Lab Tests for ECF Fluid Excess or Deficit:
An extracellular fluid (NaCl + water) excess causes the compartment
to become expanded. This most commonly occurs when kidneys are
not functioning properly as in renal disease or decreased renal
blood flow. No electrolyte test will indicate this condition.
The hematocrit value may be used to indicate extracellular
fluid excess and will be somewhat low.
(Hematocrit (HCT): Measures a volume percentage of
red blood cells in the plasma.
Normal value : Females = 37-47%; Males = 40=54%)
Why should the HCT value be low? If the normal HCT value is 40%
and the normal plasma fluid content is 3 liters, what is the
HCT % if the plasma fluid is now expanded to 3.3 liters?
First find the volume of red cells (40% of total plasma).
3 L x 40 / 100 = 1.7 L of red blood cells.
Find the new percentage of red blood cells at the increased
% HCT = 1.7 L / 3.3 L x 100 = 36.3% HCT
Therefore, a lower percent value for HCT means an INCREASE
in the fluid volume. Low %HCT means ECF excess.
A height %HCT means an ECF Deficit.