How to Define Abbreviations in Scientific and Medical Writing

Abbreviations are common in scientific and medical writing. When used appropriately, they can save space, avoid repetition, and aid understanding. But when used inappropriately, or when ill-defined, they can be confusing for readers.

To help you know when to define abbreviations, you can follow the general guidelines below. But remember that every style guide has slightly different recommendations, so check with the agency to ensure you use the correct style.

Define abbreviations at first mention in each element

In your writing, expand abbreviations at first mention in each element of the paper. In a manuscript, define the abbreviation in the abstract, main text, and each table and figure. In a proposal, define the abbreviation in the abstract, specific aims, main text, and (depending on the agency) each table and figure.

Expand abbreviations used fewer than three times

If you use a term three or more times, you can use the abbreviated form. For terms used less than three times, different style guides have different recommendations. Most indicate that when using a term fewer than three times (in any element of the paper), you should expand that abbreviation in that element of the paper. But some state that if the abbreviation is a standard abbreviation for a long, familiar term (eg, MRI, PTSD), then using the abbreviated form may be clearer and more concise, even if the term is used fewer than three times.

Follow the expanded term with the abbreviation

When you first mention a term, write the expanded form, and then immediately follow the expanded form with the abbreviation in parentheses. Thereafter, abbreviate the term in that element of the paper.

Examples

cardiopulmonary resuscitation (CPR)

clustered regularly interspaced short palindromic repeats (CRISPR)

Reverse the order for common abbreviations

For terms better known by their abbreviated form, abbreviate the term at first mention and then immediately follow the abbreviated form with the expanded form in parentheses.

Examples

MRI (magnetic resonance imaging)

HIPAA (Health Insurance Portability and Accountability Act)

Define with the singular or plural form

When defining terms, you may use either a singular or plural form (eg, EMR, EMRs), whichever is mentioned first. You do not need to redefine terms for variations.

Use an appropriate article before an abbreviation

In your writing, use an appropriate article (eg, a, an, the) before an abbreviation. The article will be based on the sound of the abbreviation rather than the letter that starts the abbreviation.

Examples

an ATPase

a US regulation

an MRI

a GFR

the FDA

Choose preciseness over the urge to abbreviate

Some abbreviations have more than one meaning. For example, AMA can mean American Medical Association, American Marketing Association, American Music Association, among others. In these instances, using the expanded form may be the most precise form to use.

Avoid defining terms in headings

Avoid introducing or defining abbreviations in titles (including table and figure titles), headings, and subheadings. Instead, introduce them in the main text following the heading with the expanded form.

Example

Avoid:
Chronic Obstructive Pulmonary Disorder (COPD)
COPD is a chronic inflammatory lung disease that obstructs airflow from the lungs. 

Preferred:
Chronic Obstructive Pulmonary Disorder
Chronic obstructive pulmonary disease (COPD) is a chronic inflammatory lung disease that obstructs airflow from the lungs. 

Avoid using abbreviations at the beginning of sentences

Avoid starting sentences with an abbreviation. Two exceptions include cumbersome expansions (eg, AAPPS, NIMHD) or abbreviations that are used often enough that they are easily understood.

Avoid defining abbreviations as possessives

When defining a term, avoid using the possessive form that would create a possessive abbreviation. Instead, reword the sentence.

Example

Avoid: The NIH’s guide to applying for funding.

Preferred: The NIH created a guide to help with applying for funding.

No need to re-expand parts of abbreviations

You do not need to re-expand abbreviations after their components have already been defined. For example, if you define “human cytomegalovirus (HCMV)”, then later mentions of “CMV” do not need to be expanded. Or if you define “cytomegalovirus (CMV)”, then later mention of “human cytomegalovirus (HCMV)” can be written as “human CMV (HCMV)”.

No need to expand units of measurement

Units of measurement do not need to be expanded if they follow a number. However, expand them if they do not follow a number or if the number is at the beginning of a sentence.

Examples

The cells were washed with 10 mL of saline.

Eighty milligrams of aspirin was given to the patient.

The size of the mass was measured in millimeters.

Common Abbreviations that Do Not Need Expansion

Some abbreviations do not need expansion. But which abbreviations fall under this category will depend on the style guide you use. Below are some common abbreviations that do not require expansion by several style guides. However, check the agency’s style guide to be sure that you follow their specific guidelines.

Abbreviation Expansion
AIDS                           acquired immune deficiency syndrome
CI                               confidence interval
CME                           continuing medical education
COVID-19                  coronavirus disease 2019
DDT                            dichlorodiphenyltrichloroethane
DNA                            deoxyribonucleic acid
dpi                              dots per inch
EDTA                         ethylenediaminetetraacetic acid 
eg                               for example
etc                              et cetera
HIV                             human immunodeficiency virus
ie                                that is
IQ                               intelligence quotient 
IQR                            interquartile range
ISSN                           International Standard Serial Number
OD                             oculus dexter
OS                             oculus sinister
PDF                            portable document format
pH                              negative logarithm of hydrogen ion concentration 
PMID                          PubMed identifier
Rh                              rhesus
RNA                            ribonucleic acid
ROM                           read-only memory
SARS-CoV-2             severe acute respiratory syndrome coronavirus 2
SAS                            Statistical Analysis System
SD                              standard deviation
SE                               standard error
SEM                           standard error of the mean
SGML                         standardized general markup language
SPSS                         Statistical Product and Service Solutions 
SSC                           standard saline citrate
SSPE                         sodium chloride, sodium phosphate, EDTA 
URL                            uniform resource locator
UV                             ultraviolet
XML                           extensible markup language