Tuxedo's blood work was normal in October, and he was near death in March. His heart was enlarged, indicating that the anemia had been slowly developing.
So IF YOUR CAT IS EATING LITTER, please take him/her FREQUENTLY to the vet to determine his/her red blood cell count! It was only a matter of five months with Tuxedo, so if we'd waited until his next annual exam, he'd have been dead. Watch for ANY changes in behavior, especially if you notice your cat is not as active as he or she was, however, by the time they're not doing much, the problem is probably already severe.
Anemia in cats can be easily treatable OR a serious potentially fatal problem - either way, the earlier the problem is caught, obviously the better.
I have since found many others that have had this problem.
So if your cat is eating litter, get him to a vet ASAP and watch those packed cell volumes closely even if there isn't a problem yet!!!!!!!!!!!!!!!!!!!!!!!