EBM - Evaluating a Diagnostic Test Study

MUSM Libraries: Evaluating a DIAGNOSTIC TEST STUDY article

Appraise | Likelihood ratios | Finding articles

Appraise

What did they do? (Methods)
1. Was there an independent, blind comparison with reference to a “gold” standard?
2. Was the test evaluated in an appropriate spectrum of patients (like those in whom we would use it in practice)?
3. Was the reference standard ascertained regardless of the diagnostic test result?
What was the answer? (Results)
1. What likelihood ratios are associated with the range possible test results?
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?
What do I do with this information?
1. Will the reproducibility of the test result and its interpretation be satisfactory in my setting?
2. Are the results applicable to my patient?
4. Will patients be better off as a result of the test?
5. Are the benefits woth the potential harms and costs?

Likelihood ratios

A Likelihood ratio for a given diagnostic test result compares the likelihood of that result in patients with disease to the likelihood of that result in patients without disease. It provides an estimate of how much a test result will change the odds of disease in a patient.

Please enter data into the 2 by 2 table to determine the likelihood ratio

	Dis Pos	Dis Neg
Test Pos
Test Neg

SEN:
SPE:
Neg LR:
Pos LR:

How much do LRs change disease likelihood?

LRs greater than 10 or less than 0.1	cause large changes
LRs 5 - 10 or 0.1 - 0.2	cause moderate changes
LRs 2 - 5 or 0.2 - 0.5	cause small changes
LRs less than 2 or greater than 0.5	cause tiny changes
LRs = 1.0	cause no change at all

Sensitivity - the proportion of patients who test positive and have the target disorder. Sen = a/a+c
Specificity - the proportion of patients who test negative and do NOT have the target disorder. Spec = d/b+d
LR + = a/(a + c) over b/(b + d)
LR - = c/(a + c) over d/(b + d)

Key Properties of LRs:

posttest Odds = LR x Pretest Odds of Disease
independent of prevalence of disease

Pretest Probabilities are estimated from published studies of prevalence, data from your practice setting, and your clinical intuition.

How much do LRs change disease likelihood?

LRs greater than 10 or less than 0.1	cause large changes
LRs 5 - 10 or 0.1 - 0.2	cause moderate changes
LRs 2 - 5 or 0.2 - 0.5	cause small changes
LRs less than 2 or greater than 0.5	cause tiny changes
LRs = 1.0	cause no change at all

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Finding articles about a diagnostic test

PubMed:

Select PubMed@Mercer and click on Clinical Queries and perform a search, then choose Diagnosis from Category

References:

JAMA 1999 Apr 7;281(13):1214-1219.

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

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