RENAL FUNCTION TESTS Flashcards by Kyra Catedral

• 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

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GFR

120-125 ml/min

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(gold standard) measures the rate by which the kidneys remove a filterable substance from the blood

Clearance tests

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Clearance tests

Sample used

24-hour urine

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• 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

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earliest glomerular filtration test

Urea

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Advantages:
• 1. Present in all urine specimens
• 2. Available lab methods
• 3. Endogenous

Urea

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• Disadvantage:
• 50% of the filtered_____ is reabsorbed by the tubules = hydration needs to be done

urea

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a polymer of fructose that is a prebiotic fiber; considered as the gold standard in measuring GFR

Inulin

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Gold standard for measuring gfr

Inulin

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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

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• Enables visualization of the filtration in the kidneys
• More labor intensive and costly

Radionuclides

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tested by their disappearance from the plasma, thereby eliminating the need for urine collection.

Radionuclides

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• Examples:
• 125I-iothalamate and 99mTc-DTPA (diethylene-triamine -pentaacetate)
• 51Cr-EDTA

Radionuclides

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• degrades in acidic environment

Beta-2-microglobulin

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• 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

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forms part of the class I MHC present in leukocytes (11,800 kda)

Beta-2-microglobulin

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• A low-molecular weight protein isolated in the CSF
• Freely filtered and reabsorbed by the PCT

Beta trace protein

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• Filtered freely and not reabsorbed
• Strong linear correlation with inulin clearance
• Measured ONLY using HPLC (expensive and time-consuming)

Tryptophan glycoconjugate

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Tryptophan glycoconjugate

Measured only using

HPLC

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• 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

1. Phenolsulfonphthalein Test: dye excretion 2. p-amino hippuric acid test (PAH). 3. 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