system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When a person is diagnosed with a terminal illness how does a doctor figure out how much time the patient has left?
gpt
Hospice RN here. We get asked this all the time.

Doctors and other medical personnel  can prognosticate (make an educated guess) about how long a patient has left, but it is not an exact science. We don't usually given a exact answer; we would tend to say "several months" or "several weeks" or "days to weeks" or "a few days" or the absolute worst "minutes to hours."

Here are a few things that affect that educated guess:

1. How advanced the disease is. Cancer only in the lungs isn't as bad as as cancer in the lungs, bones, brain, and liver. Other diseases like COPD and heart failure are evaluated on how bad the symptoms are and how frequently they happen. When the symptoms happen all the time at rest (no physical exertion.)

2.  The overall health of the person. If someone was a healthy marathon runner in their 50's diagnosed with the same terminal cancer as someone in their 80's with uncontrolled diabetes, high blood pressure, and general advanced age, guess who would live longer? The marathon runner is likely to live much longer.

3. Loss of functional status. How quickly is the patient physically declining? If someone was able to fully take care of themselves (eat, bathe, drive, pay bills) three months ago and now is bed bound and cannot perform any self care and is mentally out of it, they likely don't have long. If that same decline took two years, the  patient likely has more time because rate of decline is slower.

It's not an exact science and patients can surprise us. Sometimes the disease can cause sudden death - cancer patients are much more likely to throw a blood clot and die suddenly - cancer makes the blood hypercoagulable - basically more sticky. If a cancer eats into a major blood vessel or cuts off circulation to a major organ, it's lights out pretty quick. Likewise, heart failure makes the possibility of a heart attack much more likely. On the other hand, patients can live longer than expected - sometimes people's bodies are more resilient than we can quantify.

tl:dr - it's not an exact science but we evaluate the overall disease progression and health status of the patient.