Renal Physiology and Renal Function Tests (2) Flashcards
Measures the rate at which the kidneys can remove or clear a filterable substance from the blood
- Checks how fast the kidney removes a marker from the blood.
Clearance Test
Clearance test that requires a marker – should have specific characteristics;
- removal of a substance from plasma into urine over a fixed period,
- not bound to proteins,
- filtered, neither reabsorbed nor secreted
Markers of the clearance test
- Exogenous substance (administered)
- Endogenous substance (natural in the body)
High clearance test =
efficient excretion
Low clearance test =
inefficient excretion
Normal Value: Creatinine Clearance
in Female
75 - 112 ml/min
Normal Value: Creatinine Clearance in Male
85 - 125 ml/min
expected to have a higher muscle mass and higher body surface area why their normal values are higher
Male
Markers administered/ injected to patients
EXOGENOUS
- Reference method***
- Not routinely done because exogenous
- Soluble, freely filtered, neither secreted, nor reabsorbed
Inulin
127mL/min/1.73m2
Male inulin
118 ml/min/1.73m2
Female inulin
- 125I-iothalamate
- 99m Tc-DTPA
- Iohexol
- 51Cr-EDTA
Radioactive Markers
Creatinine: Ccrea
ENDOGENOUS
waste product of muscle metabolism that is produced enzymatically by creatine phosphokinase from creatine
- 113 Da
- Most widely used marker for GFR
- Produced constantly
- Not bound to proteins
- Not reabsorbed
- Minimally secreted
Creatinine
- Some creatinine is secreted by the tubules
- Chromogens in plasma react in the chemical analysis
- Medications inhibit tubular secretion
- Bacteria break down urinary creatinine
- A diet heavy in meat can raise creatinine
- Not a reliable indicator in patients suffering from musclewasting diseases
- Depends on the completeness of a 24-hour collection
- It must be corrected for Body Surface Area (unless normal)
CREATININE CLEARANCE: DISADVANTAGES
Methods that do not require the collection of timed (24-hour) urine specimens: (blood)
- Estimated glomerular filtration rate eGFR using serum creatinine
- Cystatin C
- Beta2-microglobulin
- Variables - age, sex, weight in kg
- Female - multiply answer by 0.85
- Females are expected to have a lower body mass
Cockcroft & Gault
Six variables: age, sex, serum urea nitrogen, serum creatinine, race, and serum albumin
Simplified MDRD based on four variables: serum creatinine, age, race, and sex
- Serum creatinine obtained with an enzymatic IDMStraceable assay (method to quantify serum creatinine)
- Correspond more closely to the isotope dilution mass spectrophotometry (IDMS)
Modification of Diet in Renal Disease (MDRD)
traceable formula- recommended by the
National Kidney Disease Education Program (NKDEP)
Modification of Diet in Renal Disease
MDRD-IDMS
IDMS
isotope dilution mass
spectrophotometry
MDRD
Modification of Diet in Renal Disease
CKD-EPI
Chronic Kidney Disease Epidemiology Collaboration
Estimates GFR from serum creatinine, age, sex, and race
CKD-EPI
- Major end product of protein and amino acid metabolism
- Freely filtered by the glomeruli but variably reabsorbed by the
tubules - Not preferred
UREA CLEARANCE: ENDOGENOUS
- For screening and monitoring GFR
- Small protein (M.W. = 13,359)
CYSTATIN C
- Produced at a constant rate by all nucleated cells
- Readily filtered by glomerulus
- Reabsorbed and broken down by the RTEs
- Not secreted by the tubules
- Serum concentration can be directly related to GFR
CYSTATIN C
- Molecular weight - 11,800
- Dissociates from human leukocyte antigens at a constant
rate - Rapidly removed from the plasma by GF
- Enzyme immunoassay
- Increased beta2- macroglobulin: decreased GFR
BETA2-MICROGLOBULIN
- Depends on the number of functioning nephrons and the functional capacity of these nephrons
GFR
- Determines the extent of nephron damage
- Monitors the effectiveness of treatment
- Determines the feasibility of administering medications
GFR Clinical Significance
- Most common test; popular
- Specific gravity: number and density
Specific Gravity
- Patients are deprived of fluids for 24 hrs prior to measuring the specific gravity; obsolete
Fishberg Test
- Compares the volume and specific gravity of day and night samples; obsolete
Mosenthal Test
Serum osmolality:
: 275 to 300 mOsm
number of particles in a solution
* Better than specific gravity as it is more specific
* Renal concentration is concerned with small particles
* Large MW molecules do not contribute to the evaluation of
renal concentration
Osmolality
INSTRUMENTS FOR MEASUREMENT OF OSMOLALITY
Freezing point Osmometers and Vapor Pressure Osmometers
Measurement of freezing point depression
Freezing point Osmometers
- No ability/ capacity to release ADH
- Unable to stimulate hypothalamus to release ADH; problem
with hormones
Neurogenic Diabetes Insipidus
- ADH is present but kidney is unresponsive to AD
Nephrogenic Diabetes Insipidus
To check Diabetes Insipidus:
* Deprive patient of water for
12 hrs
- Restrict fluid for another two hours
- Collect urine and serum for osmolality testing
If < 800 mOsm
If the osmolality normalizes then there is a _____
neurogenic
diabetes insipidus
If the osmolality decreases then there is a_____
nephrogenic diabetes insipidus
- Volume of blood plasma that is cleared of solute-free water per unit time
- To determine ability of the kidneys to respond to the state of body hydration
Free Water Clearance
indicates how much water must be
cleared each minute to produce a urine with the same osmolarity as the plasma
Osmolar clearance
