Evidence-Based Medicine Critical Appraisal of Harm. Department of Medicine - Residency Training Program Tuesdays, 9:30 a.m. - 12:00 p.m., UW Health Sciences Library. Steps in Practicing EBM. Convert the need for information into an answerable question.
Department of Medicine - Residency Training Program
Tuesdays, 9:30 a.m. - 12:00 p.m., UW Health Sciences Library
PubMed Clinical Queries
AHRQ Evidence Reports
AHRQ Preventive Services
CLINICAL RESEARCH CRITIQUES
ACP Journal Club 1996-
CASE REPORTS/SERIES, PRACTICE GUIDELINES, ETC
National Guideline Clearinghouse
ApplicabilityStrategies for Critical Appraisal of Studies of Harm
1. Did investigators assemble clearly defined groups of patients similar in all important ways other than exposure?
2. Were exposures and outcomes measured in the same ways in both groups (objective/blinded)?
3. Was follow-up sufficiently long and complete (5% and 20% rule)?
4. Do the results of the harm study fulfill some of the tests for “causation”?
*Types of Studies(in order of decreasing likelihood of being valid)
1. What is the magnitude of the treatment effect?
RR = (Exposed ER - Unexposed ER)/Unexposed ER
AR difference = Exposed ER - Unexposed ER
NNH = 1/AR difference
2. How precise is this estimate of the treatment effect?
95% CI - range of values within which we can be 95% sure that the population value lies.
1. A randomized trial of new drug “Ligatite” reveals that 25% of World Cup skiers who take the drug for one year have ACL tears whereas 50% of World Cup skiers who take the placebo for the year have ACL tears. What is the NNT?
NNT = 1/AR reduction = 1/(0.50-0.25) = 4
2. The study of the drug “Ligatite” also notes that 20% of athletes taking the drug develop clinical depression whereas 10% of athletes taking the placebo develop depression. What is the NNH?
NNH = 1/AR increase = 1/(0.20-0.10) = 10
3. An advertisement for a new drug fails to mention that it increases the relative risk of myocardial infarction by 50 % over 5 years. You read a valid study describing this finding. What is the NNH?
Unknown without knowing the event rate in the control population.
OR = (a/c)/(b/d)
Calculator available at:
1. Assign our patient the overall control event rate from the study.
2. If there is a subgroup of patients in the study with similar characteristics assign the event rate for that subgroup.
3. If a validated clinical predication guide is available use it to assign an event rate.
4. Look for a different paper that describes the prognosis of untreated patients more similar to our patient and use its results to assign an event rate.
Available at: http://hin.nhlbi.nih.gov/atpiii/calculator.asp?usertype=prof%09
1. Is our patient so different from those in the study that its results cannot apply?
2. What is our patient’s risk of benefit and harm from agent?
3. What are our patient’s preferences, concerns, and expectations from this treatment?
4. What alternative treatments are available?
The trial of the drug revealed that 25% of World Cup skiers who take the drug for 1 year have ACL tears whereas 50% of skiers who take the placebo have ACL tears. It also revealed that 20% of exposed skiers developed depression whereas 10% of unexposed skiers developed depression.
Your patient reads about this in Ski Magazine and asks you to write a prescription. In discussing the medication with her you want to provide her with an estimate of the magnitude of risk reduction she would realize. Is her NNT = 4 and should she take this medication?
1. Her risk of an ACL tear is substantially less so you have to re-estimate her expected event rate.
2. She is unlikely to be skiing year round so NNT is at least 2 to 3 times as high.
3. There is risk of developing depression (NNH = 10 over 1 year).
4. Whether or not to take the drug should take into account the relative value to the patient of preventing an ACL tear faced with the probability of developing depression.