Osometrey Flashcards
why measure OSMOLALITY
how do we measure
Screening test
WHY?
* A DETERMINANT OF OSMOTIC PRESSURE
* DETERMINES WATER DISTRIBUTION ACROSS CELL MEMBRANES
IT IS AN INDICATOR OF
* ELECTROLYTE IMBALANCE
* KIDNEY FUNCTION
* THE PRESENCE OF UNMEASURED SUBSTANCES IN THE PLASMA
SAMPLE
HOW?
* WE MEASURE OSMOLALITY INDIRECTLY BY MEASURING THE
INFLUENCE OF SOLUTE CONCENTRATION
* MEASURE ANY COLLIGATIVE PROPERTY, NOT THE CONCENTRATION
* EFFECT IS PROPORTIONAL TO THE OSMOLALITY
OSMOLALITY what is it
-TOTAL CONCENTRATION OF SOLUTE PARTICLES
MOLARITY = MOL/L OF SOLUTION -not useful expression
MOLALITY = MOL/KG OF SOLVENT (SI UNIT) - BETTER
◦ WANT THE SCALE TO REFLECT THE NUMBER OF OSMOTICALLY ACTIVE PARTICLES
LIQUIDS EXPAND WHEN HEATED
* COLLIGATIVE PROPERTIES ARE MEASURED AT DIFFERENT TEMPERATURES
* WE NEED A SCALE THAT IS INDEPENDENT OF TEMPERATURE AND PRESSURE CHANGES; HENCE, MOLALITY IS BASED ON WEIGHT OF SOLVENT, NOT VOLUME
OSMOLE
UNIT OF OSMOTIC PRESSURE
*GLUCOSE DOES NOT DISSOCIATE
* 1 GMW OF GLUCOSE DISSOLVED IN 1 KG SOLVENT PRODUCES 1 MOLE OF PARTICLES
* HAS A PRESSURE OF 1 OSMOLE
- NACL DISSOCIATES INTO 2 OSMOTICALLY ACTIVE PARTICLES
- 1 GMW OF NACL DISSOLVED IN 1 KG SOLVENT PRODUCES 2 MOLES OF PARTICLES
- HAS A PRESSURE OF 2 OSMOLES
OSMOLALITY = MOL x NUMBER OF PARTICLES/MOLECULE = OSMOL/KG
* DISSOCIABLE SOLUTES OR “OSMOTICALLY ACTIVE PARTICLES”*******
how to measure osmololity OSMOMETRY
A MEASUREMENT OF OSMOTIC PRESSURE - FUNCTION OF SOLUTE CONCENTRATION
THE MEASUREMENT OF A COLLIGATIVE PROPERTY OF A SOLUTION IN WHICH THE NUMBER OF MOLES OF SOLUTE PER UNIT VOLUME
(CONCENTRATION) IS DETERMINED
◦ MEASURES THE CONCENTRATION OF ALL DISSOLVED PARTICLES IN
SOLUTION, NOT THE CONCENTRATION OF A PARTICULAR
MOLECULE
◦ MEASURES THE NUMBER OF OSMOTICALLY ACTIVE PARTICLES IN
SOLUTION
COLLIGATIVE
PROPERTIES
- affected by SOLUTE CONCENTRATION
- DETERMINED ONLY BY THE NUMBER OF PARTICLES IN SOLUTION, AND NOT BY THEIR NATURE
- NUMBER OF PARTICLES VS. NATURE
(OSMOTICALLY ACTIVE ) OF PARTICLES - OTHER PROPERTIES OF SOLUTIONS DEPEND ON SOLUTE
CONCENTRATION (OR THE NUMBER OF PARTICLES IN SOLUTION), BUT ALSO ON THE NATURE OF THE SOLUTE
(DENSITY, ABSORBANCE, VISCOSITY, ETC.), AND SO ARE NOT COLLIGATIVE
collative properties measurement
1. BOILING-POINT (BP) ELEVATION
2. OSMOTIC-PRESSURE (OP) ELEVATION
3. VAPOUR-PRESSURE (VP) DEPRESSION **
4. FREEZING-POINT (FP) DEPRESSION
increased SOLUTE PARTICLES = increased BP
increased OP
decreased VP
decreased FP**
BOILING-POINT (BP) ELEVATION
COLLIGATIVE PROPERTIES
-eg car antifreeze
PARTICLES IN COOLANT IN AUTOMOBILE RADIATORS
RAISES TEMPERATURE AT WHICH SOLUTION BOILS
PREVENTS BOILING OVER IN WARM WEATHER
*will not freeze even at freezing temps
NOT RECOMMENDED FOR
BIOLOGICAL SAMPLES.
