Once you have done the first version of the search, you may find that you have too many results or not many results at all.
Too many results?
If you have too many results it is likely that your search was too broad and that rather than manually checking each reference, using the filters provided by the search engines could help refine the results. Some possibilities for refining the search are to limit it to a specific language (English, for example), or to just searching the abstract and title, rather than the full text.
Too few results?
This might be what you expected and could be the basis for the systematic review, highlighting the need for more studies in a particular area. However, you may want to broaden your search if you feel that not all relevant manuscripts have been captured. This could be done by including additional keywords, or removing limits (e.g. looking for all studies and not just those in humans).
What if the number of results seems fine?
Once you are happy that the search has captured all the appropriate results, you will want to save a list of the results, including references, title and abstract, as well as recording the initial number of results found. You will need to keep track of this for the PRISMA flowchart. These results should then be distributed to the individuals that have been allocated as reviewers and they should review each abstract and decide whether it meets the inclusion criteria defined for the review. These criteria should have been defined before the search and it is important that both reviewers use them. The reason why each abstract is rejected should also be recorded. This can be done according to the PICOS, with abstracts rejected according to not containing appropriate participants, intervention, comparator, outcome or setting, or for another reason which should be recorded.
In a perfect world both reviewers will select the same abstracts at this stage, however, if they do not, an additional reviewer should arbitrate whether the abstract should be kept. If there is confusion, the abstract should be kept, for full text review (the next step).
After the abstracts have been filtered, should we start writing?
No. You will want to get the full version of each reference and these will also need to be reviewed, in a similar manner to the abstracts, to ensure that no inappropriate references are included. At this stage, you can note any manuscripts in the references that also look appropriate for the systematic review. This will enable you to check that they have been included and if not they can be added (and noted on the PRISMA flowchart).
This process of filtering can be long and tedious, however, it should enable all the relevant literature for the systematic review to be identified. It is at this stage that the writing can commence!