Assessing Diagnostic Tool

Assessing Diagnostic ToolDwi Agustian20/10/2015Main QuestionDo we have good enough diagnostic tool?What is normal? Abnormal?What is the diagnosis criteria?What is the quality measures of good diagnostic tool?Example of Biological Variation

ValidityThe validity of a test is defined as its ability to distinguish between who has a disease and who does not: sensitivity and specificity.The sensitivity of the test is defined as the ability of the test to identify correctly those who have the disease. The specificity of the test is defined as the ability of the test to identify correctly those who do not have the disease.

Example

Concept

Reality [in medical practice]

Multiple TestSequential

Net Sensitivity and Net Specificity315 people of the total 500 people with diabetes in this population of 10,000 will have been correctly called positive: 315/500 = 63% net sensitivity -> a loss in net sensitivity 7,600 + 1,710 of the 9,500 nondiabetics were correctly called negative: 9,310/9,500 = 98% net specificity-> a gain in net specificity

Simultaneous Test

the net sensitivity using tests A and B simultaneously:

-> a gain in net sensitivity

the net specificity = 432/800 = 54% net specificity-> a loss in net specificity

SummaryWhen two sequential tests are used and those who test positive by the first test are brought in for the second test, there is a net loss in sensitivity, but a net gain in specificity, compared with either test alone. When two simultaneous tests are used, there is a net gain in sensitivity and a net loss in specificity, compared with either test alone.When multiple testing?Diagnostic vs ScreeningCost - InsuranceLength of StayDegree of invasivenessPredictive ValueIf the test results are positive/negative in this patient, what is the probability that this patient has/has not the disease? Example

The predictive value is affected by two factors: the prevalence of the disease in the population tested and, when the disease is infrequent, the specificity of the test being usedExample

Relationship of Predictive Value and Prevalence

Relationship between Positive Predictive Value and Specificity of the Test

RELIABILITY (REPEATABILITY) OF TESTSThe factors that contribute to the variation between test results are: intrasubject variation (variation within individual subjects), intraobserver variation (variation in the reading of test results by the same reader), andinterobserver variation (variation between those reading the test results).

Test is not valid

ReferensiWebb, Chapter 14Gordis, Chapter 5