Dear Sir!

First of all I would like to describe the major difference of narrative literature review and systematic review. All type of studies, including descriptive and analytic, can be used as the sources of narrative review. in systematic review, only analytic studies, including analytic cross-sectional, case-control, cohort and RCT, are usually used.

In the field of medicine, all types of reviews can be performed for the purpose of diagnosis, treatment and prevention.

Systematic reviews need strict systematic steps by following PRISMA. Try to check your narrative review according to PRISMA. And if its items were realized, you can convert narrative review to systematic.

Indeed, the researcher should follow the PRISMA guideline step by step in systematic review. Usually, descriptive studies can not be used in systematic review as sources

You cannot 'convert' a narrative review into a systematic review, but you can do a systematic review on the same topic. Effectively, you would need to start from scratch, as systematic reviews use a structured search and review process, but as you will already be familar with the topic area, this will be easier for you.

Also, you can include any type of study in a systematic review, the term 'systematic review' merely refers to the structured search and review process.

It is not possible to convert a narrative review into a systematic review because a systematic review will require a detailed step-by-step review protocol which outlines the following:

[1]what is the review question?

[2]where to search and how to search?

[3]strict eligibility criteria to ensure that only eligible studies will be included in the review

[4]how to perform data synthesis (i.e. either by meta-analysis or by narrative synthesis).

Furthermore, a systematic review involves collating evidence by using all of the eligible and critically appraised literature available on a certain clearly-defined topic. A narrative review, on the other hand, merely aims at identifying and summarizing what has previously been published on a broader scope of the subject.

Therefore, the major difference between a narrative review and a systematic review is that the main purpose of a narrative review is to deepen the understanding in a certain research area. This is in contrast to the results of a systematic review, which can provide the most valid evidence to guide clinical decision-making and inform policy development. Therefore, some of the high-quality systematic reviews have now become the gold standard in evidence-based medicine.

Although a majority of systematic reviews are based on interventional clinical trials, it is still possible for a systematic review to be based on observational studies such as cross-sectional studies, etc..

In fact, some of the systematic reviews are actually including meta-analysis of observational studies.

Since you already have written up a narrative review, therefore you should use it as a basis to formulate the overall rationale for preparing a systematic review protocol and then register it in PROSPERO, if you have found that no such reviews have already been written up by other authors.

This is because some of the information in the narrative review can be very useful for providing a supporting framework of a systematic review protocol.

However, in order to write up a systematic review protocol, it is first necessary to gather a team of like-minded authors, who are committed towards the writing up of both the systematic review protocol as well as the full systematic review (incorporating all the review findings).

Once you have formed a team of like-minded authors, then all the authors can either decide to register this new systematic review protocol in PROSPERO, if no such protocols and/or reviews have ever been written up by other authors before.