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To the Editor: The Research Letter by Dr Crichlow and colleagues¹ addresses an important problem faced by readers of medical literature: elusive Web-based references. In response to this challenge, the US National Library of Medicine (NLM) has recently begun to assign permanence levels to its Web-based documents² and to make these ratings available to users. Documents within this system are assigned 1 of the following ratings: permanent-unchanging content, permanent-stable content, permanent-dynamic content, or permanence not guaranteed. In the case of the first 3 (ie, permanence-guaranteed documents), the uniform resource locator (URL) link remains unchanged even if the contents are changed in the future, as may occur with documents having dynamic or stable content. This may serve as an example for other online sources of medical information. Journals may decide to stipulate that only "permanence-guaranteed" Web pages be cited as references and a brief description of the permanence level may . . . [Full Text of this Article]

Tamilarasu Kadhiravan, MD
kadhiravant@yahoo.co.in
Department of Medicine
All India Institute of Medical Sciences
Ansari Nagar
New Delhi, India

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