Imagine taking 100 ice cubes and measuring how long it took for each one to melt on an average day in an average setting. Some would melt a little faster than others and some would melt a little slower. There might an odd ball-out that took a really long time to melt for some reason, maybe the temperature was cold that day.
Average all of these together and apply them to an ice cube you haven’t seen melt yet and you would have a good guess. You can further refine it based on it’s size, how cold it is in the room based on previous observations, etc.
It’s all a best guess of averages based on those before you.
Based on a mixture of their assessment of the patient, and their prior experience/published data on the the stages and rate of progression for others who have the same illness.
The doctor will assess the patient, and using their knowledge make a judgement as to what ‘stage’ their terminal illness is at.
From there, using data on the outcome for other patients with the same illness they can make an estimation of how long the patient has left to live.
To make the estimate even more robust they will take into account other factors such as how quickly has the illness progressed in this patient vs. the ‘norm’ (is it slower, faster, pretty much typical); what other illnesses or risk factors does this particular patient have that may affect the ‘textbook’ timescales all of which can help refine the estimate.
(NB I am not a doctor/trained medical professional – hopefully one will be along soon to confirm or correct my understanding).
Doctors look at the survival rates of people with that condition. My dad had a cancerous brain tumor and the survival rate was only 40% within the first year, 17% by the end of the second year, and only 3% by the end of five years. Then doctors will make a prognosis based on the statical data combined with the patient’s overall health and how severe the condition is.
It’s a guess/estimate based on how long other people in the same condition have lasted in the past.
We’ve been collecting data from past cases for a long time now, so doctors can consider “Latino man in his 60’s, drinks moderately, doesn’t smoke, current situation is stage 2 prostate cancer” and find the average amount of time someone of that description has typically survived for.
Sometimes the estimate is very wrong. My uncle with cerebral palsy lived to be nearly 80 when doctors said he’d be dead before 20. Stephen Hawking outlived his diagnosis by decades too. Sometimes the doctor estimates 5 years and they’re dead in 6 weeks too. Everyone’s different. The estimate from doctors is just an estimate, based on people in similar situations. But health is complicated, and there are always factors in each case that make estimates inaccurate.
MD here. Based on experience… once you’ve seen hundred patients who have certain vitals and a certain assessment you can guess estimate. Like anything, once you’ve seen hundred if not thousands of the same process, you get good at recognizing patterns.
That being said, it’s only a guess. Whenever families ask me, I give a estimate but always qualify by saying I’ve been humbled because people I thought had hours lived >1 day and vice versa.
MD here.
There are 2 categories in my mind:
1. The acutely ill patient. Imagine someone who is very old, on a lot of medication to support their blood pressure, have a very bad infection and they are still in critical condition. There is not a lot left to be able to do and in these patients it is usually fairly accurate as in hours to day(s).
2. The chronically ill. Think cancer patient, neuromuscular disease, etc. You can estimate based on number of factors. Blood work, ability to eat/drink, # of ED visits in last several weeks, amount of pain meds needed, etc. Based on all these you can see a trend that is irreversible. These patients usually get the days to weeks to months diagnosis. Sometimes these category outlasts the estimate.
An experienced and attentive hospice nurse is the best guess-timator of time left. More accurate than doctors who do not usually attend the last moments. Hospice nurses also know how to make it as easy as possible for the patient.
They go by the recent past history of the patient (what is the rate of decline in the recent days/weeks, fast or slow), current symptoms and a lot of experience.
I’ve had some remarkably accurate guesses from hospice nurses. From months, weeks, days to hours. Even to ‘any moment now’.
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