There are two possibilities here - that the kidneys are damaged to the
point that they are not reabsorbing glucose, or more likely, that one of
the tests (urine or blood) is wrong.
 
The kidney has the ability to reabsorb all glucose presented to it up
to about 180 g/dl - that's 1.5x normal.  Glucose tests in the urine are
negative until they go above this threshold.  Now if the kidney is in
chronic renal failure, it will not have enough functional tissue to reclaim
all of the glucose normally presented.  But usually, all of the other
values would be seriously elevated, and other signs of renal disease would
be evident.
 
So the most likely cause is that either the blood test is wrong, or the
urine test is wrong.  If the glucose is elevated, and the urine test is
right, that means that the blood is not likely being processed fast
enough - blood that sits will use gluose as an energy source as long as
possible.  I don't know of a way for a glucose reading to be artificially
increased if a urine dipstick is used, but if properly done, I always
trust a blood test as more reliable than a urine test.  Urine sticks are
made to read glucose only in vary broad strokes.
 
If it is helpful - usually ferrets that are spilling glucose urinate a
lot - the extra solute draws water with it into the urine.  This is not
very sensitive, but if his urination level is normal - I'd start by
questioning the validity of the urine test first.
 
With kindest regards,
Bruce Williams
[Posted in FML issue 3461]