Diagnosis
Spiral computer tomography for the diagnosis of pulmonary embolism in critically ill surgical patients: A comparison with pulmonary angiography.
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Summary
Posted By: Gordon S. Doig
E-Mail: gdoig@med.usyd.edu.au
Posted Date: 14 Aug 2001
Title: Spiral computer tomography for the diagnosis of pulmonary embolism in critically ill surgical patients: A comparison with pulmonary angiography.
Authors: Velmahos GC, Vassiliu P, Wilcox A et al
Reference: Arch Surg 2001;136:505-510
Link: Click here for a direct link to the paper. A password may be required for access to fulltext.
Abstract: HYPOTHESIS: Spiral computed tomographic pulmonary angiography (CTPA) is sensitive and specific in diagnosing pulmonary embolism (PE) in critically ill surgical patients.
DESIGN: Prospective study comparing CTPA with the criterion standard, pulmonary angiography (PA).
SETTING: Surgical intensive care unit of an academic hospital.
PATIENTS: Twenty-two critically ill surgical patients with clinical suspicion of PE. The CTPAs and PAs were independently read by 4 radiologists (2 for each test) blinded to each other's interpretation. Clinical suspicion was classified as high, intermediate, or low according to predetermined criteria. All but 2 patients had marked pulmonary parenchymal disease at the time of the event that triggered evaluation for PE.
INTERVENTIONS: Computed tomographic pulmonary angiography and PA in 22 patients, venous duplex scan in 19.
RESULTS: Eleven patients (50%) had evidence of PE on PA, 5 in central and 6 in peripheral pulmonary arteries. The sensitivity and specificity of CTPA was, respectively, 45% and 82% for all PEs, 60% and 100% for central PEs, and 33% and 82% for peripheral PEs. Duplex scanning was 40% sensitive and 100% specific in diagnosing PE. The independent reviewers disagreed only in 14% of CTPA and 14% of PA interpretations. There were no differences in risk factors or clinical characteristics between patients with and without PE. The level of clinical suspicion was identical in the 2 groups.
CONCLUSIONS: Pulmonary angiography remains the gold standard for the diagnosis of PE in critically ill surgical patients. Computed tomographic pulmonary angiography needs further evaluation in this population.
 
Are the Results Valid?
1. Was there an independent, blind comparison with a reference standard?
Yes. Pulmonary angiography was used as the reference standard and angiograms and CT images were read by different radiologists who were 'blind' to the results of each other's readings.
2. Did the patient sample include an appropriate spectrum of patients to whom the diagnostic test will be applied in clinical practice?
Yes. Fairly simple, objective criteria were used to select patients for inclusion.
3. Did the results of the test being evaluated influence the decision to perform the reference standard?
No. The results of the spiral CT did not influence the decision to perform an angiogram. All patients who entered the study received both a CT and an angio.
4. Were the methods for performing the test described in sufficient detail to permit replication?
Yes. The methods appear to be described in sufficient detail.
What are the Results?
1. Are likelihood ratios for the test results presented or data necessary for their calculation provided?
Using the angiogram as the reference standard, the sensitivity of the spiral CT was reported as45% with a specificity of 82%. This yields a likelihood ratio of 2.5 for a positive test [sens/(1-spec) = .45/(1-.82)=2.5]. Unfortunately, since there were only 22 patients in this study, the confidence limits around the estimates of sensitivity and specificity are VERY wide.

From the PIOPED study, the likelihood ratio for a positive 'high probability' V/Q scan is 18.3 with an 'intermediate' positive scan LRT of 1.2 and a 'low probability' scan of 0.36.

Will the Results Help Me In Caring For My Patients?
1. Will the reproducibility of the test result and its interpretation be satisfactory in my setting?
Yes. There is no reason to belive the results obtained in any large tertiary care centre should differ from this paper's findings.
2. Are the results applicable to my patient?
Yes. The patients were fairly representative of Surgical ICU patients.
3. Will the results change my management?
Probably not. These findings do not demonstrate that spiral CT is more useful for screening PEs than V/Q scans. If you are currently using V/Q scans, this paper would not motivate a change to spiral CT.
4. Will patients be better off as a result of the test?
At the current level of performance (likelihood ratio for positive test = 2.5), spiral CT provides a 'small' (but potentially useful) improvement in information however, since there are both false positives and false negatives with spiral CTs, a significant number of patients with both negative and positive results would need to be Angio'd in order to be certain that therapy (anticoagulation) is appropriate.

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Spiral computer tomography for the diagnosis of pulmonary embolism in critically ill surgical patients: A comparison with pulmonary angiography.

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