Anemia is associated with pica (eating non-food substances) in cats. Anemia is very common in chronic renal failure since erythropoeitin, which stimulates red blood cell production, is made in the kidney and is decreased in renal failure.
MOst kitty litter products contain bentonite, which is toxic when eaten and causes anemia, low blood potassium, lethargy, muscle weakness and ataxia (loss of coordination). Bentonite may also inhibit iron absorption.
Blood work to confirm anemia and potassium levels would help determine if either iron or potassium should be supplemented. Erythropoeitin injections are quite expensive, but do a good job of increasing the red cells.
Change kitty litter to one of the paper litter products to eliminate the consumption of bentonite might be beneficial.
At home administration of subcutaneous fluids is beneficial to cats in renal failure. It helps get rid of toxic by products of normal metabolism. Some cats tolerate this well and others hate it. If they really fight it the benefits are not worth the decreased quality of life.
Let me know if you have follow up questions.