Testing thresholds Flashcards
Clinical utility:
-We. can never be certain of a diagnosis or its absence (we cant reach 100% or 0% probability)
-Every test has a cost: seeking further certainty icurs further harm
ex) time spent, money spent, radiation exposure
-We must therefore decide (ideally in advance) when to stop investigating based on the balance of the benefit from further information and the harm of further investigation
Testing thresholds “zones”:
If the probability of disease is above the testing threshold:
-further testing (questions, physical exams, labs, etc.) is needed to move the probability past a threshold
-“Test” zone
-Differentials here are often called “active alternatives”
Testing thresholds “zones”:
If the probability of disease is below the testing threshold:
-Stop doing tests related to that specific differential diagnosis
-“Trash” zone
-Differentials here are considered “ruled out” or “excluded”
Where is your threshold?
Subjective:
Factors:
-The condition: harms of ruling out a diagnosis that may be present
The test:
-Accuracy: how low is the LR; ex) how much information would it provide?
Harms: financial, time spent, other
Note: there are many potential harms; consider patient values and preferences
ex) consider the balance of accuracy of the test vs. the harms of the test
Other considerations:
-Should usually not set the testing threshold below the population prevalence:
This would suggest that you think someone taken at random from the population is in the “testing” zone, implying at least that you think everyone who walks into your office should be investigated for the condition (also see screening lecture)
-More holistically: “would I be comfortable sending the patient home with a X% chance that they had this condition when [some test] is available?”
Where is your threshold?
The test zone expands (and “trash” zone shrinks) if:
-greater harm of ruling out the condition if it is present
-More favorable balance of test harm relative to information benefit
The “trash” zone expands (and test zone shrinks) if:
-less harm of ruling out the condition if it is present
-Less favorable balance of test harm relative to information benefit
Summary:
1) testing thresholds set a probability below which we stop diagnostic work-up. We do this because complete certainty is impossible
2) Testing thresholds are set according to the harm of inappropriately ruling out a diagnosis and the balance of the harms and benefits of testing
3) When the probability of disease is above a testing threshold, further testing or other management is needed. When it is below a testing threshold, we stop diagnostic work-up for that condition.
Higher positive LR:
Helps rule in a disease
Lower negative LR:
Helps rule out a disease
