Glomerula Filtration Tests

mL/min

glomerular filtration tests measures rate in ____

glomerular filtration tests

measures the rate (mL/min) at which the kidneys are able to remove (clear) a filterable substance from the blood

substance analyzed must be not reabsorbed or secreted by the tubules

how to measure accurate glomerular filtration

urea

earliest glomerular filtration test

inulin

original reference method of glomerular filtration test

improperly timed collection

greatest source of error in glomerular filtration tests

1. creatinine

2. B2-microglobulin

3. cystatin C

4. radioisotopes

glomerular filtration: clearance tests

creatinine

single marker that denotes renal disorder

B2-microglobulin

marker of renal tubular injury

inulin

a polymer of fructose, an extremely stable substance that is neither reabsorbed nor secreted by tubules

inulin

not a normal body constituent → must be infused by IV at a constant rate throughout the testing period

cystatin c

a small protein (mw: 13,359) produced at a constant rate by all nucleated cells

cystatin C

• filtered readily by the glomerulus

• reabsorbed and broken down by the renal tubular cells → no cystatin C secreted

cystatin C

its serum concentration can be directly related to GFR

B2-microglobulin

• dissociates from human leukocyte antigens at a constant rate

• removed rapidly from the plasma by glomerular filtration

B2-microglobulin

may also be used to distinguish disorders of the kidney as either glomerular or tubular

rise in plasma B2-microglobulin

more sensitive indicator of a decreased GFR than creatine clearance

how is B2-microglobulin used to distinguish disorders of the kidney as either glomerular or tubular

acute kidney injury

besides renal tubular injury, B2-microglobulin can be a marker of

B2-microglobulin

its presence denotes proximal tubule damage, as in acute kidney injury—incomplete reabsorption

• impaired clearance by the kidneys

• protein overproduction

elevated plasma B2-microglobulin is a result of

radioisotopes

determine glomerular filtration through the plasma disappearance of injected radioactive material such as 125I-iothalamate

radioisotopes

enables visualization of filtration in one or both kidneys

radioisotopes

valuable to measure viability of transplanted kidney

120 mL/min

• men: 107-139 mL/min

• 87-107 mL/min

normal creatinine clearance values

other day / day 1 (first morning urine)

in creatinine clearance sample preparation, if the collection starts in the morning, urine formation from ___ is not included

maintained a 4 deg C

sample storage and transport for 24 hr urine

1. cockgroft and gault — WAGS: weight (kg), age, gender (sex), serum creatinine

2. modification of diet in renal disease (MDRD) system — GARBAGeS: gender (sex), age, race, BUN, serum albumin, serum creatinine

formulas used for estimated glomerular filtration rate

cockgroft and gault

most frequently used formula for estimated glomerular filtration rate

tubular reabsorption

first function affected in renal disease

1. SG

2. osmolarity

tubular reabsorption tests / concentration tests

specific gravity

tubular reabsorption test that depends on number and density of particles present in the solution

urea

contributes to SG than sodium chloride

1.025, 16hr fluid deprived

normal value of SG for tubular reabsorption / concentration test

osmolarity

tubular reabsorption test that depends on number of particles only

osmolarity

sodium = chloride = urea

≥ 800 mOsm, overnight water deprived

normal value of osmolarity for tubular reabsorption / concentration test

1. Fishberg test

2. Mosenthal

tests based on specific gravity

Fishberg test

patients were deprived of fluids for 24 hrs prior to measuring SG

Mosenthal

compares the volume and SG of day and night urine samples

osmolality test — osmometers

measurement of freezing-point depression

osmometer

converts freezing point drops → mOsm/kg

1.86ºC

1 mol (1000mOsm) of non-ionizing substance in 1kg water → lower freezing point by

tubular secretion / renal blood flow tests

closely related in that total renal blood flow through the nephron

substance that is secreted rather then filtered through the glomerulus

how to test accurate tubular secretion / renal blood flow

1. phenolsulfonphthalein test

2. p-aminohippuric acid test

tubular secretion / renal blood flow tests

urinary ammonia and titratable acidity

used to assess kidney’s ability to excrete acid (H+)

• titratable acid

• hydrogen phosphate ions

• ammonium ions

a typical person excretes approx. 70 mEq/day of acid in the form:

renal tubular acidosis

inability to produce an acid urine in the presence of metabolic acidosis

metabolic acidosis → inability to produce acid urine

• proximal convoluted tubule

• distal convoluted tubule

renal tubular acidosis may result from:

• impaired tubular secretion of hydrogen ions associated with ___

• defects in ammonia secretion associated with ___

• urine pH

• titratable acidity

• urinary ammonia

can be used to determine renal tubular acidosis