One of the main activities of clinicians, besides providing direct patient care, is documenting care in the electronic health record (EHR). Currently, clinicians document clinical findings and symptoms primarily as free-text descriptions within clinical notes in the EHR since they are not able to fully express complex clinical findings and nuances of every patient in a structured format [1, 2]. These free-text descriptions are, amongst other purposes, of interest for clinical research [3, 4], as they cover more information about patients than structured EHR data [5]. However, free-text descriptions cannot be readily processed by a computer and, therefore, have limited value in research and care optimization.

One method to make free text machine-processable is entity linking, also known as annotation, i.e., mapping free-text phrases to ontology concepts that express the phrases’ meaning. Ontologies are explicit formal specifications of the concepts in a domain and relations among them [6]. In the medical domain, SNOMED CT [7] and the Human Phenotype Ontology (HPO) [8] are examples of widely used ontologies to annotate clinical data. After the data has been annotated, it can be reused by clinicians to query EHRs [9, 10], to classify patients into different risk groups [11, 12], to detect a patient’s eligibility for clinical trials [13], and for clinical research [14].

Natural Language Processing (NLP) can be used to (semi-)automatically process free text. The literature indicates that NLP algorithms have been broadly adopted and implemented in the field of medicine [15, 16], including algorithms that map clinical text to ontology concepts [17]. Unfortunately, implementations of these algorithms are not being evaluated consistently or according to a predefined framework and limited availability of data sets and tools hampers external validation [18].

To improve and standardize the development and evaluation of NLP algorithms, a good practice guideline for evaluating NLP implementations is desirable [19, 20]. Such a guideline would enable researchers to reduce the heterogeneity between the evaluation methodology and reporting of their studies. Generic reporting guidelines such as TRIPOD [21] for prediction models, STROBE [22] for observational studies, RECORD [23] for studies conducted using routinely-collected health data, and STARD [24] for diagnostic accuracy studies, are available, but are often not used in NLP research. This is presumably because some guideline elements do not apply to NLP and some NLP-related elements are missing or unclear. We, therefore, believe that a list of recommendations for the evaluation methods of and reporting on NLP studies, complementary to the generic reporting guidelines, will help to improve the quality of future studies.

In this study, we will systematically review the current state of the development and evaluation of NLP algorithms that map clinical text onto ontology concepts, in order to quantify the heterogeneity of methodologies used. We will propose a structured list of recommendations, which is harmonized from existing standards and based on the outcomes of the review, to support the systematic evaluation of the algorithms in future studies.


This study consists of two phases: a systematic review of the literature and the formation of recommendations based on the findings of the review.

Literature review

A systematic review of the literature was performed using the Preferred Reporting Items for Systematic reviews and Meta-Analyses (PRISMA) statement [25].

Search strategy and study selection

We searched Scopus, IEEE, MEDLINE, EMBASE, the Association for Computing Machinery (ACM) Digital Library, and the Association for Computational Linguistics (ACL) Anthology for the following keywords: Natural Language Processing, Medical Language Processing, Electronic Health Record, reports, charts, clinical notes, clinical text, medical notes, ontolog*, concept*, encod*, annotat*, code, and coding. We excluded the words ‘reports’ and ‘charts’ in the ACL and ACM databases since these databases also contain publications on non-medical subjects. The detailed search strategies for each database can be found in Additional file 2. We searched until December 19, 2019 and applied the filters “English” and “has abstract” for all databases. Moreover, we applied the filters “Medicine, Health Professions, and Nursing” for Scopus, the filters “Conferences”, “Journals”, and “Early Access Articles” for IEEE, and the filter “Article” for Scopus and EMBASE. EndNote X9 [26] and Rayyan [27] were used to review and delete duplicates.

The selection process consisted of three phases. In the first phase, two independent reviewers with a Medical Informatics background (MK, FP) individually assessed the resulting titles and abstracts and selected publications that fitted the criteria described below.

Inclusion criteria were:

  • Medical language processing as the main topic of the publication

  • Use of EHR data, clinical reports, or clinical notes

  • Algorithm performs annotation

  • Publication is written in English

Some studies do not describe the application of NLP in their study by only listing NLP as the used method, instead of describing its specific implementation. Additionally, some studies create their own ontology to perform NLP tasks, instead of using an established, domain-accepted ontology. Both approaches limit the generalizability of the study’s methods. Therefore, we defined the following exclusion criteria:

  • Implementation was not described

  • Implementation does not use an existing established ontology for encoding

  • Not published in a peer-reviewed journal (except for ACL and ACM publications)

In the second phase, both reviewers excluded publications where the developed NLP algorithm was not evaluated by assessing the titles, abstracts, and, in case of uncertainty, the Method section of the publication. In the third phase, both reviewers independently evaluated the resulting full-text articles for relevance. The reviewers used Rayyan [27] in the first phase and Covidence [28] in the second and third phases to store the information about the articles and their inclusion. In all phases, both reviewers independently reviewed all publications. After each phase the reviewers discussed any disagreement until consensus was reached.

