http://www.afip.org/ferrets/Clin_Path/ClinPath.html
 
including:
>White blood cell counts in the ferret are often confusing to vets who
>are unfamiliar with this species.  While there is some variation between
>fitch and albinos, and males and females, the normal white count of the
>ferret is low, from 2500 to 7000/cm3.  This occasionally leads to an
>erroneous diagnosis of leukopenia.  When considering leukopenia in the
>ferret, look for elevated numbers of bands neutrophils in the peripheral
>blood.  Values for calculating a left shift in the dog are valid for
>that in a ferret.  Another finding that may support a diagnosis of
>leukopenia would be a concomitant anemia and/or decrease in platelets,
>such as might be seen in bone marrow suppression in estrogen toxicosis
>or the very rare case of adrenal-associated endocrinopathy.
[Posted in FML issue 5008]