If the test continues to be abnormal, additional testing should be done to diagnose diabetes insipidus
• Patient is injected with ADH
• Serum and urine are collected after 2 and 4 hours
Fluid deprivation test
• Result interpretation:
• If test becomes normal (> 800 mOsm)=_______
• If test result is below 400 mOsm or ratio is 1:1 =_______
Neurogenic/Cranial diabetes insipidus
Nephrogenic diabetes insipidus
• Measure urine SG after fluid deprivation: greater than or equal to 1.025 is normal
Fishberg test
Fishberg test
SG: greater than or equal to____ is normal
1.025
•
• Normal diet and fluid intake
• 24-hour urine: 12 hours (day), 12 hours (night)
• Measure SG:
• Urine volume of day urine must be greater than night urine
• Night urine SG must be greater than or equal to 1.020
Mosenthal test
Mosenthal test
• Measure SG:
• Urine volume of day urine must be greater than night urine
• Night urine SG must be greater than or equal to____
1.020
TUBULAR SECRETION AND RENAL
BLOOD FLOW TEST
• Test to measure tubular secretion of non-filtered substances and renal blood flow
- Phenolsulfonphthalein Test: dye excretion
- p-amino hippuric acid test (PAH).
- Titratable Acidity (depends on phosphate in the filtrate) & Urinary
Ammonia
• Can cause anaphylactic shock to patients
Phenolsulfonphthalein Test: dye excretion
• Loosely bound to plasma proteins
• Infused intravenously
• Performed in renal labs
p-amino hippuric acid test (PAH).
• Measures the ability of kidney to produce acid urine depends on tubular secretion/excretion of ammonia by the cells of the DCT
Titratable Acidity (depends on phosphate in the filtrate) & Urinary
Ammonia
• measures the inability to produce acid urine (renal tubular acidosis)
• Measurement of total hydrogen ion excretion in urine after intake of oral ammonium chloride (urine collection every 2 hours for 8-10 hours)
Ammonium chloride test