Data extraction and categorization

Both reviewers categorized the implementations of the found algorithms and noted their characteristics in a structured form in Covidence. The objectives of the included studies and their associated NLP tasks were categorized by way of induction. The results were compared and merged into one result set.

We collected the following characteristics of the studies, based on a combination of TRIPOD [21], STROBE [22], RECORD [23], and STARD [24] statement elements (see Additional file 3): year, country, setting, objectives, evaluation methods, used NLP systems or algorithms, used terminology systems, size of datasets, performance measures, reference standard, language of the free-text data, validation methods, generalizability, operational use, and source code availability.

List of recommendations

Based on the findings of the systematic review and elements from the TRIPOD, STROBE, RECORD, and STARD statements, we formed a list of recommendations. The recommendations focus on the development and evaluation of NLP algorithms for mapping clinical text fragments onto ontology concepts and the reporting of evaluation results.


The literature search generated a total of 2355 unique publications. After reviewing the titles and abstracts, we selected 256 publications for additional screening. Out of the 256 publications, we excluded 65 publications, as the described Natural Language Processing algorithms in those publications were not evaluated. The full text of the remaining 191 publications was assessed and 114 publications did not meet our criteria, of which 3 publications in which the algorithm was not evaluated, resulting in 77 included articles describing 77 studies. Reference checking did not provide any additional publications. The PRISMA flow diagram is presented in Fig. 1.

Fig. 1
figure 1

PRISMA flow diagram

The induction process resulted in eight categories and ten associated NLP tasks that describe the objectives of the papers: computer-assisted coding, information comparison, information enrichment, information extraction, prediction, software development and evaluation, and text processing. Our definitions of these NLP tasks and the associated categories are given in Table 1 and Table 2.

Table 1 Induced objective tasks with their definition and an example
Table 2 Induced objective categories with their definition and associated NLP task(s)

Table 3 lists the included publications with their first author, year, title, and country. Table 4 lists the included publications with their evaluation methodologies. The non-induced data, including data regarding the sizes of the datasets used in the studies, can be found as supplementary material attached to this paper.

Table 3 Included publications and their first author, year, title, and country
Table 4 Included publications and their evaluation methodologies

Table 5 summarizes the general characteristics of the included studies and Table 6 summarizes the evaluation methods used in these studies. In all 77 papers, we found twenty different performance measures (Table 7).

Table 5 Characteristics of the included studies
Table 6 Evaluation methods of the included studies
Table 7 Performance measures used in the included studies


In this systematic review, we reviewed the current state of NLP algorithms that map clinical text fragments onto ontology concepts with regard to their development and evaluation, in order to propose recommendations for future studies.

Main findings and recommendations

We identified 256 studies that reported on the development of such algorithms, of which 68 did not evaluate the performance of the system. We included 77 studies. Many publications did not report their findings in a structured way, which made it challenging to extract all the data in a reliable manner. We discuss our findings and recommendations in the following five categories: Used NLP systems and algorithms, Used data, Evaluation and validation, Presentation of results, and Generalizability of results. A checklist for determining if the recommendations are followed in the reporting of an NLP study is added as supplementary material to this paper.

Used NLP systems and algorithms

A variety of NLP systems are used in the reviewed studies. Researchers use existing systems (n = 29, 38%), develop new systems with existing components (n = 25, 33%), or develop a completely new system (n = 23, 30%). Most studies, however, do not publish their (adapted) source code (n = 57, 74%), and a description of the algorithm in the final publication is often not detailed enough to replicate it. To ensure reproducibility, implementation details, including details on data processing, and preferably the source code should be published, allowing other researchers to compare their implementations or to reproduce the results. Based on these findings, we formulated three recommendations (Table 8).

