Cochrane systematic reviews of interventions (intervention reviews) and Cochrane diagnostic test accuracy reviews (DTA reviews) are two types of systematic review.
They share the following characteristics:
- They aim to answer a specific healthcare question to help people make decisions based on up-to-date evidence from research.
- They look for all the relevant studies to answer the review question.
- They appraise the reliability of the studies.
- They summarize the results to produce a picture of the body of evidence available.
- They are systematic because they search for and analyze evidence in a systematic way, according to predetermined and published methods.
Intervention reviews seek to answer questions about the effectiveness of healthcare interventions (medicines, treatments or policies) on the people who receive them. For example: ‘How effective is acupuncture for treating depression’; or ‘Which treatment is most effective for treating psoriasis?’. To answer such questions, the authors of intervention reviews identify studies that compare one intervention with either another intervention, an inactive intervention (placebo), or no intervention. Depending on the number and reliability of the studies identified, intervention reviews may provide information on whether the intervention works, or whether we need more evidence before we can draw a conclusion. They may identify for whom the intervention works best, which version of the treatment works best, whether another option is just as effective, and whether it causes any unwanted effects.
DTA reviews evaluate how well diagnostic tests (index tests) identify or exclude a particular disease or condition (the target condition). We know that diagnostic tests make errors, even when they are correctly performed. There are two types of test errors: false positive test errors (the index test suggests the target condition is present when it is not) and false negative test errors (the test suggests the target condition is absent when it is not). Cochrane DTA reviews can cover all types of diagnostic tests, from antibody tests to X-rays, for any disease or condition. It is really important that diagnostic tests provide accurate results so that people can receive prompt treatment or take preventive measures if necessary, and to avoid unnecessary testing, treatment and anxiety
DTA reviews combine the results of all available test accuracy studies to determine the best possible estimate of the accuracy of an index test. Test accuracy studies most often report accuracy using sensitivity and specificity.
Sensitivity means the proportion of people who are correctly diagnosed by the index test. Specificity means the proportion of people without the target condition who are correctly identified by the index test. Therefore, the nearer the sensitivity and specificity are to 100%, the better the test.
An alternative way to report test accuracy is using positive and negative predictive values, which tell us about the usefulness of a positive index test result and a negative index test respectively – this helps patients understand how reliable their test results are. Predictive values measure the number of positive index test results that will be true positives and the number of negative index test results that will be true negatives. The nearer the positive and negative predictive values are to 100%, the better the test.
Cochrane DTA reviews are concerned with evaluating test accuracy. A new test may benefit patients because it offers an improvement in accuracy compared to existing tests. Alternatively, a new test may have similar accuracy to existing tests but with the advantage of being quicker, cheaper or easier to perform so that tests are available to more patients and results are obtained more quickly.