EBM - Evaluating a Harm/Etiology Study

MUSM Libraries: Evaluating a HARM/ETIOLOGY article

Appraise | Strength of inference | Finding articles

Appraise

What did they do? (Methods)
1. Were there clearly defined groups of patients, similar in all important ways other than exposure to the treatment or other cause?
2. Were the treatments/exposures and clinical outcomes measured in the same ways in both groups? (Was the assessment of outcomes either objective or blinded to exposure?
3. Was the follow-up of the study patients sufficiently long (for the outcome to occur) and complete?
4. Do the results of the harm study fulfill some of the diagnostic tests for causation?
5. Were the outcomes and exposures measured in the same way in the groups being compared?
What was the answer? (Results)
1. How strong is the association between exposure and outcome?
2. How precise is the estimate of risk?
What did they say about the answer? (Conclusion)
1. Do the results of the study support the author/s conclusions?
2. Does the conclusion reflect the discussion?
3. Are there study limitations, and do these impact the conclusions?
4. Are potential discrepancies mentioned?
What do I do with this information?
1. Were the study patients similar to the patient in my practice?
2. Was the duration of follow-up adequate?
3. What was the magnitude of the risk?
4. Should I attempt to stop the exposure?

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Strength of inference

	Adverse Outcome Yes	Adverse Outcome No
Exposed Yes	a	b
Exposed No	c	d

RCT or Prospective cohort studies:

Relative Risk (RR) is the risk of the outcome in the exposed group divided by the risk of the outcome in the unexposed group: RR = a/(a + b) divided by c/(c + d)
"RR of 3.0 means that the outcome occurs 3 times more often in those exposed versus unexposed."

Case-control studies:

Odds Ratio (OR) is the odds of previous exposure in subjects with the outcome divided by the odds of exposure in subjects without the outcome: OR = (a/b) divided by (c/d) = ad/bc
"OR of 3.0 means that cases were 3 times more likely to have been exposed than were control patients."

Confounding variable is one whose influence distorts the true relationship between a potential risk factor and the clinical outcome of interest.

Number Needed to Treat (NNT) The number of patients that need to be treated with a specified therapy in order to prevent one additional bad outcome. Calculated as the inverse of the absolute risk reduction (1/ARR)

Absolute Risk Reduction (ARR) is the difference in risk between the control group (X) and the treatment group (Y). ARR = X-Y

Control Event Rate (CER)
The proportion of patients in the control group who experience the studied event.

Experimental Event Rate (EER)
The proportion of patients in the experimental treatment group who are observed to experience the outcome of interest.

Relative Risk Reduction (RRR) is the percent reduction in risk in the treated group (Y) compared to the control group (X). RRR = 1-RR x 100%

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Finding articles about harm/etiology

PubMed:

Select PubMed@Mercer and click on Clinical Queries, perform a search, then choose Etiology from Category
Select PubMed@Mercer and enter your search term/s AND risk.

References:

JAMA 1994 271:1615-1619
ACP Journal Club 1994 Nov-Dec; A10-A11

From: Guyatt, G. Users' Guides to the Medical Literature: Essentials of Evidence-based Clinical Practice. AMA Press, 2002 and Strauss. Evidence-Based Medicine. How to Practice and Teach EBM. Churchill-Livingstone, 3rd edition, 2005 (pocket cards).

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Experimental group size:
Control group size:
Events in experimental group:
Events in control group:

EER
CER
ARR
RRR
NNT
OR/RR