Medical Evidence: Where to Turn?

Medical Evidence: Where to Turn?

Not all evidence is created equal when it comes to making health care decisions.

Steve is a 59-year-old active man who has developed an inguinal (groin) hernia that needs repairing. Steve hasn’t had surgery before, doesn’t have a relationship with a surgeon and is anxious about the whole matter. Having heard about both open and laparoscopic surgery, he would like to visit the surgeon he was referred to armed with some knowledge of the benefits and drawbacks of the choices he faces.

Choosing the best surgical approach for Steve involves carefully integrating the best medical evidence (about the techniques) with the clinical expertise of the surgeon and Steve’s personal preferences about trade-offs involving safety, anesthesia, inpatient versus outpatient, recovery time, likelihood of recurrence and such. This decision-making starts with the evidence and is termed, “evidence-based medical practice.”

Isn’t all medicine based on evidence? In fact, no. Many common practices are rooted in long-standing traditions and beliefs. Health care providers and patients alike may believe, for example, that knowing is always better than not knowing, or that early detection and early treatment always improve health outcomes. For many conditions these practices have not been tested.

MEDICAL DECISION-making based on evidence is relevant for less urgent health questions than the ones mentioned above. You may have heard that antioxidants are good for you, but that how you prepare broccoli or garlic affects the antioxidant content. Or you may have heard that antioxidants can be harmful and wonder about emphasizing them in your regular diet. You may have heard that various foods or supplements can raise or lower your risk of cancer and other diseases. Nutritional recommendations for prevention of aging and disease abound in our culture — and yet, the evidence for much of this is shaky. Which ones should you follow?

Many consumers want to participate in their own health care by learning about the current recommendations for prevention and treatment. They want to know what the evidence (scientific literature) actually supports. Hearing about the latest research finding on the radio or reading about prevention measures in a magazine is not enough for those who want to make informed decisions and choices. Regrettably, not all the recommendations touted are based on good evidence.

The overall level (quality) and strength of the evidence is key. Evidence has differing meaning depending on its form and strength. Evidence is not the same as proof. It can be weak or point in the wrong direction. It’s not enough to hear, “evidence indicates that…” It’s very important what kind (level) of evidence it is, how strong it is and how consistent it is.

SOME TYPES of scientific studies are not intended to yield conclusive evidence, but, rather, to provide clues or support about what warrants further study. These studies are published so that other researchers in the field can benefit from the results, but their “evidence” is not at a level for guiding patient care or public health policy.

So, consider all the evidence. Lots of new reports of research findings are carried in the media — both broadcast and printed sources. However, rarely can a single medical research study, even if new or large, be definitive all by itself. Research results must be considered in the light of what is known, not known, or controversial in that field in order to understand what the new research outcomes mean. The scientific literature is large, complex, growing fast and not organized. Understanding the meaning of a new report without knowing the whole field is difficult for anyone, even physicians and scientists.

So how can the health nut or interested consumer get the most complete information about health measures such as screening for colon cancer, taking aspirin to prevent cardiovascular disease, or dietary measures for preventing cancer? One way is starting with sources that gather and review all the available evidence about a particular topic and that then issue a report, statement, recommendation or guideline based on that analysis.

WHERE TO GO for evidence-based information about prevention and treatment. On the Internet, you can find evidence reviews, recommendations based on evidence or other documents (sometimes called guidelines, clinical practice guidelines, practice parameters, clinical policies, position statements, reports). They are posted by the original source of the work, or are available through second parties who compile lists of evidence-based documents. Some sources use systematic evidence reviews, which are the most formal, comprehensive, unbiased ones and have the highest standards. We expect these reviews to provide the most valid product that the evidence justifies.

Some of these documents are intended for health-care professionals, others for patients/consumers or both. You may not find a systematic review of the effects of different ways to cook broccoli, but you can find reviews of acupuncture for migraine headaches and for depression, postmenopausal hormone therapy, screening for prostate cancer, diet and risk of heart disease, prevention of breast cancer, St John’s Wort for depression, or techniques for repairing hernias.

A readily available source of information on a variety of topics is the Agency for Healthcare Research and Quality, or AHRQ, an agency of the U. S. Department of Health and Human Services ( It has a wealth of information oriented to either health care professionals or interested consumers, though all of it is available to everyone. The home page has a heading entitled, “Consumers and Patients.” Another heading, “Clinical Information,” has several sources of information, including evidence reports and technology assessments. Selecting “Preventive Services” will take you to the U. S. Preventive Services Task Force, an “independent panel of experts in primary care and prevention that systematically reviews the evidence of effectiveness and develops recommendations for clinical preventive services.”

