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|Year : 2017 | Volume
| Issue : 1 | Page : 8--9
Systematic reviews of diagnostic tests: A primer
Thambu David1, Prathap Tharyan2,
1 Department of Medicine - Unit II, Christian Medical College, Vellore, Tamil Nadu, India
2 Department of Phychiatry - Unit II, South Asian Cochrane Network, Christian Medical College, Vellore, Tamil Nadu, India
Department of Medicine - Unit II, Christian Medical College, Vellore, Tamil Nadu
|How to cite this article:|
David T, Tharyan P. Systematic reviews of diagnostic tests: A primer.Indian J Med Microbiol 2017;35:8-9
|How to cite this URL:|
David T, Tharyan P. Systematic reviews of diagnostic tests: A primer. Indian J Med Microbiol [serial online] 2017 [cited 2017 Mar 27 ];35:8-9
Available from: http://www.ijmm.org/text.asp?2017/35/1/8/202351
What Are Systematic Reviews?
Systematic reviews are reviews that use prespecified and transparent methods to systematically identify, reliably evaluate and synthesise the evidence from all eligible primary research studies to answer a given question. Systematic reviews can address questions regarding disease prevalence, aetiology, diagnosis, therapy, harm and prognosis. Well-done systematic reviews are commonly considered as providing the best evidence of the effectiveness of health-care interventions. The essential steps of all well-done systematic reviews include formulating a focused clinical question; developing a systematic and comprehensive search strategy (without language restrictions) to identify all relevant published and unpublished primary studies; locating and selecting the studies; describing the included studies and detailing the reasons for excluding studies; critically appraising all included studies for the risk of bias; reliably extracting the data for prespecified outcomes; synthesising the results using meta-analysis (if appropriate); assessing the results for inconsistency (heterogeneity), and attempting to explain important heterogeneity; summarising and interpreting the results based on the size and precision of the estimates and the confidence one can place in these estimates. These explicit methods result in more reliable results than those obtained by traditional ways of reviewing evidence.
What Are Diagnostic Systematic Reviews?
Interest in systemically evaluating the results of diagnostic studies in health has increased over the past decades. In a nutshell, systematic reviews of diagnostic tests use the systematic review process to evaluate the evidence for the usefulness of a diagnostic test(s) for a particular disease in a specified patient group.
Types of Diagnostic Tests and Systematic Reviews of Diagnostic Evaluations
Initially, when a researcher develops a new diagnostic test for a disease, the test is first done on those with proven disease. If this shows promise, the new test is tried on patients with proven disease as well as those with no disease using a case–control design. If still promising in accurately diagnosing those with disease and ruling out those without, research moves on to compare the accuracy of the new test with the currently accepted reference standard test using a cohort design, since without such a comparison, the test's performance would not reflect its accuracy in clinical practice. For this, consecutive patients suspected to have the disease in question are prospectively recruited and independently subjected to the new test(s) as well as the reference standard. Systematic reviews of these cross-sectional studies comparing the new test(s) with the reference standard in a cohort with the given disease condition (and stage of disease) are referred to as systematic reviews of diagnostic test accuracy (DTA). These types of systematic reviews are ideal since they include study designs that provide valid estimates of DTA that can aid clinical practice. Since the ultimate goal of using diagnostic tests is to improve health outcomes, systematic reviews could also be done of studies evaluating whether the use of one or more diagnostic tests improves health outcomes. These are actually reviews of effectiveness; and the methods used in systematic reviews of interventional studies differ from those used in DTA reviews.
How Are Research Questions for a Systematic Review of Diagnostic Test Accuracy Framed?
Framing the question for a systematic review of DTA properly is important as it guides the review process. The traditional PICO format for systematic reviews as applied to DTA reviews include: the patients or population with suspected disease (P), with specifications of the stage of disease, if considered relevant; the test (s) of interest or index test (s) (I), the comparator test (C) that often is the reference standard and the outcome or outcomes of interest (O), which in diagnostic tests are the sensitivity and specificity, positive and negative predictive values, diagnostic odds ratios, likelihood ratios and summary receiver operating characteristic curves. How the index test is likely to be used in the diagnostic pathway is also important to consider as part of the review question. If the test is to be used as a screening test, then the population studied and the consequences of testing will differ from if the test is to be used for confirmation and to initiate treatment, or as an add-on test that would require higher sensitivity and/or specificity than screening tests.
Where Do Diagnostic Test Accuracy Systematic Reviews Search for Studies?
To ensure that the search for relevant studies is comprehensive, multiple sources are searched. At the very least, the search usually includes not only general databases such as Medline and EMBASE but also more specific databases such as MEDION, a database of diagnostic literature (http://www.mediondatabase.nl). This is often supplemented by scanning bibliographies of included studies, contacting authors of studies and experts in the field to locate ongoing and unpublished studies and even contacting manufacturers of the diagnostic tests being studied.
How Do Diagnostic Test Accuracy Systematic Reviews Assess Risk of Bias?
Most diagnostic studies are retrospective and use a case–control design. Systematic reviews based on such studies often provide unreliable estimates of diagnostic accuracy. Even if the DTA review included only studies with cross-sectional comparisons of index tests versus the reference standard in cohorts of people with the disease in question, the studies may differ in where, in whom, and how the tests were performed and also in the methods used that could bias the results for test accuracy. The tool many DTA systematic reviews use to assess the quality of studies included in systematic reviews is the revised version of QUADASS. This tool assesses four domains: patient selection, index test, reference standard and flow and timing. Each domain is assessed for risk of bias, and the first three domains are also assessed in terms of concerns regarding applicability. DTA reviews often present summary quality assessment results in graphs or tables; some may exclude studies considered to be of low quality while others may assess the effects of study quality on test accuracy in stratified analyses.,
The quality of primary DTA studies themselves can be improved by adherence to the recommendations laid down in the updated Standards for Reporting Diagnostic Accuracy statement (STARD 2015) that provides a list of thirty essential items that should be included in every report of a diagnostic accuracy study.
Do Diagnostic Test Accuracy Systematic Reviews Have Any Role in Policy Decisions
The real success of DTA systematic reviews lies in changing health-care decisions. A well-quoted example of a DTA systematic review that influenced policy is the one that evaluated the diagnostic accuracy of commercial serological tests for pulmonary and extra-pulmonary tuberculosis (TB), with a focus on the relevance of these tests in low- and middle-income countries. The review found that the overall quality of evidence for pulmonary TB (67 studies, 5147 participants) and extra-pulmonary TB (25 studies, 1809 participants) was low and that commercial serological tests produced inconsistent and imprecise estimates of sensitivity and specificity. This led to the WHO recommending against their use.
How Can One Learn More About Diagnostic Test Accuracy Systematic Reviews?
The Cochrane Methods Group for Screening and Diagnostic Tests provides a Handbook for DTA reviews as well as software to undertake DTA reviews that can be accessed via the Cochrane website, where other training resources are also available. Cochrane also publishes DTA systematic reviews on various topics in the Cochrane Database of Systematic Reviews that are labelled as DTA reviews. Cochrane South Asia may also be able to offer guidance.
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