What it is
Red blood cells carry oxygen around the body, and to do their job they need energy, which the enzyme pyruvate kinase helps them make. Without enough of it the cells weaken and break down faster than the body can replace them.
The anaemia can be mild and go unnoticed for a long time, or come in episodes when the cat is weaker and sleepier for a few days and then recovers. Sometimes the spleen enlarges, or the gums and the whites of the eyes turn yellow.
Because the signs come in waves, it is worth noting down when your cat seemed listless and for how long: notes like these help a vet build a picture.
Signs you may notice
A cat is listless, eats less, loses weight, and its gums look pale. Such spells can come and go.
When to see a vet
Book a visit if your cat becomes listless or eats less for no clear reason, and see a vet as soon as possible if its gums turn very pale or yellow. When getting a kitten, ask the breeder about the parents' health testing.
This is general information, not veterinary advice. For your own cat's health, tests and treatment, see a vet.