Below is a list of various publication types as defined by the National Library of Medicine (NLM). These descriptions are used as Limits when searching PubMed.

Abstracts
Works consisting of lists of publications on a subject and that provide full annotated bibliographical information together with substantive summaries or condensations of the facts, ideas, or opinions presented in each publication listed. (From LC Subject Cataloging Manual)

Case Reports
Clinical presentations that may be followed by evaluative studies that eventually lead to a diagnosis.

Classical Article

Works of a current presentation of a previously printed seminal article marking a milestone in the history of medicine or science and is usually accompanied by introductory remarks heralding its reprinting, often on the anniversary of its original publication or on an anniversary of the author’s birth or death. It is usually reprinted in full, with complete bibliographical reference to the original appearance.

Clinical Conference
Work that consists of a conference of physicians on their observations of a patient at the bedside, regarding the physical state, laboratory and other diagnostic findings, clinical manifestations, results of current therapy, etc. A clinical conference usually ends with a confirmation or correction of clinical findings by a pathological diagnosis performed by a pathologist. “Clinical conference” is often referred to as a “clinico-pathological conference.”

Clinical Trial
Work that is the report of a pre-planned clinical study of the safety, efficacy, or optimum dosage schedule of one or more diagnostic, therapeutic, or prophylactic drugs, devices, or techniques in humans selected according to predetermined criteria of eligibility and observed for predefined evidence of favorable and unfavorable effects. While most clinical trials concern humans, this publication type may be used for clinical veterinary articles meeting the requisites for humans. Specific headings for specific types and phases of clinical trials are also available.

Comparative Study
Comparison of outcomes, results, responses, etc. for different techniques, therapeutic approaches or other inputs

Consensus Development Conference, NIH
Work consisting of summary statements, from a conference sponsored by NIH, representing the majority of current opinion of physicians, scientists, and other professionals on a selected subject.

Controlled Clinical Trial
Work consisting of a clinical trial involving one or more test treatments, at least one control treatment, specified outcome measures for evaluating the studied intervention, and a bias-free method for assigning patients to the test treatment. The treatment may be drugs, devices, or procedures studied for diagnostic, therapeutic, or prophylactic effectiveness. Control measures include placebos, active medicine, no-treatment, dosage forms and regimens, historical comparisons, etc. When randomization using mathematical techniques, such as the use of a random numbers table, is employed to assign patients to test or control treatments, the trial is characterized as a RANDOMIZED CONTROLLED TRIAL.

Formularies
Works that consist of lists of drugs or collections of recipes, formulas, and prescriptions for the compounding of medicinal preparations

Guideline

Work consisting of a set of statements, directions, or principles presenting current or future rules or policy Guidelines may be developed by government agencies at any level, institutions, organizations such as professional societies or governing boards, or by the convening of expert panels. The text may be cursive or in outline form, but it is generally a comprehensive guide to problems and approaches in any discipline or activity. This concept relates to the general conduct and administration of health care activities rather than to specific decisions for a particular clinical condition. For that aspect, PRACTICE GUIDELINE is available.

Meta-Analysis
Works consisting of studies using a quantitative method of combining the results of independent studies (usually drawn from the published literature) and synthesizing summaries and conclusions which may be used to evaluate therapeutic effectiveness, plan new studies, etc. It is often an overview of clinical trials. It is usually called a meta-analysis by the author or sponsoring body and should be differentiated from reviews of literature.

Multicenter Study
Work consisting of a controlled study executed by several cooperating institutions

Randomized Controlled Trial
Work consisting of a clinical trial that involves at least one test treatment and one control treatment, concurrent enrollment and follow-up of the test- and control-treated groups, and in which the treatments to be administered are selected by a random process, such as the use of a random-numbers table.

Review
An article or book published after examination of published material on a subject. It may be comprehensive to various degrees and the time range of material scrutinized may be broad or narrow, but the reviews most often desired are reviews of the current literature. The textual material examined may be equally broad and can encompass, in medicine specifically, clinical material as well as experimental research or case reports. State-of-the-art reviews tend to address more current matters. A review of the literature must be differentiated from HISTORICAL ARTICLE on the same subject, but a review of historical literature is also within the scope of this publication type.