13 is certainly a very low PCV. Severe anemia can have many different causes. The two main types of anemias are a regenerative anemia (in which the bone marrow is still working to create new red blood cells), or a nonregenerative anemia (the bone marrow is not creating new red blood cells).
If your cat has not yet had a reticulocyte count done....then this should be done first. Mainly because if the anemia is regenerative, then we know that there really isn't anything wrong with the bone marrow.
The causes of regenerative anemia...are things that are causing red blood cell destruction. They include the following:
Immune-medited hemolytic anemia
Lead poisoning
Infectious diseases that attack red blood cells (Hemobartonella)
Oxidant injury (chronic ingestion of oonions or garlic, tylenol, zinc (usually in the form of a swallowed penny), moth balls, etc...)
Heartworm disease
Vasculitis
Blood loss inside the body from trauma
Bleeding cancers
Rodenticide poisoning (warfarin)
Bloodsucking parasites (fleas, ticks, and hookworms)
I know that this last one may sound crazy to you, but I have actually seen cats so riddled with either fleas or hookworms (or both) that it has made them dangerously anemic.
A nonregenerative anemia would be a much different set of causes. The list would include:
Chronic renal failure (your cat should certainly have other changes on the blood and urine panels if this were the case).
Chronic liver disease (same)
Aplastic anemia
or Pancytopenia (caused by proliferative cancers that invade the bone marrow)
With a HCT as low as 13....a blood transfusion is certainly in order. I recommend that you ask your vet for a referral to a specialist in veterinary internal medicine. Most specialists are located at referral centers or colleges of veterinary medicine. The benefit of seeing a specialist, is that you are more likely to get a quick and accurate diagnosis of your animal's problem, and thus a faster resolution of the symptoms. It is also more likely that a specialist has seen this problem before and treated it successfully.
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