Question 1

What is the purpose for calculating an osmolal gap?
A)This serves as a quality-control measure of osmolality measurement.
B)This serves as an indication of abnormal concentrations of unmeasured substances in the blood stream.
C)This is the method utilized to calibrate the osmometer.
D)This removes the need for direct measurement of osmolality.

Accepted Answer

The osmolal gap allows for the comparison between the measured osmolality and the calculated osmolality.The benefit of this comparison is to identify the presence of abnormal concentrations of unmeasured substances in the patient sample.Frequently an elevated osmolal gap can be associated with the presence of some toxic material such as acetone in diabetics or ethanol in intoxicated individuals.

Question 2

Calculate the osmolal gap, given the following data: measured osmolality = 366 mOsm/kg; sodium = 144 mEq/L; glucose = 5.0 mmol/L; BUN = 9.0 mg/dL.
A)64 mOsm/kg
B)70 mOsm/kg
C)74 mOsm/kg
D)208 mOsm/kg

Accepted Answer

To determine the osmolal gap, the calculated osmolality must be determined first.In this calculation, the BUN must be divided by 2.8 to convert mg/dL to mmol/L.Glucose does not require this conversion.The calculated osmolality is 296 mOsm/kg.Osmolal gap is determined by: Osmolal gap = measured osmolality - calculated osmolality.Osmolal gap = 366 mOsm/kg - 296 mOsm/kg = 70 mOsm/kg

Question 3

In the equation to estimate osmolality by calculation, why are glucose and BUN divided by 18 and 2.8, respectively?
A)to provide an estimate of the contribution of glucose and BUN to the total number of particles of each
B)to convert each from mg/L to mmol/L
C)to account for the number of charges on each molecule
D)both a and b

Accepted Answer

Glucose and BUN are molecules that contribute to osmolality.The purpose for dividing glucose by 18 and BUN by 2.8 is to convert each from mg/L to mmol/L.Doing this provides an estimate of the contribution of glucose and BUN to the total number of particles in solution.

Question 4

Suppose a lab measured serum osmolality on the same patient sample by freezing-point and vapor-pressure methods, and the osmolality measurement by the freezing-point method was found to be greater than the osmolality measurement by the vapor-pressure method.Which of the following is the most likely explanation for this discrepancy?
A)The difference in the two methods will always result in significantly different osmolalities.
B)overhydration of the patient
C)technologist error
D)the presence of unexpected volatile solutes in the patient sample

Accepted Answer

The presence of unexpected volatile solutes is known to produce this type of discrepant result between these two methods.Vapor-pressure techniques will not detect the presence of all solutes, because volatile solutes will be measured as part of the solvent and not cause vapor-pressure depression.Ethanol is an example of a solute that will not be detected by vapor-pressure depression, because it will not suppress evaporation of solvent.Instead, because of the volatile nature of ethanol, it will add to vapor pressure produced by the solvent.Freezing-point methods will detect the presence of volatile materials like ethanol.Thus when volatile solutes are present, the two methods will produce different results.

Question 5

Given that a normal albumin is approximately 40 g/L, a normal glucose is approximately 900 mg/L, and a normal BUN is approximately 100 mg/L, why is albumin (which has a much higher concentration) not considered in the equation to estimate osmolality?
A)Albumin concentration is so much higher than other components of the blood stream, its effect on osmolality is considered constant and thus not included in the calculation.
B)When considering the molecular weight of albumin, the high concentration becomes a very small number of moles, causing albumin not to be included in the calculation.
C)Albumin controls the movement of fluids in the body and does not influence osmolality.
D)Albumin is a transport protein in the blood stream, and osmolality is only influenced by components that are being transported by albumin, not albumin itself.

Accepted Answer

Osmolality is a measure of the number of particles in a solution, not the concentration.The approximate normal level for albumin is 40g/L, which is equivalent to 0.62 mmol/L.Glucose, on the other hand, has an approximate normal level of 900 mg/L, which is equivalent to 5 mmol/L.Thus when it comes to the number of moles of albumin present, albumin does not contribute significantly to osmolality and is not included in the equation.

