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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.
He should be able to wait until Monday. He should last, but if he doesn't being seen a day earlier most likely won't be the reason. The stress of emergency clinics is high and adversely affects the animal. It's a game of risk and benefit. From what you describe the risk is higher going to an emergency clinic than waiting to see the regular vet.
If he wants people food, let him have some. Chicken, eggs, a small amount of beef will provide high quality protein and will not not ruin his low protein diet. Cats require protein to metabolize glucose. It is one of the unique aspects of their metabolism. Eating something is better than nothing even if that means the diet isn't perfect.