Re: How do you test for (fat-soluble) vitamin deficiencies?

It is very astute of you to know that there is a difference between having 
a deficiency of a vitamin and having symptoms of that deficiency.  There is
not a single test that tells if a person has a deficiency of a fat-soluble 
vitamin.  Instead the tests are specific for that vitamin.  For vitamin A, 
the usual test is to measure serum retinol (a form of vitamin A) levels 
(deficient levels are less than 10 micrograms/dl, marginal levels are 
10-20 micrograms/dl, and acceptable levels are above 20 micrograms/dl).  
For vitamin D, one of the more common tests is to measure serum levels of 
25-OH vitamin D (normal ranges are from 8 to 60 ng/ml).  There is not an 
accurate measure of vitamin E status (or deficiency).  Some of the tests 
that are used are the serum vitamin E to serum total lipids ratio (greater 
than 0.8 is normal), the erythrocyte hemolysis test (above 20% indicates a 
deficiency), and the erythrocyte malondialdehyde test (measures the 
peroxidation of PUFA of erythrocytes exposed to hydrogen peroxide).  
Vitamin K deficiency is also not simple to detect.  The usual measurement 
is prothrombin time (the length of time it takes for blood to clot)(normal 
is 11-13 seconds).  Other tests for vitamin K status are the ratio of 
des-gamma-carboxyglutamyl prothrombin to prothrombin in the plasma and the 
urinary excretion of gamma-carboxyglutamic acid.