ClinicalTrials.gov search query (Nov 16, 2014): Closed Studies | Interventional Studies | Germany | United Kingdom | France | Phase 3 | received from 01/01/2002 to 01/01/2014
Results: 4496 studies – 67.2% of studies registered in Europe using the above search criteria
View the updated map for this search query on ClinicalTrials.gov
HOW TO ESTIMATE THE PERCENTAGE OF UNPUBLISHED TRIALS?
1. Download the Search Results
2. Extract the zip file (search_result.zip).
3. Open the archive (study_fields.csv) using Google Spreadsheet or Microsoft Excel.
4. Copy and paste NCT numbers in a different column – replace the letter “N” for “OR N”
5. Copy & Paste expert search terms and perform a PubMed Search Query
6. Analyze the results:
– Records not found (PubMed): 1189 out of 1995 ClinicalTrials.gov records (60%)
– 806 ClinicalTrials.gov records found —> matching 1349 PubMed articles
7. Perform a second search query on Embase for records not found in PubMed (n = 1189)
Filter: publication type = article
Results = 332 articles
FINAL NOTES & COMMENTS:
Manual revision of identified articles in PubMed and Embase is necessary to estimate precisely the percentage of published and unpublished clinical trials. Some articles reporting the results of trials not found on PubMed and Embase can be identified using other academic databases (e.g., EBSCO Discovery and Google Scholar) and performing search queries with different registry fields (e.g., other IDs). Nonetheless, I know from previous analysis that PubMed & Embase can identify the majority of published clinical trial registries.
Unpublished US phase 3 ClinicalTrials.gov registries (intervention: drugs and biologicals) completed before January 2011 and registered between Jan 1st, 2002 to Jan 1st, 2014.(1, 2)
Total number of registries: 2957
1. Published 48.2% (n = 1450)
– PubMed 80% (n = 1160)
– Embase 18.5% (n = 269)
– EBSCO Discovery 1.5% (n = 21)
2. Unpublished 50.8% (n = 1507)
It is important to understand that the estimation of unpublished studies in a sample of clinical trial registries is only a minor part of the total analysis. Other variables such as the time of registration and the beginning of patient recruitment are also very important (see Table Below).
Other variables such as the location of patient recruitment can be also analyzed. For example, agomelatine registered studies (Figure below). However, the analysis of locations is easier to carry out downloading registries from WHO ICTRP (ClinicalTrials.gov locations registry fields are not available for download).(3)
With this information it can be estimated that the percentage of unpublished phase 3 clinical trials, evaluating drugs and biologicals, recruiting patients in locations from three different European countries (i.e., Germany, United Kingdom, and France), registered between January 1st 2002 and January 1st 2014, and completed before January 1st, 2011 is…
Around 50% ——-> UNPUBLISHED
A more precise estimate will be disclosed after manual verification of published and unpublished clinical trial registries.