urinalysis and body fluids

©2008 F. A. Davis
CHAPTER 2 • Renal Function 23
The osmolarity of a solution can be determined by
measuring a property that is mathematically related to the
number of particles in the solution (colligative property) and
comparing this value with the value obtained from the pure
solvent. Solute dissolved in solvent causes the following
changes in colligative properties: lower freezing point, higher
boiling point, increased osmotic pressure, and lower vapor
pressure.
Because water is the solvent in both urine and plasma,
the number of particles present in a sample can be determined
by comparing a colligative property value of the sample
with that of pure water. Clinical laboratory instruments
are available to measure freezing point depression and vapor
pressure depression.
Freezing Point Osmometers
Measurement of freezing point depression was the first principle
incorporated into clinical osmometers, and many instruments
employing this technique are available. These
osmometers determine the freezing point of a solution by
supercooling a measured amount of sample to approximately
27C. The supercooled sample is vibrated to produce crystallization
of water in the solution. The heat of fusion produced
by the crystallizing water temporarily raises the temperature
of the solution to its freezing point. A temperature-sensitive
probe measures this temperature increase, which corresponds
to the freezing point of the solution, and the information is
converted into milliosmoles. Conversion is made possible by
the fact that 1 mol (1000 mOsm) of a nonionizing substance
dissolved in 1 kg of water is known to lower the freezing
point 1.86C. Therefore, by comparing the freezing point
depression of an unknown solution with that of a known
molal solution, the osmolarity of the unknown solution can
be calculated. Clinical osmometers use solutions of known
NaCl concentration as their reference standards because a
solution of partially ionized substances is more representative
of urine and plasma composition.
Vapor Pressure Osmometers
The other instrument used in clinical osmometry is called the
vapor pressure osmometer. The actual measurement performed,
however, is that of the dew point (temperature at
which water vapor condenses to a liquid). The depression of
dew point temperature by solute parallels the decrease in
vapor pressure, thereby providing a measure of this colligative
property.
Samples are absorbed into small filter paper disks that
are placed in a sealed chamber containing a temperaturesensitive
thermocoupler. The sample evaporates in the chamber,
forming a vapor. When the temperature in the chamber
is lowered, water condenses in the chamber and on the thermocoupler.
The heat of condensation produced raises the
temperature of the thermocoupler to the dew point temperature.
This dew point temperature is proportional to the vapor
pressure from the evaporating sample. Temperatures are compared
with those of the NaCl standards and converted into
milliosmoles. The vapor pressure osmometer uses microsamples
of less than 0.01 mL; therefore, care must be taken to
prevent any evaporation of the sample prior to testing. Correlation
studies have shown more variation with vapor pressure
osmometers, stressing the necessity of careful technique.
Technical Factors
Factors to consider because of their influence on true
osmolarity readings include lipemic serum, lactic acid, and
volatile substances, such as ethanol, in the specimen. In
lipemic serum, the displacement of serum water by insoluble
lipids produces erroneous results with both vapor pressure
and freezing point osmometers. Falsely elevated values
owing to the formation of lactic acid also occur with both
methods if serum samples are not separated or refrigerated
within 20 minutes. Vapor pressure osmometers do not detect
the presence of volatile substances, such as alcohol, as they
become part of the solvent phase; however, measurements performed
on similar specimens using freezing point osmometers
will be elevated.
Clinical Significance
Major clinical uses of osmolarity include initially evaluating
renal concentrating ability, monitoring the course of renal
disease, monitoring fluid and electrolyte therapy, establishing
the differential diagnosis of hypernatremia and hyponatremia,
and evaluating the secretion of and renal response to
ADH. These evaluations may require determination of serum
in addition to urine osmolarity.
Normal serum osmolarity values are from 275 to
300 mOsm. Normal values for urine osmolarity are difficult to
establish, because factors such as fluid intake and exercise can
greatly influence the urine concentration. Values can range
from 50 to 1400 mOsm. 2 Determining the ratio of urine to
serum osmolarity can provide a more accurate evaluation.
Under normal random conditions, the ratio of urine to serum
osmolarity should be at least 1:1; after controlled fluid intake,
it should reach 3:1.
The ratio of urine to serum osmolarity, in conjunction
with procedures such as controlled fluid intake and injection
of ADH, is used to differentiate whether diabetes insipidus
is caused by decreased ADH production or inability of the
renal tubules to respond to ADH. Failure to achieve a ratio of
3:1 following injection of ADH indicates that the collecting
duct does not have functional ADH receptors. In contrast, if
concentration takes place following ADH injection, an inability
to produce adequate ADH is indicated. Tests to measure
the ADH concentration in plasma and urine directly are available
for difficult diagnostic cases. 10
Free Water Clearance
The ratio of urine to serum osmolarity can be further
expanded by performing the analyses using water deprivation
and a timed urine specimen and calculating the free water
clearance. The free water clearance is determined by first