DIAGNOSTIC TEST

From: Users’ Guides to the Medical Literature Evidence-Based Medicine Working Group

Are the Results Valid?

Was there an independent, blind comparison with a reference standard?

Did the patient sample include an appropriate spectrum of patients to whom the test will be applied?

Did the results of the test being evaluated influence the decision to perform the reference standard?

Were the test’s methods described clearly enough to permit replication?

What are the Results?

What are the likelihood ratios for the test results?

Will the Results Help Me in My Patients' Care?

Will the test be reproducible and well interpreted in my practice setting?

Are the results applicable to my patients?

Will the test results change my management?

Will my patients be better off because of the test?

References: JAMA 1994; 271:389-391 JAMA 1994; 271:703-707

The Likelihood Ratio for a test result compares the likelihood of that result in patients with disease to the likelihood of that result in patients without disease.

Dz+ Dz-

Test (+) a b LR(+) = a/(a+c)/b/(b+d)

Test (-) c d LR(-) = c/(a+c)/d/(b+d)

LR(+) = sen/(1-spec) = TPR/FPR

LR(-) = (1-sen)/spec = FNR/TNR

Key Properties of LRs:

a. Posttest Odds = LR x Pretest Odds of Disease

b. Independent of Prevalence of Disease

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

How much do LRs change disease likelihood?

LRs >10 or <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 <2 or >0.5 cause tiny changes

LRs = 1.0 cause no change at all

Finding Articles About a Diagnostic Test

· Explode Sensitivity and Specificity

· Diagnosis& (px)

[see ACP J Club 1994 Sept-Oct; A10-A12]