Question 6

Determine the calculated osmolality, given the following data: sodium = 140 mEq/L; glucose = 32 mmol/L; BUN = 30 mmol/L.
A)202 mOsm/kg
B)292 mOsm/kg
C)342 mOsm/kg
D)404 mOsm/kg

Accepted Answer

The equation to use for this calculation is:
mOsm/kg = 2•Na (mEq/L) + glucose (mmol/L) + BUN (mmol/L)
mOsm/kg = 2(140 mEq/L) + 32 mmol/L + 30 mmol/L
mOsm/kg = 342 mOsm/kg

Question 7

The two methods used in the clinical laboratory to determine osmolality are:
A)osmotic pressure and boiling point
B)boiling point and freezing point
C)vapor pressure and osmotic pressure
D)freezing point and vapor pressure

Accepted Answer

Freezing-point depression and vapor-pressure depression are the two methods utilized in the clinical laboratory to measure osmolality.Freezing-point depression is the most common.It is not possible to measure osmotic pressure directly, since there is no membrane that is permeable to water but impermeable to all solutes.Likewise, boiling-point elevation is not suitable for clinical laboratory use, because boiling would cause proteins to coagulate.

Question 8

As the osmolality of a sample increases, the osmotic pressure ______, the boiling point _____, the freezing point ______, and the vapor pressure ______.
A)increases, increases, decreases, decreases
B)increases, decreases, increases, decreases
C)decreases, increases, decreases, increases
D)decreases, decreases, increases, increases

Accepted Answer

As osmolality increases, the osmotic pressure increases, the boiling point increases, the freezing point decreases, and the vapor pressure decreases.

Question 9

The major clinical use for measuring urine osmolality is to evaluate ______, and the major clinical use for measuring colloid osmotic pressure is to detect conditions leading to ______.
A)the kidney's ability to dilute the urine, the kidney's ability to concentrate the urine
B)the kidney's ability to dilute the urine, pulmonary edema
C)the kidney's ability to concentrate the urine, the kidney's ability to dilute the urine
D)the kidney's ability to concentrate the urine, pulmonary edema

Accepted Answer

Urine that is delivered to the bladder is typically one to three times more concentrated than the blood stream.Measurement of urine osmolality will determine the ability of the kidney to concentrate the urine.The major use of colloid osmotic pressure measurements is to detect conditions leading to pulmonary edema.

Question 10

The major measured contributor to serum osmolality is:
A)sodium
B)glucose
C)BUN
D)protein

Accepted Answer

The major measured contributor to serum osmolality is sodium.

Question 11

Osmometers utilizing the vapor-pressure colligative property of solutions are based on what principle?
A)An increase in the concentration of dissolved solutes decreases the vapor pressure generated because of a lack of volatility of the typical solutes.
B)An increase in the concentration of dissolved solutes increases the vapor pressure generated because of a lack of volatility of the typical solutes.
C)An increase in the concentration of dissolved solutes decreases the vapor pressure generated because of an increased volatility of the typical solutes.
D)An increase in the concentration of dissolved solutes increases the vapor pressure generated because of an increased volatility of the typical solutes.

Accepted Answer

In vapor-pressure measurements, the pressure created by volatile molecules (such as solvent molecules) is measured.Generally a solute molecule is not volatile.As the concentration of dissolved solutes increases in a solution, there is a greater chance that solute molecules in the solution will be on the surface of the solution being tested.The increased numbers of nonvolatile molecules on the surface of the solution will decrease the amount of evaporation of the solvent.Thus vapor pressure from the solvent evaporating will be decreased, resulting in vapor-pressure depression.

Question 12

All of the following variables are needed to calculate a stool osmolal gap except:
A)stool glucose level
B)stool sodium level
C)stool potassium level
D)measured stool osmolality

Accepted Answer

Stool osmolal gap is determined by the following equation: stool osmolal gap = measured stool osmolality - 2([Na] + [K]).From the equation, the stool glucose level is not required for the calculation.

