Learning Unit 2: SUPPLEMENT TO CHAPTER 3

 

OSMOMETRY

 

This supplement to Chapter 3 will provide information on laboratory instrumentation that is used to measure serum or urine osmolality.  The instrument used to do this is an osmometer.  The science of measuring osmolality is called osmometry.  This supplement will help answer the following questions:

 

 

1.      What is osmolality?

2.      What is the clinical significance of measuring osmolality?

3.      What is an osmometer?

4.      What are the principle components of an osmometer?

5.      How does an osmometer work?

 

 

1)      What is osmolality?

 

First, we must define osmosis.  Osmosis is the process of moving water across a semipermeable membrane ( the cell wall) until there is an equal concentration of fluid  on both sides of the membrane.  This is an essential process of moving nutrients, drugs ,antibiotics, etc. into a body cell as well as allowing waste materials to pass out from the interior of the cell.  This process keeps the cell function in balance and allows it to function properly.  If an imbalance occurs, cell function may become impaired or non-functional.  This creates a disease state in the patient.

 

Osmolality is the measurement of the concentration of particles in solution.  The “solution” is a clinical fluid such as serum or urine.  The “particles” are such things as electrolytes (sodium, potassium, calcium, iron, etc.), glucose, drugs, antibiotics, and other biologic materials.  In other words, we are looking at the amount of particles (chemicals) in serum or urine. 

 

 

2)      What is the clinical significance of measuring osmolality?

 

Serum osmolality is controlled partly by a hormone called antidiuretic hormone (ADH), also called vasopressin.  ADH is produced in the brain and is released into the blood.  If a person does not drink enough water (dehydration), the concentration of chemicals in the blood or urine (osmolality) increases because water constantly leaves the body when a person breathes, sweats, or urinates.   In response to an increase in serum osmolality, the brain releases more ADH in order to reduce the amount of urine produced by the kidneys.  This helps prevent the osmolality from increasing even further and causing serious illness from dehydration. 

 

When a person drinks too much water, the opposite process occurs and the kidneys release more water into urine.

3)      What is an osmometer?

 

The osmometer is a laboratory instrument that measures the osmolality of a solution---usually a body fluid such as serum or urine. 

 

Science tells us that there are four measurable properties of solutions that can tell us about the osmolality.  These four properties are:

                       

·        Osmotic pressure

·        Vapor pressure depression

·        Elevation of boiling point

·        Freezing point depression

 

Virtually all osmometers in clinical laboratories determine osmolality by measuring freezing point depression.

 

 

4)      What are the principle components of an osmometer?

 

·        A thermostatically controlled cooling bath capable of accurately maintaining sub-freezing temperatures.

·        A rapid stir mechanism.

·        A thermistor probe connected to a Wheatstone bridge circuit to measure the temperature of the clinical sample.  The usual thermistor is a glass bead attached to a metal stem whose resistance varies rapidly and predictably with temperature.

·        A galvanometer which displays the freezing curve which is used as a guide when the measuring potentiometer is used.

·        A measuring potentiometer (variable resistor) is used to null the current in the Wheatstone bridge circuit.

·        NOTE:  The galvanometer and measuring potentiometer are present in older instruments.  New instruments combine these components in a thermistor readout circuit that results in an LED or similar display on the instrument panel.

 

 

The following illustration shows a diagram of the chief components of an osmometer.

 

 

 

The following shows a graph of the freezing point depression that is translated into a measurement of osmolality.

 

 

5.       How does an osmometer work?

 

The patient’s specimen is poured into an analysis vial and the thermistor probe and stirring wire are lowered into the center of the analysis vial.  This assembly is then lowered into the freezing bath and with gentle stirring, is supercooled to a temperature several degrees below the freezing point.

 

When the readout indicates sufficient supercooling has occurred, the sample is raised to a point above the liquid in the freezing bath.  The wire stirrer is changes from a gentle rate of stir to a timed (usually one second) vigorous rate which in turn initiates freezing the supercooled solution in the sample vial.  The patient sample is frozen only to the “slush” stage.  The galvanometer reverses direction, as the solution warms to equilibrium temperature.  During this equilibrium phase of about 2-3 minutes, the balancing potentiometer is  adjusted, thus changing the variable resistance of the Wheatstone bridge to bring the galvanometer to the null position.  At the end of the temperature equilibrium plateau, the galvanometer again indicates decreasing temperature as the sample freezes further toward a complete solid.  The balancing potentiometer readings can be related to the degree of freezing point depression.  This data is used in the electronic calculation of the osmolality of the patient’s specimen.

 

The timing phases of the analysis are strictly controlled automatically by the instrument’s timing circuitry.   

 

 

 

References

 

1)      Tietz, N.W., Freier, E.F.  Textbook of Clinical Chemistry. pp. 129-135.  WB Saunders  1996.

 

2)      Science University of Technology, Sydney.  No. 91326 Analytical Biochemistry. 2000

 

3)      Advanced Instruments Technical Bulletin; Norwood MA  2000

 

4)      WebMD.  An internet service. (adam.com) 1999

 

 

 

 

Supplement to Chapter 3

Study Questions

 

1)      What is osmosis ?

 

 

 

 

 

2)       What is osmolality?

 

 

 

 

 

3)      What is an osmometer?

 

 

 

 

 

4)      Of the four properties of chemical solutions, which is the most commonly used to determine osmolality?

 

 

 

 

 

5)      What are the main components of an osmometer?

 

Supplement to Chapter 3

Study Questions & Answers

 

1)      What is osmosis ?

 

Osmosis is the process of moving water across a semipermeable membrane until there is an equal concentration of fluid on both sides of the membrane.  Osmosis moves chemicals in and out of body cells through the semipermeable membrane of the cell wall.

 

 

2)      What is osmolality?

 

Osmolality is the measurement of particles (chemicals) in solution.

 

 

3)      What is an osmometer?

 

An osmometer is a clinical laboratory instrument that measures osmolality in serum or urine.

 

 

4)      Of the four properties of chemical solutions, which is the most commonly used to determine osmolality?

 

Freezing point depression.

 

5)      What are the main components of an osmometer?

 

1)      A thermostatically controlled cooling (freezing) bath.

2)      A rapid stir mechanism.

3)      A thermistor probe.

4)      A galvanometer

5)      A measuring potentiometer.

NOTE:  # 4 and 5 are commonly combined in a thermistor readout circuit in newer osmometers.