It is common for multiple guidelines to be made by different developers on the same topic. Problems arise though when different guidelines make differing recommendations. Which one should you believe?

The guideline development process is complex. The Institute of Medicine  published a framework for trust worthy guideline development as have other groups. Guidelines could differ by performing any of the steps along the development pathway differently from each other. A lot of judgment and decision-making goes into developing guidelines explaining why each of the steps could be performed differently.

What are some of the main reasons why different guidelines might have different recommendations?

1. They attempt to provide guidance on different clinical questions. The guidelines just address different things even though they seem similar. Obviously you would choose the one that best fits the clinical scenario.
2. Different evidence bases were used to make recommendations. It could be that one guideline is newer than another and contains more updated evidence. What is more problematic is when guidelines are released at about the same time but have differing evidence bases. As I mentioned earlier a lot of judgments and decisions are made during the guideline development process. An important one is which studies to include/exclude from the evidence review. This is more subjective than you might realize. It’s easy to develop study selection criteria to include only that evidence supporting your point of view while excluding that which doesn’t. Pick the guideline with the most comprehensive literature search. Also make sure the exclusion criteria make sense and don’t just help to support a biased point of view.
3. Different outcomes were considered. Recommendations are made to improve care with the hope of improving some outcome.  It could be that one guideline focused on a surrogate marker (e.g. LDL levels) while another focused on hard clinical outcomes (MI and stroke event rates). Go with the one focused on hard clinical outcomes.
4. Values, biases and conflicts of interest of the guideline developers is probably the main reason for disparate recommendations. Almost every guideline panel is biased in some way. The best you can hope for is that multiple biases and conflicts of interest are represented and essentially cancel each other out (by assembling a multidisciplinary panel to develop the guideline). Just disclosing conflicts of interest does nothing to lessen their impact. Moving from evidence to recommendations involves value judgments. The recommendations in the guideline are shaped by these values. Different groups will weigh the benefits and harms differently even if the same exact evidence base is reviewed. One need look no further than breast cancer screening guidelines to find very different values structures between cancer organizations and the USPSTF.  Unfortunately the values structure of the panel is usually not explicitly stated and one must infer it from the recommendations that are made.   You should choose the guideline that best matches the values of the patient.

I tried to give a very basic overview of why guidelines can differ. There are other reasons but these are the main ones. Clinicians should look for and use guidelines that are trustworthy. They should not follow recommendations uncritically but seek to understand what values and judgments shaped the recommendations.