Likelihood ratios increase diagnostic certainty in pulmonary embolism

Kevin Chu, Anthony FT Brown

Pulmonary embolism (PE) is a difficult diagnosis to confirm. The choice of tests has led to a myriad of algorithms. Diagnostic uncertainty can be quantified by the application of the tests' likelihood ratios (LR). Positive and negative LR enable the conversion of a pretest to a post-test probability, given a positive and negative test result, respectively. Thus, a pretest probability of <17% and a negative D-dimer with a negative LR of 0.05 (sensitivity 98%, specificity 40%) lead to a post-test probability of PE of <1%. Ventilation perfusion (V/Q) scans with a normal, very low, low, intermediate and high probability result have an LR of 0, 0.125, 0.25, 1 and 17, respectively. Also, patients with a V/Q scan result other than normal or high probability still have a post-test probability of PE from 3 to 65%. Positive and negative computed tomography pulmonary angiograms (CTPA) have an LR of 8.6 and 0.06, respectively (sensitivity 95%, specificity 89%). Patients with a high pretest probability and negative CTPA again still have a post-test probability of more than 10%. However, as the post-test probability after one test becomes the pretest probability for the next, test results used cumulatively progressively narrow the gap to a final diagnosis. The post-test probability after a D-dimer, V/Q scan, CTPA, leg ultrasound or pulmonary angiography, alone or in any combination or in any order, can be calculated using their LR. Use of LR thus assists in the precise interpretation of test results, such as in complex algorithms for PE.