We, of course, are concerned about selection bias and immortal time bias,
1
and a cautious interpretation is appropriate. However, it is important to obtain as much evidence as possible by observational trials.2
Unsupportable costs prevent the performance of a randomized controlled trial of inferior vena cava (IVC) filters in stable patients with pulmonary embolism who receive thrombolytic therapy.Unstable patients in our investigation were defined in the Methods as those in shock or on ventilator support.
3
Such patients, as indicated in the Methods, were identified on the basis of International Classification of Diseases, Ninth Revision, Clinical Modification codes, which are shown in Table 1.3
Our investigation was a retrospective cohort study, not a case-control study.
4
The use of relative risk, therefore, was appropriate.4
, 5
, 6
The term “relative risk” is also called “risk ratio.”,6
, 7
Whether we use the term “relative risk” or “risk ratio,” therefore, is of no consequence.It is recommended that absolute risk reduction should be reported together with relative risk.
7
Absolute risk reduction has a clear meaning and is appealing to the practitioner.7
The use of number needed to treat has also been encouraged.7
The number needed to treat is an absolute measure, as is absolute risk reduction, and both a relative and absolute measure should be reported to portray a more complete picture.7
There is no reason to reserve absolute risk and number needed to treat for prospective randomized trials.Regarding external validity, the investigations cited by Dr. Bergl are not relevant to our investigation. The investigation by Bikdeli et al
8
of IVC filters in Medicare patients did not address the subset of patients who received thrombolytic therapy. The investigation by Meyer et al9
of fibrinolysis for patients with intermediate-risk pulmonary embolism was not powered to detect differences in rates of death. The prospective randomized trial by Mismetti et al10
of the effect of retrievable IVC filters plus anticoagulation vs anticoagulation alone did not address the subset of patients who received thrombolytic therapy.References
- Problem of immortal time bias in cohort studies: example using statins for preventing progression of diabetes.BMJ. 2010; 340: b5087
- Requiem for liberalizing indications for vena caval filters?.Circulation. 2016; 133: 1992-1994
- Inferior vena cava filters in stable patients with acute pulmonary embolism who receive thrombolytic therapy.Am J Med. 2018; 131: 97-99
- Observational research methods. Research design II: cohort, cross sectional, and case-control studies.Emerg Med J. 2003; 20: 54-60
- What's the relative risk? A method of correcting the odds ratio in cohort studies of common outcomes.JAMA. 1998; 280: 1690-1691
- Odds ratios and risk ratios: what's the difference and why does it matter?.South Med J. 2008; 101: 730-734
- Odds ratio, relative risk, absolute risk reduction, and the number needed to treat–which of these should we use?.Value Health. 2002; 5: 431-436
- Vena caval filter utilization and outcomes in pulmonary embolism: Medicare hospitalizations from 1999 to 2010.J Am Coll Cardiol. 2016; 67: 1027-1035
- Fibrinolysis for patients with intermediate-risk pulmonary embolism.N Engl J Med. 2014; 370: 1402-1411
- Effect of a retrievable inferior vena cava filter plus anticoagulation vs anticoagulation alone on risk of recurrent pulmonary embolism: a randomized clinical trial.JAMA. 2015; 313: 1627-1635
Article info
Footnotes
Funding: This investigation was supported by grant 24212.ll from the Blue Cross Blue Shield of Michigan Foundation.
Conflicts of Interest: None.
Authorship: All authors had access to the data and participated in preparation of the manuscript.
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© 2018 Published by Elsevier Inc.
