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Purpose of investigation: Difference between revisions
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== Diagnosis == | == Diagnosis == | ||
When an investigation is ordered for diagnosis, one should already have a list of differential diagnoses before ordering the test, and the test should be able to narrow down the list of differential diagnoses. It's important to consider the test's [[specificity]], [[sensitivity]], [[positive predictive value]], and [[negative predictive value]] in this. There is no reason to perform an investigation if it doesn't help narrow down the number of differential diagnoses or doesn't validate your tentative diagnosis. | When an investigation is ordered for diagnosis, one should already have a list of differential diagnoses before ordering the test, and the test should be able to narrow down the list of differential diagnoses. It's important to consider the test's [[specificity]], [[sensitivity]], [[positive predictive value]], and [[negative predictive value]] in this. There is no reason to perform an investigation if it doesn't help narrow down the number of differential diagnoses or doesn't validate your tentative diagnosis. | ||
This article leans heavily on statistical terms like probability and thresholds. In real-life, the exact probability and thresholds cannot be known in most circumstances. Instead, we think of these factors in approximate terms based not on statistical analysis but our own experience and knowledge. | |||
=== Approach to diagnosis === | === Approach to diagnosis === | ||
When a patient is presented with symptoms or clinical findings, one should formulate a list of differential diagnoses which could be underlying. Often, this is not a conscious process but rather an unconscious one | When a patient is presented with symptoms or clinical findings, one should formulate a list of differential diagnoses which could be underlying. Often, this is not a conscious process but rather an unconscious one. | ||
The model "a safe diagnostic strategy", made by professor John Murtagh, involves five questions one should ask themselves during patient presentations: | The model "a safe diagnostic strategy", made by professor John Murtagh, involves five questions one should ask themselves during patient presentations: | ||
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=== Considering test sensitivity === | === Considering test sensitivity === | ||
Using a test with low sensitivity for the diagnosis of a disorder can lack value. Consider, for example, a rib fracture. One might think that a rib fracture should be evaluated with a radiograph, as most fractures are. However, a radiograph has very low sensitivity for rib fracture (60%), and so 40% of people with rib fracture will have no findings on an x-ray. Knowing this, you could not trust a negative radiograph to rule out rib fracture, and so it is not recommended for the evaluation of this disorder. | Using a test with low sensitivity for the diagnosis of a disorder can lack value. Consider, for example, a rib fracture. One might think that a rib fracture should be evaluated with a radiograph, as most fractures are evaluated. However, a radiograph has very low sensitivity for rib fracture (60%), and so 40% of people with rib fracture will have no findings on an x-ray. Knowing this, you could not trust a negative radiograph to rule out rib fracture, and so it is not recommended for the evaluation of this disorder. | ||
=== Considering how pre-test probability changes the test characteristics === | === Considering how pre-test probability changes the test characteristics === | ||
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The more severe the suspected disorder is, the lower the pre-test probability needs to be before one should consider testing for it. This is because the consequences of missing a diagnosis is more severe, so the disadvantages of the test are weighed up for. | The more severe the suspected disorder is, the lower the pre-test probability needs to be before one should consider testing for it. This is because the consequences of missing a diagnosis is more severe, so the disadvantages of the test are weighed up for. | ||
For example, one would not test every person with a sore throat for [[streptococcal tonsillitis]], only if the pre-test probability is sufficiently high, usually more than 30%, as determined by a Centor score of 3 or more. However, one would test every person with significant microscopic haematuria for bladder cancer, despite a pre-test probability of only 5% (only 5% of people with significant microscopic haematuria have bladder cancer). | For example, one would not test every person with a sore throat for [[streptococcal tonsillitis]], only if the pre-test probability is sufficiently high, usually more than 30%, as determined by a Centor score of 3 or more. However, one would test every person with significant microscopic haematuria for [[bladder cancer]] (usually by [[cystoscopy]]), despite a pre-test probability of only 5% (only 5% of people with significant microscopic haematuria have bladder cancer). | ||
=== Considering the treatment threshold === | |||
The treatment threshold is a theoretical level of disease probability (pre-test or post-test) where the disease is so likely that you would consider treatment to be indicated. Another way to think of it is "at what degree of disease uncertainty am I comfortable with initiating treatment?". | |||
The treatment threshold is low if the treatment is harmless for healthy people and very efficacious for people with the assumed disease, for example [[Antihistamine|antihistamines]] in suspected [[allergic rhinitis]]. | |||
The treatment threshold is high if the treatment is harmful for healthy people ''or'' it is not very efficacious for people with the disease. This can occur for example in case of cancer (where treatment is very harmful, so the disease probability should be close to 100% before considering treatment) or in case of bacterial infections which are usually uncomplicated and self-limiting (like uncomplicated [[acute otitis media]] or [[acute sinusitis]]). | |||
If, based on the clinical findings, you find the pre-test probability to be sufficiently high that even a negative test will yield a post-test probability higher than the treatment threshold, testing does not influence the outcome, and so testing is unnecessary. | |||
Alternatively, if you find the pre-test probability to be so low that despite a positive test the post-test probability remains lower than the treatment threshold, testing also does not influence the outcome, and so testing is unnecessary in this case as well. An example of this can be if a patient has a sore throat but you find no evidence of streptococcal tonsillitis, in which case even a positive rapid strep test won't make the diagnosis of streptococcal tonsillitis high enough that you would consider antibiotics. | |||
== Follow-up and monitoring == | == Follow-up and monitoring == | ||
Follow-up and monitoring of a disorder is also a common use of laboratory and imaging investigations. Examples include: | Follow-up and monitoring of a disorder is also a common use of laboratory and imaging investigations. Examples include: | ||
* Serial CRP or leukocyte count following antibiotic prescription to evaluate the treatment response | * Serial [[CRP]] or [[leukocyte]] count following [[antibiotic]] prescription to evaluate the treatment response | ||
* Serial plasma sodium measurement following fluid restriction in assumed SIADH | * Serial plasma [[sodium]] measurement following fluid restriction in assumed [[SIADH]] | ||
* Yearly brain MRI following removal of a meningeoma, to try and catch a recurrence early, before it causes symptoms | * Yearly brain [[MRI]] following removal of a [[meningeoma]], to try and catch a recurrence early, before it causes symptoms | ||
However, it's important to keep in mind the requirement of value for the patient. In the first scenario, there is value as a lack of normalisation of inflammatory parametres in a patient treated with antibiotic for a bacterial infection may be a sign that the antibiotic is ineffective, which may require administration of a different antibiotic. In the | However, it's important to keep in mind the requirement of value for the patient. In the first scenario, there is value as a lack of normalisation of inflammatory parametres in a patient treated with antibiotic for a bacterial infection may be a sign that the antibiotic is ineffective, which may require administration of a different antibiotic. In the third scenario, there is also value, as the earlier one can catch recurrence of a meningeoma, the better the prognosis after treatment. However, if the patient is in a condition where they will not receive treatment anyway (for example, if they are preterminal), monitoring is not of value to the patient. | ||
== Evaluation of prognosis == | == Evaluation of prognosis == | ||