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What is the difference between a disease (e.g. Covid-19) and a diagnosis (e.g. Covid-19)?

By HUW LLEWELYN, May 10 2021 06:07PM

A ‘disease’ is what a person experiences and what others may observe. Many of its features may be shared with other diseases and many may be hidden, especially in its early stages (e.g. as in Covid-19). It may therefore be therefore impossible to identify and attach a name to all those and only those people with a disease. In order to do this we would need to have an impossibly perfect test that can be performed on a perfect random sample or everyone in the population. The best that can be done therefore is to assume that someone has a disease based on limited information and to consider the consequences of that assumption. This is what is meant by a diagnosis. It is a hypothesis applied to an individual. If it is acted upon, then it is called a working diagnosis or working hypothesis.

Diagnosis and imagination

A diagnosis is therefore not simply a group of observable phenomena or the 'disease name' given to the uncomfortable experiences of an individual but also a process of recognizing the latter and its implications. The term is derived from the ancient Greek word of ‘diagignoskein’ (‘dia’ – between) ‘gignoskein’ (recognize). A ‘diagnosis’ is also the title to various tentative predictions or hypotheses. These predictions may be experienced mentally as imagining what may be happening now (such as a virus multiplying inside cells), imagining what may have happened in the past (such as inhaling virus laden droplets) and imagining what may happen in future with and without various interventions (e.g. death or survival after intensive care). It also involves imagining a degree of severity.

Decisions arising from a diagnosis

The initial diagnosis may suggest a variety of outcomes such as spread of disease to others, a mild illness with spontaneous recovery or severe illness requiring hospitalization and various treatments. In order to decide the best way forward, more information is usually needed in the form of immediate tests or waiting to see how things progress. This will include assessing the current degree of severity and its rate of change with or without various interventions. When considering treatments, the information required might be the presence of findings shown to predict that patients in a randomized control trial fared better on treatment than on a control such as a placebo.

Provisional and final diagnoses

If the progress of the illness differs from what is suggested by the diagnosis then another diagnosis might be considered and different treatments tried. This may happen many times during the diagnostic process. However, if there is a predicted or satisfactory outcome with no reason to question the diagnosis further, it becomes ‘final’. This does not mean it is confirmed or true. It simply means that it continues to be assumed to be ‘true’ and becomes a ‘theory’.

Pattern recognition

An experienced diagnostician may be do all this silently as a process of pattern recognition and a ‘gut feeling’ as to what should be done. This is also what usually happens during ordinary situations, predictions and decisions made by people in their day to day lives. It is a challenge to make some of these thought processes transparent especially the diagnostic thought process (see Chapter 1 of the Oxford Handbook of Clinical Diagnosis for more details (it can be accessed via this link).

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