You have diagnosed a patient clinically as having SLE and ordered 6 tests. Out of which 4 tests have come positive and 2 are negative. To determine the probability of SLE at this point, you need to know-
Prior probability of SLE; sensitivity and specificity of each test
Incidence of SLE and predictive value of each test
Incidence and prevalence of SLE
Relative risk of SLE in this patient
Ans. is 'a' i.e. Prior probability of SLE; sensitivity and specificity of each test o Probability allows for an accurate method of calculating how much the likelihood of changes as new information (i.e., a test result) becomes available. o Probability of disease after a given test (post-term probability) can be calculated by (Bayes' theorem) Post test probability = Pretest probability x Likelihood ratio Pre test probability o It is the probability of disease before getting a test. o There is a principal "The interpretation of new information depends on old information". o That means the interpretation of a test result (post-test probability) depends on the probability of disease before test (pre-test probability). o Pre-test probability depends on prevalence, infact prevalence rate is described as pre-test probability of having a disease. Likelihood ratio o The likelihood ratio provides a direct estimate of how much a test result will change the probability of having the disease. o Tests results are often expressed as either positive or negative. o So, there is A likelihood ratio for a positive test result (LR +), and A likelihood ratio for a negative test result (LR-) o The positive likelihood ratio (LR +) tells us how much to increased the probability of a disease if the test is positive. It is calculated by following formula. The likelihood ratio is generally a better descriptor than sensitivity or specificity because it more directly describes the effect of a test result on the probability of disease and it incorporates both the sensitivity and specificity. So, in sho o To know the probability of a disease after a test (post test probability) depends upon : i) Pretest probability --> Prevalence ii) Likelihood ratio --> Specificity & sensitivity