Also under “Clinical Information” at the AHRQ site is a link to the “National Guideline Clearinghouse” (NGC,, a “comprehensive database of evidence-based clinical practice guidelines and related documents.” This very extensive compilation of documents from many sources has a glossary of terms and is updated weekly. While all guidelines listed by the NGC must meet specific criteria to be included, it includes only those submitted to it. Though it is available to the public, the content is intended for expert audiences.

ANOTHER IMPORTANT source is the Cochrane Collaboration (, an international, not-for-profit organization that carefully reviews health care issues. Its main product is a database of regularly updated, evidence-based, systematic reviews of interventions including preventions. The database is available through the Cochrane Library that makes abstracts of its systematic reviews available to the public ( or Full text is available by subscription, or free to the public in some countries.

Of the many reviews in the Cochrane database, nearly 200 address complementary/alternative medicine interventions. For those people who would like to see the complementary/alternative medicine topics separated out from the others, a list (no text) is at

The National Cancer Institute of the National Institutes of Health (NIH) maintains PDQ (Physician Data Query), which is a comprehensive cancer database containing peer-reviewed summaries on cancer prevention, treatment and other topics. Though it evaluates the evidence, it does not make recommendations ( NCI’s PDQ provides separate information for health professionals and for patients.

INVESTIGATING HEALTH information on the Internet or elsewhere may raise two issues. One is how current the information is, and the other is how to look at inconsistencies among different sources.

Is the information up to date? You will usually find dates accompanying the evidence summaries, recommendations and practice guidelines found at these and other websites. Inevitably, some time will have passed since they were issued. Though some sources like PDQ, AHRQ, NGC, and Cochrane work actively to update their postings, some delay is likely even there. Consumers may wonder whether these documents are still relevant, especially if several years have passed. Should you worry?

The answer is, “It depends.” Medical knowledge generally advances slowly and in small steps. Even when newer information becomes available, it may not warrant revision of an earlier statement or summary. For a single new study to dramatically alter the public health approach to an issue is unusual. It may take quite a bit of newer, high quality evidence to change a recommendation or guideline.

Sometimes, where evidence is lacking or conflicting, the issues are controversial and opinions may be diverse. In that setting, when a new, long-awaited study is completed, the results may indeed provide a clearer picture, leading to important revisions in thinking. Then, new statements are likely to be issued. For example, currently there is controversy about whether screening for prostate cancer with serum prostate-specific antigen (PSA) leads to longer life. Two very large, carefully designed clinical trials, one in the U. S. and one in eight countries in Europe, have been underway for over 10 years. Results from these trials are expected to clarify this controversy, and revisions of existing statements and recommendations are likely to follow. When this happens, considerable publicity is likely even before formal recommendations are revised.

How consistent are the recommendations? One group may recommend regular screening for a disease, while another may say that evidence doesn’t support regular screening at this time. One source may state that increased consumption of certain foods or supplements may increase or decrease risk of a particular cancer, while another source does not agree.

Which is correct? Inconsistent recommendations may occur because some sources haven’t used a systematic review of the evidence as a basis for the statement. It can also happen because the evidence itself is inconsistent, flawed, or just unclear. Furthermore, honest differences of opinion among experts do occur. For example, some may judge that the evidence for benefit is good, while others may hold that the size of the health benefit is too small compared to the risk of harm.

WHICH SOURCES should you use? The ones mentioned above offer high quality, evidence-based statements about important health issues. Many evidence-based documents exist, but be aware that not all published guidelines are based on an unbiased, systematic evidence review. Some may be based on less thorough considerations of the evidence.

Some help with this variation in quality is available. EBMsources Directory of Evidence-Based Information Websites (Laval University Department of Family Medicine, Quebec, Canada) is a list of over 100 websites that is updated periodically ( A special feature of the EBMsources directory is that each site is described as well as scored for quality using a formal appraisal process. This directory provides quick, direct access to many sites with some information about both content and quality.

Steve went to the Internet seeking evidence about the risks and benefits of various approaches to hernia repair. The Cochrane Library has at least three reviews about techniques for hernia repair and one about antibiotic prophylaxis for repair. The National Guideline Clearinghouse lists several reviews about surgical repair. The EBMsources directory has ASERNIP-S, a Royal Australasian College of Surgeons site with two versions of a recent review addressing laparoscopic versus open repair. This is just a sampling.

These resources may make your decision-making easier and more informed. Hope this helps — and happy hunting.