The prognosis and the diagnosis

Two words are often used as though they meant the same thing. A diagnosis says what the illness is. A prognosis says how it is likely to develop. Confusing them causes real trouble, because they carry different kinds of certainty.

A diagnosis can frequently be checked. There is a test, or a scan, or a pattern of symptoms that points clearly in one direction. A prognosis cannot be checked in the same way, because it is a statement about a future that has not happened. It is built from what happened to large numbers of other people with a similar condition, and those people were not this person.

This is why careful doctors give prognoses as ranges rather than dates, and why patients so often remember a single number that was never said. A sentence such as 'most people in this situation improve within three months' gets stored as 'three months', and when the fourth month arrives the patient believes something has gone wrong.

The same confusion appears outside medicine. A school tells a family that students with this reading level usually catch up by the end of the year. That is a prognosis about a group, offered to one child, and it will be remembered as a promise.

The honest way to give such information is to say plainly which kind of statement it is: this is what we have found, and this is what tends to happen next, and the second sentence is much less certain than the first.