The hazards of quick searches

A sound systematic review can be performed only if most or all of the available data are examined. Simply performing a quick Medline search using "clinical trial" as publication type is rarely adequate because complex and sensitive search strategies are needed to identify all potential trials, and because clinical trials that are published in a journal not listed by Medline will be missed. Potential sources for finding studies about treatment include: The Cochrane Central Register of Controlled Trials (CENTRAL), which is part of the Cochrane Library, Medline, Embase, bibliographies of studies, review articles and textbooks, symposia proceedings, pharmaceutical companies and contacting experts in the field. Searching the literature is discussed in more detail in Chapter 6.

The CENTRAL is a database of over 300 000 controlled clinical trials and is now the largest single and most complete database of clinical trials worldwide. The CENTRAL has been compiled through several complex searches of the Medline and Embase databases, and by hand searching many journals, a process that is quality controlled and monitored by the Cochrane Collaboration in Oxford, UK. Hand searching of journals to identify controlled clinical trials and randomised controlled clinical trials was undertaken because members of the Cochrane Collaboration noticed that many trials were incorrectly classified in the Medline database. As an example, Adetugbo et al. hand searched the Archives of Dermatology from 1990 to 1998 and identified 99 controlled clinical trials. Nineteen of the trials were not classified as controlled clinical trials in Medline and 11 trials that were not controlled clinical trials were misclassified as controlled clinical trials in Medline.9

Medline is the National Library of Medicine's (NLM) bibliographic database covering the fields of medicine, nursing, dentistry, veterinary medicine, the healthcare system and the preclinical sciences. The Medline file contains bibliographic citations and author abstracts from approximately 3900 current biomedical journals published in the US and 70 other countries. The file contains approximately 9 million records dating back to 1966.10

Medline searches have inherent limitations that make their reliability less than ideal.11 For example, Spuls et al. conducted a systematic review of systemic treatments for psoriasis.12 Treatments analysed included UVB, PUVA, methotrexate, ciclosporin A and retinoids. The authors used an exhaustive strategy to find relevant references, including Medline searches, contacting pharmaceutical companies, polling leading authorities, reviewing abstract books of symposia and congresses, and reviewing textbooks, reviews, editorials, guideline articles and the reference lists of all papers identified. Of 665 studies found, 356 (54%) were identified by a Medline search (range 30-70% for different treatment modalities).12

Embase is Excerpta Medica's database covering drugs, pharmacology and biomedical specialties.1 Embase has better coverage of European and non-English language sources and may be more up to date than Medline.1 The overlap in journals covered by Medline and Embase is about 34% (range 10-75% depending on the subject).1,13,14

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