Likelihood ratios increase diagnostic certainty in pulmonary embolism

Abstract

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.

Annual outpatient hysteroscopy and endometrial sampling (OHES) in HNPCC/Lynch syndrome (LS)

“…For comparison the PLR and NLR for mammography are 9.4 and 0.19 [22], and for CT pulmonary angiogram in diagnosing pulmonary embolism are 8.6 and 0.06 respectively. [23] OHES performance in asymptomatic women in our series is consistent with the previously reported performance in symptomatic women (PLR 60.9, NLR 0.15) for diagnosing EC, [24] with a negative hysteroscopy reducing EC probability to 0.6%. [24] TVS alone would have missed 2 of 4 cases of EC/AEH, and did not seem to add to the performance of OHES.…”

Evidence‐based practice: Where next? What, now?

“…An entertaining trip down history lane shows it arose mostly over intervention questions with the emergence of the randomized controlled trial 2 . A paper on diagnosis is welcome, with a careful dissection of standard characteristics of tests in suspected pulmonary embolus 3 . Understanding the clinical epidemiology of diagnostic tests is counter‐intuitive and difficult, and I predict many readers will find this paper by Chu and Brown tough.…”

Emergency department overcrowding: Time for a quantitive measure

“…In their article on the association between access block and analgesia for renal colic, Chu and Brown correctly suggest that access block is too crude a measure of ED overcrowding 1 …”