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Abbreviations, contractions, acronyms, initialisms: What do you need to know as a medical writer?

‘Abbreviations’ is often used as a blanket term to cover abbreviations, contractions, acronyms and initialisms. So, what’s the difference?

Abbreviation: the shortened form of a word, where start and/or end letters are removed.

  • etc. (et cetera/etcetera)*

  • exam (examination)

  • flu (influenza)

  • vol (volume)

Contraction: the shortened form of a word or phrase, where middle letters are lost but the start and end are retained.

  • Dr (doctor)

  • govt (government)

  • no. (numero, number)

  • vs (versus)†

Acronym: the shortened form of a phrase, usually (but not always) using the first letter of each word, and pronounced as a single word. Not necessarily restricted to uppercase letters.

  • AIDS or Aids (acquired immunodeficiency syndrome)

  • laser (light amplification [by] stimulated emission [of] radiation)

  • radar (radio detection and ranging)

  • TENS (transcutaneous electrical nerve stimulation)

Initialism: the shortened form of a phrase, using the first letter of each word or word part, and pronounced as the individual letters themselves. Not necessarily restricted to uppercase letters.

  • CPR (cardiopulmonary resuscitation)

  • i.e.‡ (id est, that is)

  • HIV (human immunodeficiency virus)

  • WHO (World Health Organization)§

Nobody’s going to test you, though, and for simplicity, we’re going to sacrifice these distinctions and revert to the general term ‘abbreviation(s)’ for the majority of this piece.

Photo by Gemma Evans on Unsplash

How can medical writers use abbreviations in their writing to avoid the document becoming cluttered, haphazard or ambiguous?

Whenever you write, you need to ask yourself what might trip up the reader.

If you use abbreviations with care, they act as an efficient code, signalling the (now understood) definitions throughout your work. This tidies up clunky areas of writing and communicates your message more clearly. Conversely, littering your document with repeated capitals and brackets is distracting at best, so you have to strike a balance.

To help you edit your work, I’ve listed a few considerations below on where, when and how (not) to use abbreviations.

When to define

You should define abbreviations at the first mention and use the abbreviated form thereafter, unless instructed otherwise. This also applies to the abstract of journal articles, so that it can be read independently from the main article.

Certain common abbreviations may not need defining, so refer to the style guide first.

Some clients may ask you to reiterate the definition at the first mention in each major section of a manuscript, report or slide deck, depending on its length, so always check the brief.

Abbreviations in titles

It’s typically best to avoid using abbreviations in titles (and often subtitles) unless the abbreviation is widely recognised.

Abbreviations in journal submissions

A journal’s style guide should stipulate how to use abbreviations in manuscripts. This often means adopting a leaner approach, with good practice being to use an abbreviation if it appears three times or more.

Abbreviations beneath visual elements

When listing abbreviations under tables, images, graphs, etc., aim for consistent usage and styling throughout the entire document.

Check each instance: have you gone straight in with the first abbreviation, or did you write Abbreviations: beforehand? Is there a colon or a comma between the abbreviation and the definition? Are you going with end punctuation or not? Have you accidentally abbreviated type 2 diabetes mellitus as T2D and T2DM interchangeably? Are all items listed alphabetically?

Using full points with abbreviations, contractions and initialisms

*British English sometimes uses a point after abbreviations like etc. and co., but modern styles are leaning towards omitting this. If in doubt, always refer to the style guide or check requirements with your client.

†American English generally uses a full point (period) after certain contractions (Dr., vs.), whereas British English tends not to (except in confusing contexts where no. without the full point could be read no). Again, check the style guide.

‡Most initialisms don’t use points (DNA, COPD) but where they do, make sure you use a point after each letter (a.m. rather than a.m or am.) and don’t add a further point when the initialism falls at the end of the sentence.

Using only one abbreviation per term

To avoid confusion, never use the same abbreviation to apply to two different terms, e.g. TE for both transient elastography and thromboelastography. This kind of error can often creep in with longer documents, as the writer may lose track of the terms they’ve already abbreviated.

Equally, it’s important to use the same abbreviation throughout the document – I’ve come across SD and Std Dev used for standard deviation in the same pre-edited document.

Plural versions of abbreviations

To make acronyms and initialisms plural, make sure the ‘s’ you add is lowercase to avoid it seeming like a final, significant abbreviated word. Never use an apostrophe unless it is in the possessive.

e.g. Talk to your GP about screening for STIs. (N.B. not STIS or STI’s.)

Correctly naming official bodies in defined terms

When defining abbreviations for institutions, organisations and other official bodies, double-check that you have the correct ‘filler words’ (in/on/for/of). It’s easy to misremember or miscopy the true version of a name.

Initialisms as nouns and adjectives

Some style guides set different rules for using initialisms as nouns vs adjectives. For instance, often United States (in full) is preferred over US when used as the noun, but US is preferred adjectivally.

e.g. In the US cohort, we found … but The condition is widespread in the United States.

As ever, refer to the style guide if unsure.

Global changes for abbreviations

When using similar abbreviations in the same document, e.g. US and U.S. for ultrasound and the United States, be aware that it may be trickier to implement global changes to align the style. Look up each instance to ensure it’s correct in context.

Abbreviations lists

If you’ve compiled a separate abbreviations list, ensure you’ve styled it to client requirements (and, if alphabetical, that all terms are correctly placed) and that the list appears at the agreed point in the document.

Uppercase versus lowercase letters in the definition

Unless instructed otherwise, don’t use uppercase letters in the definition unless it’s a named entity.

e.g. You’d use lowercase for standard definitions such as chronic obstructive pulmonary disease (COPD) or glomerular filtration rate (GFR), but you’d use uppercase initial letters for named entities like the Health Insurance Portability and Accountability Act (HIPAA) or the World Health Organization (WHO).

Retaining the accepted spellings of any definitions

§Always retain the accepted spelling of a name, rather than adapting it to the spelling preference of your style guide. For instance, even in a document whose style has -ise/-isation spellings, the final word in the World Health Organization would not change to Organisation.

Typos in acronyms and initialisms

It’s extremely common to type letters in the wrong order in acronyms and initialisms (e.g. ECRP vs ERCP) and these errors are harder to find when you’ve been writing for hours. Take a proper break from your work and then check each instance assiduously, or use macros to help you identify discrepancies.

Using articles (a, an, the) with abbreviations

Articles like a or an should align with the sound of the abbreviation itself, not the first word it represents.

e.g. an HDL level uses an to go before the sound aitch rather than a before the word high; whereas a NASA report uses a before (pronunciation of) nasser rather than an before the sound en for the initial N.

Bear in mind that articles are sometimes omitted before initialisms and acronyms, even though they may be used when the initialism or acronym is used adjectivally or when it’s been spelt out in full.

e.g. CDC (not the CDC) or NATO (not the NATO); but the CDC delegate or the North Atlantic Treaty Organization.


Referring to these pointers should ensure that any abbreviations you’ve included are used consistently and, importantly, that they enhance the clarity and readability of your medical writing.


If you or your agency would like some editorial support with med comms writing, please get in touch to chat through what your project involves.


Georgina Fradgley is a professional web editor, copyeditor and proofreader, based in Hampshire and offering a range of services. She is an Intermediate Member of the Chartered Institute of Editing (CIEP).

Explore her website at, connect via LinkedIn and Facebook, or send her a message.

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