
Evaluating the validity of a Diagnostic study

Key issues for Diagnostic Studies:

What likelihood ratios (LRs) were associated with the range of possible test results?
How much will different levels of the diagnostic test result raise or lower the pretest probability of disease?
Reference Standard 
Reference Standard 

New Test 
a 
b 
New Test 
c 
d 
Sensitivity: measures the proportion of patients with the disease who also test positive for the disease in this study. It is the probability that a person with the disease will have a positive test result.
Sensitivity = true positive / all disease positives [ a / (a + c) ]
Specificity: measures the proportion of patients without the disease who also test negative for the disease in this study. It is the probability that a person without the disease will have a negative test result.
Specificity = true negative / all disease negatives [d / (b + d) ]
Sensitivity and specificity are characteristics of the test but do not provide enough information for the clinician to act on the test results.
Likelihood ratios (LR): indicate the likelihood that a given test result would be expected in a patient with the target disorder compared to the likelihood that the same result would be expected in a patient without that disorder.
Likelihood ratio of a positive test result (LR+) increases the odds of having the disease after a positive test result.
Likelihood ratio of a negative test result (LR) decreases the odds of having the disease after a negative test result.
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 
How to use a nomogram with a likelihood ratio
More about likelihood ratios: Diagnostic tests 4: likelihood ratios. JJ Deeks & Douglas G Altman BMJ 2004 329:168169
Will the reproducibility of the test result and its interpretation be satisfactory in your clinical setting?
Does the test yield the same result when reapplied to stable participants?
Do different observers agree about the test results?
Are the study results applicable to the patients in your practice?Does the test perform differently (different LRs) for different severities of disease?
Does the test perform differently for populations with different mixes of competing conditions?
Will the test results change your management strategy?
What are the test and treatment thresholds for the health condition to be detected?
Are the test LRs high or low enough to shift posttest probability across a test or treatment threshold?
Will patients be better off as a result of the test?
Will patient care differ for different test results?
Will the anticipated changes in care do more good than harm?
Based on: Guyatt, G. Rennie, D. Meade, MO, Cook, DJ. Users' Guide to Medical Literature: A Manual for EvidenceBased Clinical Practice, 2nd Edition 2008.
Note: For criteria for other types of studies, see the following
supplements:
Therapy  Prognosis  Etiology/ Harm  Systematic Review
Revised July 2010