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Essentials of Clinical Research - part 8

Phương pháp nghiên cứu Nghiên cứu của các xét nghiệm chẩn đoán Bảng 14,1 Các mối quan hệ giữa bệnh tật và kết quả xét nghiệm kiểm tra bất thường kiểm tra bệnh thông thường hiện nay bệnh vắng mặt | 14 Research Methodology for Studies of Diagnostic Tests 249 Table 14.1 The relationship between disease and test result Abnormal test Normal test Disease present TP FN Disease absent FP TN test. Note that the FP percentage is 1-specificity that is if the specificity is 90 -in 100 patients without the index disease 90 will have a negative test which means 10 will have a positive test - i.e. FP is 10 . Predictive Value Another concept is that of the predictive value PV and PV- of a test. This is asking the question differently than what sensitivity and specificity address - that is rather than asking what the TP and TN rate of a test is the PV of a test result is asking how likely is it that a positive test is a true positive TP i.e. TP TP FP for PV- it is TN tN FN . Ways of Determining Test Accuracy and or Clinical Usefulness There are at least six ways of determining test accuracy and they are all interrelated so the determination of which to use is based on the question being asked and one s personal preference. They are Sensitivity and specificity 2 X 2 tables Predictive value Bayes formula of conditional probability Likelihood ratio Receiver Operator Characteristic curve ROC Bayes Theorem We have already discussed sensitivity and specificity as well as the tests predictive value and the use of 2 X 2 tables and examples will be provided at the end of this chapter. But understanding Bayes Theorem of conditional probability will help provide the student interested in this area with greater understanding of the concepts involved. First let s discuss some definitions and probabilistic lingo along with some shorthand. The conditional probability that event A occurs given population B is written as P A B . If we continue this shorthand sensitivity can be written as P T D and PV as P D T . Bayes Formula can be written then as follows The post test probability of disease 250 S.P. Glasser Sensitivity disease prevalence Sensitivity disease prevalence 1-specificity disease