Phil help, please! anemia with severely low hematocrit

Not just Phil, but anyone who might have experience with this type of
problem in cats....

My brother just sent me the following email:

--begin paste--
Shiro is in the hospital. He has severe anemia, extremely low
hematocrit levels. We've pumped almost $2000 in tests but so far the
tests - liver, kidney, viral, radiological views - all show a healthy
cat. The vet is puzzled as to what is causing it. Our toxics are all
stored well above his (and Tessa's reach). No one has dropped any
acetaminophen on the floor lately (ever) and besides, he wouldn't eat
it anyways. I'm afraid we may be nearing the end for Tessa's first
best friend L. Our next step is to run to a marrow biopsy...
--end paste--

Does anyone have an idea what might be wrong? Shiro is also an obese
cat (last I heard, more than 20lbs, but he may have lost some weight
recently), if that might influence anything.

I can't find anything on the internet--pretty much everything about
low hematocrit levels points to anemia, although I did find one source
regarding humans that had it related to a hemorrhage. I would just
rather my brother and his wife go into the vet's office armed with any
information they can get, rather than have the vet blindly run test
after test. Shiro is one of the sweetest cats in the world (although
his sleeping on my head/neck did send me into physio for three months
several years ago...), and I want him to get better!

Any help will be much appreciated!