DUMC Library: Evaluating a DIAGNOSTIC TEST article
Validity issues | Likelihood ratios | Finding articles
Validity issues
Are the results valid?
- Did the clinicians face diagnostic uncertainty?
- Was there blind comparison with an independent gold standard?
- Did the results of the test being evaluated influence the decision to perform the gold standard?
What are the results?
- What likelihood ratios are associated with the range of possible test results?
How can I apply the results to my patient care?
- Will the reproducibility of the test result and its interpretation be
satisfactory in my setting?
- Are the results applicable to my patient?
- Will the results change my management?
- Will patients be better off as a result of the test?
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.
| | Disease
Pos | Disease
Neg |
| Test Pos | a | b |
| Test Neg | c | d |
- 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
|
Finding articles about a diagnostic test
PubMed:
- Use Clinical Queries for Diagnosis
- search: sensitivity OR specificity
Ovid:
- explode "sensitivity and specificity"/ (MeSH)
- /di (diagnosis as subheading)
- EBM Filter/Saved Search: EBMDX1 (comprehensive/sensitivity)
- EBM Filter/Saved Search: EBMDX2 (relevance/specificity)
References:
- JAMA 1994 271:389-391
- JAMA 1994 271:703-707
- ACP Journal Club 1994 Sept-Oct; A10- A12
From: Guyatt, G. Users' Guides to the Medical Literature: Essentials of Evidence-based Clinical Practice.
AMA Press, 2002 and Sackett, D.L. Evidence-Based Medicine. How to Practice and Teach EBM. Churchill-Livingstone,
2nd edition, 2001 (pocket cards).
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Last modified on: 16-Dec-04