"Evidence-based health care" means health care that is based on the best scientific research, combined with what's important to patients, and doctors' knowledge and experience. People are usually surprised to learn that not all medical care is based on the best evidence, but unfortunately it's true.
Much of medical practice is handed down from generation to generation of clinicians, and is accepted as knowledge, but has not been shown to be both effective and safe by the most rigorous research. And because health care always comes with a possible mix of benefits and harms, it's important to understand what those benefits and risks are.
One example that shows why evidence-based health care is so important was the introduction of high-dose chemotherapy with bone marrow or stem cell transplantation for the treatment of metastatic breast cancer in the mid-1980s and 1990s. Early research results had indicated that patients with metastatic breast cancer who had undergone these treatments fared better than those given standard chemotherapy drugs. However, more rigorous research studies later found that this new treatment was not more effective than the standard chemotherapy drugs, did not increase survival among women with metastatic breast cancer, and in fact, had significant harms. Because the health care system (doctors, insurance companies, and patients) rushed to accept this new treatment without a strong evidence base to support its effectiveness and understand the risks, tens of thousands of women were harmed.
New studies on breast cancer come out literally every day. But they don't all carry the same weight, or mean that you should act on the results. This is not only because some studies are better than others. It's mostly because useful knowledge grows very slowly. In fact, most research that is published or presented at meetings is really part of a conversation among scientists. Another way to think of a single study is as one stitch in a whole fabric of what's known about a certain topic.
But of course, we do hear about new studies in the news all the time. So how do we figure out whether the results of a study are worth acting on? Read KnowBreastCancer.org and join the conversation!
There are thousands of studies of different types that have led to what we know about breast cancer. These include everything from chemistry studies to purify drugs from natural products, to studies in animals, to randomized trials – which are actually experiments – in people. You can think of levels of evidence as a pyramid, with lab studies at the bottom (there are a lot of them), observational studies in the middle (somewhat fewer), randomized trials near the top, and finally systematic reviews (very few) at the very top.
Systematic reviews are the strongest level of evidence because they summarize multiple studies on a topic, taking into account the different study designs, their strengths and limitations, and the quality of both the study design and how the research was conducted. Systematic reviews provide the broadest perspective on what we know about a particular topic, and what remains unknown and requires more research.
One example that shows how different types of research studies provide different levels of evidence, and potentially different answers to the same question, is question of whether abortion increases a woman's risk for breast cancer. Several past case-control studies have suggested that abortions may be associated with small increases in the risk of breast cancer. However, these studies were all criticized for their potential for reporting bias – women who have breast cancer may be more likely to give accurate reports of their abortion histories than women who do not have breast cancer. Prospective cohort studies found no association between abortion and breast cancer risk, and this type of study design is considered to be stronger since it is based on medical registry data and not individual self-report, and so the potential for reporting bias is much smaller.