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. Repeat from Nov 06 & May 06 To know the probability of ds. after a given test/or tests (i.e. post-test probability) we need to know two things I .the pre test probability & 2. the likelihood ratios (LR) of tests. Post-test probability = Pre-test probability x likelihood ratio The pre test probability depends on the prevalence of the disease and the clinical assessment. The likelihood ratio provides a direct estimate of how much a test result will change the probability of having the disease. Because tests can be positive or negative, there are two likelihood ratios for each test. The 'positive likelihood ratio' tells us how much to increase the probability of a disease if the test is positive, while the 'negative likelihood ratio tells us how much to decrease it if the test is negative. The likelihood ratio incorporates both the sensitivity and specificity of the test and can be calculated by following formulae: LR (+) sensitivity / 1 - specificity LR (-) = 1 - sensitivity / specificity