Table 8 Recommendation regarding the use of systems and algorithms

Used data

Most authors evaluate their algorithms with manual annotations (n = 40, 52%) and use data present in their institutions (n = 55, 71%). However, it is not clear what these datasets consist of. Most studies describe the data as ‘reports’, ‘notes’, or ‘summaries’, but do not list the contents or example rows from the dataset. It is, therefore, not clear what types of patients and what specific types of data are included, making the study hard to reproduce. Finally, we found a wide range of dataset sizes and formats. The training datasets, for example, ranged from 10 clinical notes to 636.439 discharge reports. The use of small datasets can result in an overfitted algorithm that either performs well on the dataset, but not on an external dataset, or performs poorly, for the algorithm was only trained on a specific type of data. More difficult recognition tasks require more data, and therefore sample size planning is recommended [106]. To improve the description and availability of datasets used in NLP studies, we formulated three recommendations (Table 9).

Table 9 Recommendation regarding the use of data

Evaluation and validation

Evaluation of the algorithm determines its performance on the dataset, and validation determines if the algorithm is not overfitted on that dataset and thus if the algorithm might work on other datasets as well. Over one-fourth of the studies (n = 68, 27%) that we identified did not evaluate their algorithms. In addition, 22 included studies (29%) did not validate the developed algorithm. A statement claiming that an algorithm can be used in clinical practice can be questioned if the algorithm has not been evaluated and validated. Across all studies, 20 performance measures were used. To harmonize evaluation and validation efforts, we formulated three recommendations (Table 10).

Table 10 Recommendation regarding the evaluation and validation of Natural Language Processing algorithms

Presentation of results

Authors report the evaluation results in various formats. Only twelve articles (16%) included a confusion matrix which helps the reader understand the results and their impact. Not including the true positives, true negatives, false positives, and false negatives in the Results section of the publication, could lead to misinterpretation of the results of the publication’s readers. For example, a high F-score in an evaluation study does not directly mean that the algorithm performs well. There is also a possibility that out of 100 included cases in the study, there was only one true positive case, and 99 true negative cases, indicating that the author should have used a different dataset. Results should be clearly presented to the user, preferably in a table, as results only described in the text do not provide a proper overview of the evaluation outcomes (Table 11). This also helps the reader interpret results, as opposed to having to scan a free text paragraph. Most publications did not perform an error analysis, while this will help to understand the limitations of the algorithm and implies topics for future research.

Table 11 Recommendation regarding the presentation of results

Generalizability of results

88% of the studies did not perform external validation (n = 68). Of the studies that claimed that their algorithm was generalizable, only 22% (n = 5) assessed this claim through external validation. However, one cannot claim generalizability without testing for it. Moreover, in 19% (n = 3) of the cases where external datasets were used, the datasets were not referenced and only listed in the text of the article, making it harder to find the used data and reproduce the results. Algorithm performance should be compared to that of other state-of-the-art algorithms, as this helps the reader decide whether the new algorithm could be considered useful for clinical practice. However, only 24 studies (31%) made this comparison, and four of those studies (17%) tested the performance difference for statistical significance. We also found that the authors’ descriptions of generalizability are rather ambiguous and unclear. We formulated five recommendations regarding the generalizability of results (Table 12).

Table 12 Recommendation regarding the generalizability of results


Our study has three main strengths: First, to our knowledge, this is the first systematic review that focuses on the evaluation of NLP algorithms in medicine. Second, we used a large number of databases for our search, resulting in publications from many different sources, such as medical journals and computer science conferences. Third, we used existing statements and guidelines and harmonized them to induce our findings and used these findings to propose a list of recommendations.


Several limitations of our study should be noted as well. First, we only focused on algorithms that evaluated the outcomes of the developed algorithms. Second, the majority of the studies found by our literature search used NLP methods that are not considered to be state of the art. We found that only a small part of the included studies was using state-of-the-art NLP methods, such as word and graph embeddings. This indicates that these methods are not broadly applied yet for algorithms that map clinical text to ontology concepts in medicine and that future research into these methods is needed. Lastly, we did not focus on the outcomes of the evaluation, nor did we exclude publications that were of low methodological quality. However, we feel that NLP publications are too heterogeneous to compare and that including all types of evaluations, including those of lesser quality, gives a good overview of the state of the art.


In this study, we found many heterogeneous approaches to the development and evaluation of NLP algorithms that map clinical text fragments to ontology concepts and the reporting of the evaluation results. Over one-fourth of the publications that report on the use of such NLP algorithms did not evaluate the developed or implemented algorithm. In addition, over one-fourth of the included studies did not perform a validation and nearly nine out of ten studies did not perform external validation. Of the studies that claimed that their algorithm was generalizable, only one-fifth tested this by external validation. Based on the assessment of the approaches and findings from the literature, we developed a list of sixteen recommendations for future studies. We believe that our recommendations, along with the use of a generic reporting standard, such as TRIPOD, STROBE, RECORD, or STARD, will increase the reproducibility and reusability of future studies and algorithms.