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This is an algorithm for deciding whether a patient suffers from a specific disease or not:

A patient has rheumatoid arthritis if at least four out of the following seven symptoms are present:

  • Symmetric swelling (arthritis)
  • Rheumatoid nodules
  • The presence of rheumatoid factor (in serum)
  • ...

There are similar classifications for many conditions. I remember seeing a specific term used for this kind of definition. It was fairly technical and could be unique to the fields of medicine or medical informatics.

What is this definition type called?

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3 Answers

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The word was polythetic, as opposed to monothetic.

(of a class of things) having many, but not all properties in common (wiktionary)

A polythetic class is defined in terms of a broad set of criteria that are neither necessary nor sufficient. Each member of the category must possess a certain minimal number of defining characteristics, but none of the features has to be found in each member of the category. (iva.dk)

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Wow, "fairly technical" is right! – Gnawme Oct 27 '11 at 21:45
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They're probably normally called diagnostic criteria, individually and collectively.

Any one symptom associated with a medical condition may also be called an indicator, but that has undesirable resonance with contra-indications (reasons why you might not adopt some particular treatment in some particular case).

I think most healthcare professionals wouldn't particularly want to encourage the use of a specialist term for 'Tick box' medicine, since they see this as something fundamentally undesirable.

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The word I'm looking for specifies that the diagnostic criteria require at least a specific fraction of the symptoms to be present, as opposed to all of them being present. – Tim N Oct 27 '11 at 17:06
I'll add that this is quite a common way for diagnosing certain "fuzzy" conditions. The DSM-IV lists three major symptoms of depression, two of which should be present for the diagnosis to be given. Of course there'd be an overall judgement, too -- I'll edit my question if it seems a bit too crass. – Tim N Oct 27 '11 at 17:16
Yes, I'm familiar with the methodology. But say there's some particular disease with a well-known list of 10 "most common" indicators, and someone tries to promote the idea that if you have 6 of them you have the disease, but with 5 you don't. I think most doctors would poo-poo the idea of using such a "tick box/cutoff point" style of diagnosis - they'd rather trust their own judgement on a case-by-case approach. It's more relevant to governments - when deciding if you qualify for disability payments, for example. – FumbleFingers Oct 27 '11 at 17:20
And there are always people who take one of these "5 out of 10" lists, pick the 5 most general or vague criteria, and then claim that everyone or everything qualifies. Like hypocondriacs who conclude they have some rare disease because they met "#2 headaches, #5 trouble sleeping, and #7 excessive tiredness". Or conversely someone will say that diagnostic criteria were obviously invented by people with some ulterior motive because almost everyone in the world could meet them. – Jay Oct 27 '11 at 18:41
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These are called classification criteria, and are used in performing a differential diagnosis. For example, a differential diagnosis is performed to determine whether someone has myasthenia gravis.

For your specific example of rheumatoid arthritis (RA), I found this statement:

The standard and accepted means of defining RA is by use of classification criteria. Classification criteria enable the stratification of groups of individuals into those with and those without RA... [italics mine]

(The classification criteria for RA were revised in 2010; the methodology makes fascinating reading.)

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The example presented is of a simple diagnosis (patient has RA vs. patient does not have RA), not a differential diagnosis (patient has RA vs. patient has some other condition). – Lord Torgamus Oct 27 '11 at 18:43
@LordTorgamus Are you referring to the OP's example? Perhaps I misread it, but I thought the OP was asking what the term is when using some set of symptoms to differentiate conditions. – Gnawme Oct 27 '11 at 18:52
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@Gnawme: The term might apply in both cases, but where I saw it, it was used to differentiate between having RA and not having RA. Classification criteria might be what I'm looking for, although it doesn't immediately ring a bell. – Tim N Oct 27 '11 at 18:56
FYI, read the paper at the link referenced in my answer. The determination of whether RA is present is much more fine-grained than in your example. (The decision tree shown in the paper is fairly involved.) – Gnawme Oct 27 '11 at 19:05
I can't seem to find a definition for classification criteria. Are you certain that it doesn't apply to a diagnostic rule like "you have scarlet fever if and only if you have a red tongue"? – Tim N Oct 27 '11 at 19:06
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