The recent correspondence on both responsibilities for method performance
and request forms have a common thread.
Roger Ekins states that fitness for purpose is a responsibility of the
manufacturers. This is true provided we use a kit for that purpose for which
it has been produced and verified. Trevor Gray, however, points out, often
requests generated by juniors follow a 'blunderbuss' mentality.
Blunderbusses have a value, as does a rifle with telescopic sights but they
are very different. Both, nevertheless, use the same principle of metals
projectiles forced through a barrel by an explosive force. Manufacturers
cannot be expected to take responsibility when we use a sniper's rifle as a
blunderbuss.
How any test perfoms will depend upon where, within the decision tree, it is
applied - just as the effectiveness of a blunderbuss depends on population
density.
Personally I wonder if early use of the laboratory 'blunderbuss' might now
be more effective (both medically and financially) than 'history and
examination' (other than the basic 'Sex? Age? Complaining of?') in directing
early decision events.
It is an inevitable consequence of the nature of information that tests of
exclusion are more powerful than tests of inclusion until the proportion of
population that is targeted exceeds 50%. With very few exceptions a positive
test result in a population where p is less than 0.5 increases diagnostic
uncertainty while a negative result decreases uncertainty.
Only occasionally do we see tests designed and verified for exclusion.
Directing our efforts at shortening the decision process has many advantages
for both patients and providers. One of the questions for the future may be
which tests are best for exclusion in the 'general unwell' (i.e.
self-selected) patient populations that present to GPs or hospitals.
Trevor Tickner,
Norwich
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