MUSM Libraries: Evaluating a HARM/ETIOLOGY article

Validity issues |  Strength of inference | Finding articles

Validity issues

Are the results valid?

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 lon (for the outcome to occur) and complete?
4. Do the reults of the harm study fulfill some of the diagnostic tests for causation?

Did experimental and control groups retain a similar prognosis after the study started?

• Were the outcomes and exposures measured in the same way in the groups being compared?
• Was follow-up sufficiently complete?

What are the results?

1. How strong is the association between exposure and outcome?
2. How precise is the estimate of risk?

How can I apply the results to my patient care?

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?

Strength of inference

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%

Finding articles about harm/etiology

PubMed:
PubMed for Handhelds (Memorial On-Campus/Remote One and Mercer On-Campus access)
PubMed for Handhelds (Mercer Off-Campus access)

• Select Search MEDLINE/PubMed and choose Etiology from the Categories under Clinical Queries.
• SelectSearch MEDLINE/PubMed and use the Search with no filters 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). Top of page

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