Question 13

Which of the following anticoagulants will contribute the least to plasma osmolality?
A)indoacetic acid
B)sodium fluoride-potassium oxalate
C)heparin
D)EDTA

Accepted Answer

Heparin works by activating antithrombin III, which inhibits thrombin and factor Xa, crucial in the blood coagulation cascade. It does not contribute significantly to plasma osmolality because it does not dissociate into ions in solution, unlike the other options which involve salts or compounds that dissociate and can affect osmolality.

Question 14

Osmometers utilizing the freezing point colligative property of solutions are based on what principle?
A)1 osmole of nonionizing substance dissolved in 1 kilogram of water raises the freezing point 1.86°C.
B)1 osmole of nonionizing substance dissolved in 1 kilogram of water lowers the freezing point 1.86°C.
C)Increased solute concentration will raise the freezing point of water in direct proportion to a NaCl standard.
D)Deceased solution concentration will lower the freezing point of water in direct proportion to a NaCl standard.

Accepted Answer

The freezing point temperature of a solution is decreased 1.86°C for each mole of particles in the solution.Based on this relationship, the freezing point determined by an osmometer can be readily converted to osmolality.

Question 15

Osmolality measurements depend on:
A)the molecular weight of particles in solution
B)the kind of particles in solution
C)the number of particles in solution
D)all of the above

Accepted Answer

Osmolality is a measurement of the total particle concentration in a solution.This is a measurement of the concentration not of a single particle, but of all particles in a solution.

Question 16

In the equation to estimate osmolality by calculation, why is the sodium multiplied by two? A)Sodium is present in body fluids in twice the concentration of other ions. B)Sodium has a valence of two. C)The equation accounts for sodium and its corresponding anion. D)This factor accounts for the fact that sodium may be bound to chloride (NaCl) or to bicarbonate (NaHCO₃).

Accepted Answer

Recall that osmolality depends on the number of particles in solution.Thus 1 millimolal of NaCl is equivalent to 2 milliosmolal because NaCl will separate into sodium ions and chloride ions in solution.

Question 17

Determine the calculated osmolality given the following data: sodium = 135 mEq/L; glucose = 95 mg/dL; BUN = 10 mg/dL.
A)240 mOsm/kg
B)279 mOsm/kg
C)375 mOsm/kg
D)480 mOsm/kg

Accepted Answer

The equation to use for this calculation is:
mOsm/kg = 2•Na (mEq/L) + glucose (mg/dL)/18 + BUN (mg/dL)/2.8
mOsm/kg = 2(135 mEq/L) + 95 mg/dL/18 + 10 mg/dL/2.8
mOsm/kg = 279 mOsm/kg

Quiz 11: Measurement of Colligative Properties

What is the purpose for calculating an osmolal gap?

Calculate the osmolal gap, given the following data: measured osmolality = 366 mOsm/kg; sodium = 144 mEq/L; glucose = 5.0 mmol/L; BUN = 9.0 mg/dL.

In the equation to estimate osmolality by calculation, why are glucose and BUN divided by 18 and 2.8, respectively?

Given that a normal albumin is approximately 40 g/L, a normal glucose is approximately 900 mg/L, and a normal BUN is approximately 100 mg/L, why is albumin (which has a much higher concentration) not considered in the equation to estimate osmolality?

Determine the calculated osmolality, given the following data: sodium = 140 mEq/L; glucose = 32 mmol/L; BUN = 30 mmol/L.

The two methods used in the clinical laboratory to determine osmolality are:

As the osmolality of a sample increases, the osmotic pressure ____, the boiling point _, the freezing point , and the vapor pressure __.

The major clinical use for measuring urine osmolality is to evaluate , and the major clinical use for measuring colloid osmotic pressure is to detect conditions leading to .

The major measured contributor to serum osmolality is:

Osmometers utilizing the vapor-pressure colligative property of solutions are based on what principle?

All of the following variables are needed to calculate a stool osmolal gap except:

Which of the following anticoagulants will contribute the least to plasma osmolality?

Osmometers utilizing the freezing point colligative property of solutions are based on what principle?

Osmolality measurements depend on:

In the equation to estimate osmolality by calculation, why is the sodium multiplied by two?

Determine the calculated osmolality given the following data: sodium = 135 mEq/L; glucose = 95 mg/dL; BUN = 10 mg/dL.