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Finally, prognostic factors arise not only from the web of causes of
principal disease outcomes but also from concurrent diseases or newly
appearing comorbid states (such as cross-infections, metabolic imbalances,
etc.). Indeed, all may act upon the outcome of the disease of main inter-
est and/or on any other comorbidity or additional new disease or health
problem.
Nonetheless, probabilistic estimations and analyses of exposure to risk
or to prognostic factors and their outcomes are still often merged and
confounded in current literature. 36,38 Let us always read with care.
3.4.2 What Do We Need to Know about Prognostic
Events and Outcomes?
A good or bad (qualitative) prognosis also requires quantification in terms of
probabilities. A probabilistic prognosis can be expressed in many different
ways, such as
By establishing a survival rate after 5 years of observation, as is usually
done in studies of cancer patients
By determining a case fatality rate at any moment (N.B. Mortality may
also be a measure of risk and prognosis since it is the product of inci-
dence density and case fatality rate) 7
By detecting a response rate to treatment at any moment of a follow-up
period (such as an improvement rate after the treatment of leukemia
patients)
- As a remission rate following a defined treatment (provided that a
remission period and its criteria are well defined)
- As a relapse rate (with the same conditions as above)
- By establishing a longitudinal picture of events during the natural or
clinical course of disease , known as a survival curve.
A survival curve is essentially a sequence of rates (proportions) of
events of interest in time. Originally, a proportion of patients (usually
cancer sufferers) surviving or dying at a given moment were the subject
of study. Today, the terms “survival” or “survival studies” are rather awk-
ward or misleading because, in addition to deaths, any event appearing
during the natural or clinical course of disease may be the subject of a
“survival” analysis: disease spells, recurrence, comorbid events, adverse
reactions, and so on.
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