APPENDIX
D
Special Application Notes
You must be aware of this phenomenon if you are to correctly inter-
pret results. By way of illustration, the table below depicts the
results of solution osmolality measurements made by both vapor
pressure and freezing point osmometers for varying amounts of
ethanol in human blood serum. Note that in the vapor pressure
instrument, concentrations of ethanol anywhere within the clinically
significant range do not appreciably affect the indication of osmolali-
ty. This is because the vapor pressure of a water-ethanol solution
does not change measurably with small concentrations of ethanol.
On the other hand, the freezing point osmometer tends to overesti-
mate the actual number of ethanol particles in the solution, as the
freezing point falls disproportionately with increasing amounts of
ethanol. Thus, neither instrument faithfully reports osmolality in the
case of water-ethanol mixtures. In clinical practice, the unique
response of the vapor pressure osmometer is usually an advantage
inasmuch as it allows the clinician or attending physician to monitor
the patient's serum metabolites (other than alcohol) independently
of the patient's blood alcohol level.
TABLE
ETHANOL IN HUMAN BLOOD SERUM
VAPOR PRESSURE VERSUS FREEZING
POINT OSMOLALITY DETERMINATION
(1) (2) (2) (2) Calculated Measured Measured
Serum Ethanol Ethanol Ethanol Total F.P. V.P.
Osmolality Added / kg Added / kg Osmolality Osmolality Osmolality
(mmol/kg) (µL) (mg) (mmol) (mmol/kg) (mmol/kg) (mmol/kg)
289 2500 1953 42 331 340 287
289 5000 3905 85 374 392 285
289 10000 7810 170 459 501 282
289 25000 19525 424 713 798 277
289 50000 39050 849 1138 1400 250
(out of cal.)
(1) Instruments gave identical results on serum alone.
(2) Assuming 100% ethanol, with a relative gravity of 0.78.
90
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