1. Make a clinical estimate (before testing) of the likelihood of the disease as odds.
Clue : Odds = Probability /(1- Probability)
How do we come up with pre-test probability?
Careful history and physical examination, and a few simple labs
Knowledge Epidemiology, History (age,gender, prevalence, natural course…)
Traditional? textbooks ,studies (Table 1), report
Experience Many cases? (last case?)
Judgement Clinical reasoning (Heuristics…)
2. Estimate or calculate the action threshold for the treatment you would advise. Use your patient's subjective preferences in estimating the impact of the various outcomes.
Clue : Harm = Probability of harm times its impact score
Clue : Improvement = Probability of improvement of disease multiplied by its impact score.
Clue : Action threshold = Harm / Improvement (as probability)
3. Select the test that best suits the scenario if more than one test is available. Seek the test that has the greatest power to change your mind; if your pre-test estimate is above the action threshold , choose the test with the lowest negative likelihood ratio (rule-out). If it is below the action threshold, select the test with the highest positive likelihood ratio(rule-in).
These are just the "numbers". If a given test entails disutility such as great cost, risk,
pain, or other negative implications, clinical judgment (the final arbiter of all things
clinical ) may rightfully override the mathematically "correct" decision in favor of a more
humane one.
How do we keep up to date with the best evidence concerning tests?
Texts such as Ref.2
Electronic texts Best Evidence… Internet PubMed "Clinical Queries"
4. Calculate what the post-test estimates of likelihood would be if the test results were positive and if the test results were negative.
5. If your test would take you from below the action threshold (pre-test odds) to above the action threshold ( post-test odds) or from above the action threshold to below the action threshold, go ahead and test. If it would not do so, there is no point in testing because the test results would not change your mind.
6. Based on the actual test result, if your post-test estimate of disease likelihood is above the action threshold, treat. If below, observe.
References
1. Making Medical Descisions An approach to clinical decision making for practicing physicians.
Richard Gross ACP
2. Diagnostic strategies for common medical problems 2nd. ACP
3. Evidence-based Medicine D.L. Sackett 2nd
4. Learning Clinical Reasoning
(Table 3) Strep pharyngitis probability score: Score Interpretation
0 0.64% probability
1 1.71% probability
2 4.53% probability
3 11.42% probability
4 25.95% probability
10 1.34% probability
11 3.55% probability
12 9.1% probability
13 21.39% probability
14 42.52% probability
20 2.96% probability
21 7.65% probability
22 18.38% probability
23 37.97% probability
24 62.47% probability
Tonsillar exudate?
Yes (1 points) No (0 points)
Swollen or tender anterior cervical lymph nodes?
Yes (1 points) No (0 points)
Fever?
Yes (1 points) No (0 points)
Absence of cough (except in smokers)?
Yes (1 points) No (0 points)
Prevalence of group A strep pharyngitis in patient's community
5% (0 points) 10% (10 points) 20% (20 points)