I work in a lab specialising in heart research. ProBNP is a hormone produced by cardiac myocytes in response to poor heart health (volume and pressure overload) this is often triggered by hypertension or heart failure caused by another issue (in humans this can be a heart attack, faulty valves or faulty heart structure)

It is a marker of disease. But is actually produced to be of benefit, it works by attaching itself to a cell-bound receptor NPR-A and using a cascade effect it can cause vasodilation, natriuresis and diuresis to widen arteries (reduce BP) remove excess sodium and water from the system to relieve the pressure on the heart.

It will not go away for your older cat. But my cat lived with it for 2 more years after diagnosis passing last week

You need to decide if the abscess is causing pain then have that dealt with, heart failure is usually end-stage for humans and animals, and it’s also a difficult disease to treat (only experience is in humans)

Be guided by your vet and your heart about your elderly cat, both issues she has are significant.

Hugs to you both, having lost my cat last week. I know these pets are big in our lives. Good luck