First, rule out any Medical issues
Litter box avoidance can have a number of causes, but first, you have to rule out any medical causes, keeping in mind that even once the medical condition that led a cat outside of their box is solved, the cat may have developed a habit of urinating outside the box, and this habit needs to be addressed separately with Litter box retraining.
Medical issues that can lead cats out of their boxes
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Urinary tract infection
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FLUTD (Feline Lower Urinary Tract Disorder, an umbrella term for many conditions that affect the bladder or urethra)
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Blockages/Bladder stones
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Feline Idiopathic Cystitis (abbrev. FIC, and a type of FLUTD)
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Endocrine disorders like Hyperthyroidism
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Medical conditions affecting nerves, muscles or joints, like arthritis or Cerebellar Hypoplasia
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Kidney disease or failure
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Neoplasia or tumor
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Anatomical abnormalities
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Any illness that stresses them out
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Remember, some issues may come and go, so even though your cat's lab work looks good one time, this doesn't mean there isn't a serious health issue that is leading her out of the box. Be your cat's advocate: do some research, ask questions, and keep in contact with your vet when your cat's health or behavior changes.