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What is test of treatment?
Though ideally we should have a clear diagnosis before starting treatment, such certainty is not always possible. Sometimes this uncertainty can be resolved by using the treatment as the test that confirms the diagnosis. 1 For example, if we are unsure if a patient's airway obstruction has a reversible element, a trial of steroids can test this: a sufficient response is then considered evidence of reversibility. At other times the test of treatment is not planned, but the failure to respond to treatment as expected leads to a rethink of the diagnosis. In this brief review we discuss different uses of the "test of treatment," its reliability as a diagnostic tool, and how its use might be improved (for a specific example of its use, see the accompanying article on chronic cough 2 ).
When is it used?
As illustrated in figure 1 , a "test of treatment" is one strategy for the final stage of arriving at a diagnosis. It is appropriate when a single diagnosis is highly probable but not certain, when an available treatment works for most patients if the diagnosis is correct, and when there is a measurable short term outcome or surrogate. Such tests are more common and more useful in chronic or recurrent conditions rather than in acute conditions.
A test of treatment is likely to be useful:
To make a diagnosis when the clinical features are atypical-for example, using glyceryl trinitrate for atypical chest pain or prednisone for suspected polymyalgia rheumatica in patients with a low or normal erythrocyte sedimentation rate
To make a definitive diagnosis when various differential diagnoses are possible-for example, chronic cough 2
To see if a particular treatment-for example, switching antihypertensive drugs to minimise adverse effects-is appropriate in someone with the diagnosis.
One early example of a test of treatment is the physostigmine test for myasthenia gravis, 3 used since 1937. The treatment used for testing is not always the treatment used in the long term: short acting edrophonium (Tensilon) has become the standard agent to test for myasthenia gravis, although it is not suitable for long term treatment. 4 Similarly with...