OSMOTIC-PRESSURE (OP)
ELEVATION
COLLIGATIVE PROPERTIES
◦ OSMOTIC MOVEMENT OF WATER ACROSS A SEMIPERMEABLE
MEMBRANE
◦ WATER BALANCE, DISTRIBUTION BETWEEN FLUID COMPARTMENTS
-low to high solute movement - measuring pressure
COLLOID OSMOTIC PRESSURE (COP)
*used to COP of high molecular weight solutes that are not diffusible across the membrane - looking for protein content aka Alb
* OSMOTIC PRESSURE ELEVATION IS SLOW TO EQUILIBRATE, LARGE VOLUME REQUIRED, MEMBRANE
BEHAVIOR NOT REPRODUCIBLE
* USED ONLY IN ESTIMATES OF
COLLOID (PROTEIN) CONTENT,
called COLLOIDAL OSMOTIC PRESSURE (COP)
* SAMPLE SIZE 350 UL (WHOLE
BLOOD)
SALINE IS SEPARATED FROM PATIENT SAMPLE BY A SEMIPERMEABLE MEMBRANE WITH CONTROLLED PORE SIZE
MEMBRANE: PERMEABLE TO SMALL MOLECULES (CRYSTALLOIDS) YET IMPERMEABLE TO LARGE MOLECULES (PRIMARILY PROTEINS, COLLOIDS)
COP measures PROTEIN MOLECULES ONLY
WATER MOVES TO BALANCE OSMOTIC PRESSURE
BACK-PRESSURE (NEGATIVE PRESSURE) STOPS FURTHER FLOW
RESULTS REPORTED IN UNITS OF PRESSURE (MM HG)
USED TO DETERMINE POTENTIAL FOR PULMONARY EDEMA
3) VAPOUR-PRESSURE (VP) DEPRESSION
- VOLATILES ARE CHARACTERIZED BY THEIR EASE OF EVAPORATION
- PARTICLES DECREASE THE AMOUNT OF EVAPORATION - SALT IN OCEANS
- VAPOUR PRESSURE IS THE PRESSURE LIQUID SOLVENT IS IN EQUILIBRIUM WITH WATER VAPOUR
- VAPOUR PRESSURE DEPRESSION IS MOST ACCURATE BUT SLOW,
REQUIRES PRECISE TEMPERATURE CONTROL - VP IS LOWERED BY 0.3 MM HG PER MOLE OF SOLUTE.
not used for legal
VAPOUR-PRESSURE DEPRESSION
OSMOMETRY
-NON-VOLATILE SOLUTES REDUCE
THE ABILITY OF THE SURFACE
SOLVENT MOLECULES TO ESCAPE
THE LIQUID. Lowering VAPOR PRESSURE depending on the amount of solute
VAPOR PRESSURE IS PROPORTIONAL TO PURITY OR THE NUMBER OF PARTICLES IN SOLUTION.
- MEASURE THE EXTENT TO WHICH THE DEWPOINT IS DEPRESSED TO
DETERMINE THE NUMBER OF PARTICLES IN SOLUTION (OSMOLALITY)
-specialized pipette used . when the sample is introducted at RT it will cool it down and rise the temps
dewpoint - temperature at which saturation is reached
4) FREEZING-POINT (FP) DEPRESSION*****
- EXAMPLE: ROAD SALT
- PARTICLES DECREASE THE TEMPERATURE AT WHICH SOLUTION FREEZES
- LESSENS ICE FORMATION
- FREEZING POINT DEPRESSION IS CONVENIENT RAPID, REQUIRES ONLY SMALL SAMPLE VOLUMES
- FAVORED METHOD
WHAT HAPPENS, FP DEPRESSION?
The actual temperature at which freezing takes place is measured at this plateau
FREEZING-POINT DEPRESSION OSMOMETRY
CRYOSCOPIC METHOD
- MEASURE THE TEMPERATURE AT WHICH SPECIMEN FREEZES TO
DETERMINE THE NUMBER OF PARTICLES IN SOLUTION (OSMOLALITY) - THE CRYOSCOPIC OSMOMETER
-the wire cools the solution down past the freezing point but stirs and forms a slush which then slowly raises the temp to its freezing point
-decrease in temperature is proportional to the particles present
-delta temperature is proportional to the osmolality value
-therefore freezing point is proportional to osmo
KNOW THE FORMULA
CRYOSCOPIC OSMOMETRY SPECIMENS
- SERUM OR HEPARINIZED PLASMA
- AVOID (EDTA, CITRATE)
- MAY GIVE FALSELY ELEVATED VALUES
- ANTICOAGULANT SALTS MAY DISSOCIATE INTO OSMOTICALLY ACTIVE PARTICLES
- URINE
- RANDOM OR TIMED
- CLEAN CONTAINER
- NO PRESERVATIVE REQUIRED
- FRESH LIQUID STOOL
SOURCES OF ERROR
- STANDARD PREPARATION AND STORAGE
- SAMPLE STORAGE
- PARTICULATE MATTER IN SPECIMENS
- CONTAMINATION
- SAMPLE TEMPERATURE
- SAMPLE VISCOSITY- high protein high lipid
- INADEQUATE INSTRUMENT MAINTENANCe
CALCULATION OF OSMOLALITY
- ALL DISSOLVED PARTICLES IN PLASMA INCLUDING
ELECTROLYTES, CARBOHYDRATES, VITAMINS, WASTE PRODUCTS,
HORMONES AND DRUGS CONTRIBUTE TO OSMOLALITY - SODIUM CONTRIBUTES TO ROUGHLY HALF OF PLASMA OSMOLALITY
- CHLORIDE, GLUCOSE, UREA, PROTEINS AND ETHYL ALCOHOL
(WHEN PRESENT) ARE THE MAJOR CONTRIBUTORS
A DISCREPANCY BETWEEN MEASURED AND CALCULATED IS CALLED THE OSMOLAL GAP -ESTIMATE OF THE SERUM OSMOLALITY
-measured minus calculated
- AVERAGE OSMOLAL GAP IS NEAR ZERO (<10 MMOL/KG)
- INDICATES THE PRESENCE OF UNMEASURED SUBSTANCES
- VOLATILES SUCH AS ETHANOL, METHANOL, ISOPROPANOL, ETHYL ETHER
- “HIDDEN ANIONS” SUCH AS LACTATE OR “KETONE BODIES” -keto diet
- ACETONE, ACETOACETATE, BHB
- MANNITOL, PROPYLENE GLYCOL
KNOW THE CALCULATION
