What is the difference between lifetime risk, lifetime morbid risk, and lifetime prevalence, and lifetime cumulative incidence?

Question

Would appreciate any references to specific places where these terms are clearly defined. Thanks!

Answer 1

These terms describe various longitudinal measures of disease severity using units of time or occurrences in both the denominator and numerator of the quantity measured. Consider herpes as an example. Someone experiencing an outbreak of herpes once in their life contributes one event to the denominator of lifetime risk, regardless of the number of recurrences, or whether total outbreak time is a day or a year.

Morbid risk refers specifically to disease or unfavorable outcomes (aside from mortality) whereas some other risk outcomes might be favorable like adoption of a child.

Lifetime prevelance is the proportion of time someone spends in a specific outcome state. Returning to the herpes example, this measure would be different for an individual experiencing a one-day outbreak (defined as visible open sores) or a one-year outbreak. This has in the numerator total person-time in morbid state and in the denominator total-person time observed.

Cumulative incidence measures repeated binary outcomes like hemorrhagic strokes or stubbed toes. Number of events is in the denominator and person-time at risk is in the numerator.

Answer 2

Since you asked for a reference regarding the terms I use Porta's "A Dictionary of Epidemiology" when I need to look up epidemiological terms. I found him through one of Rothmans reference in his Epidemiology: An Introduction where he uses the Porta's definition of cohorts.

I don't think any of the books cover lifetime in any detail but I looked into Porta that I like. Unfortunately I can't recommend Rothmans introductory book but I think I've seen a lot of people praise his Modern Epidemiology but I haven't bought it... yet...

The following are from Porta

Lifetime risk

The risk to an individual that a given health effect will occur at any time after exposure without regard fro the time at which that effect occurs

Lifetime morbid risk

I couldn't find this as a definition. I would look into the different definitions and combine them but it seems like a strange measure to use when you have the lifetime incidence & prevalence. I guess as @Adam Omidpanah notes the key is that morbidity is a unfavorable event.

Lifetime prevalence

The proportion of individuals who have had the disease or condition for at least part of their lives at any time during their life-course.

$Prevalence = \frac{number\ of\ cases}{population\ at\ risk}$

Lifetime cumulative incidence

I couldn't find a clear definition for lifetime but in the cumulative incidence you can extract this definition:

The number or proportion of a group (cohort) of people who experience the onset of a health-related event during a life-course.

$Incidence\ rate = \frac{number\ of\ new\ events}{time*population\ at\ risk}$

When you have a lifetime the time is irrelevant but you may experience issues with events that can recur and therefore you might get a nonsense number. Usually you use the first event (ex. first episode of low-back pain) as your counter in these cases but it's much better to use event/person/time.