Creating Effective Search Strategies is a Lot Like Freshman Algebra

As we discussed in my last post, brainstorming synonyms for each of your search concepts is critical for capturing the myriad ways that authors express those concepts in the medical literature. But now that we have our synonym sets, how do we string these together to form search strategies? Just like in Freshman Algebra, there is a very precise logic that must be employed to instruct the search engine about how these terms relate to one another and how to perform the search. That’s where our “Boolean operators,” AND, OR, and (sometimes) NOT come into play.

Constructing Search Strings

Let’s get back to our original research question: “What are the safety issues around using VEGF inhibitors for neovascular age-related macular degeneration?” We identified the main search concepts as: “safety,” “VEGF inhibitors,” and “neovascular age-related macular degeneration.” Beginning with the concept of “safety,” let’s think about how to link our synonyms together. Here’s the list:

• “Safety”[Mesh]
• Adverse event
• Complication
• Side effects
• Eye redness
• Floaters
• Blurry vision
• Detached retina

When setting up a search string for synonyms, the Boolean operator will be “OR,” as in, either “this” OR “this.” And in order to indicate that the string is a set, it’s necessary to put parentheses around it. Similar to algebra, parentheses indicate that what’s inside them must be considered before moving to what lies outside of them.? And exact phrases must be in quotes. Here’s what a PubMed-ready search string would look like:

(“Safety”[Mesh] OR “adverse event” OR complication OR “side effects” OR “eye redness” or floaters OR “blurry vision” OR “detached retina”)

Next, VEGF inhibitors. This set is going to be a little more complex because in MeSH terms, this phrase actually represents two concepts: vascular endothelial growth factors and inhibitors. And while there is a MeSH term for “vascular endothelial growth factors,” there isn’t one for “VEGF inhibitors.” The way we deal with this is to “AND” the MeSH term with the word, inhibitors, and surround it with parentheses (so it will be considered a mini-set). AND-ing terms means that both concepts must be present. Then we’ll “OR” the other synonym phrases to the string and close with a final parenthesis, indicating the whole string is a set:

?((“Vascular Endothelial Growth Factors”[Mesh] AND inhibitors) OR “VEGF inhibitors” OR “Anti-VEGF treatments”)

And finally, neovascular age-related macular degeneration. The closest MeSH term for this is “macular degeneration.” Definitely not an exact match, since there are different types of macular degeneration, so we’ll need to “AND” the MeSH term with the additional qualifiers of either “neovascular” or “wet.” Get your eyeglasses out because there are going to be a lot of parentheses in this string! Every opening parenthesis must be matched by a closing parenthesis! Here’s what it would look like:

?(((“Macular Degeneration”[Mesh] AND (wet OR neovascular)) OR (maculopathy AND (wet OR neovascular)) OR “neovascular age-related macular degeneration” OR “wet age-related macular degeneration” OR nAMD OR wAMD)

Combining Search Strings to Create the Search Strategy

Just like in algebra, every element in this “equation” has a very important function! Here’s the entire search strategy with each individual set linked together with “ANDs”:

“Safety”[Mesh] OR “adverse event” OR complication OR “side effects” OR “eye redness” or floaters OR “blurry vision” OR “detached retina”)

AND

((“Vascular Endothelial Growth Factors”[Mesh] AND inhibitors) OR “VEGF inhibitors” OR “Anti-VEGF treatments”)

AND

(((“Macular Degeneration”[Mesh] AND (wet OR neovascular)) OR (maculopathy AND (wet OR neovascular)) OR “neovascular age-related macular degeneration” OR “wet age-related macular degeneration” OR nAMD OR wAMD)

Kind of involved, huh? And one misplaced OR or AND changes the entire meaning of the search strategy. (Try it, you’ll see!) But if you plug this exact strategy into PubMed, you will see some results that are right on target. That’s not to say that every citation will be a “direct hit.” It will still be necessary to curate your results, but you should find many relevant abstracts with this strategy.

Next time, just for kicks, let’s see what happens to your results when you inadvertently AND when you should've OR-ed!