Commas are the least forceful of the 3 marks. There are definite rules for using commas; however, usage is often subjective. Some writers and editors use the comma frequently to indicate what they see as a natural pause in the flow of words, but commas can be overused. The trend is to use them sparingly. Follow the accepted rules and use commas only when breaks are needed for sense or readability or to avoid confusion or misinterpretation.
Separating Groups of Words.
The comma is used to separate phrases, clauses, and groups of words and to clarify the grammatical structure and the intended meaning.
Use a comma after opening dependent clauses (whether restrictive or not) or long opening adverbial phrases.
If the infection recurs within 2 weeks, an additional course of antibiotics should be given.
A comma is not essential if the introductory phrase is short.
In some patients midazolam produces paradoxic agitation.
Use commas to set off nonrestrictive subordinate clauses (see 7.2.2, Grammar, Pronouns, Relative Pronouns) or nonrestrictive participial phrases.
Ms Frederick, who had been waiting on hold for more than an hour, abandoned all hope of having her questions answered.
The numbness, which had been apparent for 3 days, disappeared after drug therapy.
The delegates, attaining consensus, passed the resolution.
But avoid setting a phrase off with commas where it would make the meaning ambiguous.
Although numerous investigators have called for measures to improve sight in nursing home residents, to our knowledge, none have attempted a study of the effect of a vision restoration-rehabilitation program on function and quality in this population.
In the example above, it is not clear whether the phrase “to our knowledge” applies to what precedes it or what follows it. Removing the comma after “to our knowledge” makes the meaning clear.
Although numerous investigators have called for measures to improve sight in nursing home residents, to our knowledge none have attempted a study of the effect of a vision restoration-rehabilitation program on function and quality in this population.
Outside, the ambulance siren shrieked.
Still, noting the trends and highlighting the lack of funding for achieving world health goals does not translate into more positive actions.
Use commas to set off appositives. (Note: Commas precede and follow the apposition.)
Two colleagues, John Smith and Perry White, worked with me on this study.
The battered-child syndrome, a clinical condition in young children who have experienced serious physical abuse, is a frequent cause of permanent injury or even death.
In a simple coordinate series of 3 or more terms, separate the elements by commas. (See 7.1.1, Grammar, Nouns, Modifiers [Noun Strings].)
Each patient was asked to complete a 21-item, 7-point, self-administered questionnaire.
Use a comma before the conjunction that precedes the last term in a series to prevent ambiguity (this is often referred to as a serial comma).
Outcomes result from a complex interaction of medical care and genetic, environmental, and behavioral factors.
The physician, the nurse, and the family could not convince the patient to take his medication daily.
While in the hospital, these patients required neuroleptics, maximal observation, and seclusion.
However, a series of 3 or more modifiers should not be separated by commas when the modifiers are seen as 1 term or entity:
The patient has chronic progressive multiple sclerosis.
Gray matter magnetic resonance imaging was used to predict longitudinal brain atrophy.
Judgment and common sense are required in the interpretation of this rule. If the order of the adjectives can be rearranged without loss of meaning or clarity, use the comma.
The studies selected for inclusion were English-language, randomized, double-blind, controlled trials of newer atypical antipsychotic medications.
Data from several large, multicenter, administrative databases were analyzed.
Note: When fewer than 3 modifiers are used, avoid adding a comma if the modifiers and the noun are read as one entity:
We conducted a randomized placebo-controlled trial.
Data from multicenter administrative databases were analyzed.
Names of Organizations.
When an enumeration occurs in the name of a company or organization, the comma is usually omitted before the ampersand. However, follow the punctuation used by the individual firm, except in references. (See 3.12.9, References, References to Print Books, Publishers.)
Farrar, Straus & Giroux Inc
Little, Brown & Co
Mayer, Brown, Rowe & Maw
Houghton Mifflin Co
Sidley Austin Brown & Wood
Setting Off ie, eg, viz.
Use commas to set off ie, eg, and viz and the expanded equivalents, that is, for example, and namely.
The use of standardized scores, eg, z scores, has no effect on statistical comparisons.
The most important tests, that is, the white blood cell and platelet counts, were unduly delayed.
Our double-blind study compared continuous with cyclic estrogen treatment; ie, estrogens for 4 weeks were compared with estrogens for 3 weeks followed by placebo for 1 week.
Separating Clauses Joined by Conjunctions.
Use commas to separate main clauses joined by coordinating conjunctions (and, but, or, nor, for).
Plasma lipid and lipoprotein concentrations were unchanged after low-intensity training, but high-intensity training resulted in a reduction in triglyceride levels.
No subgroup of responders could be identified, and differences between centers were so great that no real comparison was possible.
If both clauses are short, punctuation can be omitted.
The test may be useful or it may be harmful.
I have read the article and I am concerned about the data collection methods.
Be careful not to confuse the coordinating conjunction used between independent clauses with a coordinating conjunction used to link a compound predicate.
These facilities are beginning to resemble “minihospitals” and are losing their identity as freestanding ambulatory surgery centers.
Clauses introduced by yet and so and subordinating conjunctions (eg, while, where, after, whereas) are preceded by a comma. (See 11.1, Correct and Preferred Usage, Correct and Preferred Usage of Common Words and Phrases.)
He taught medical students, performed careful research, and wrote thoughtful articles, yet was denied tenure.
The United States spends more than $1000 per capita per year on paperwork related to health care, whereas Canada spends only about $300 per capita.
One recent study found that low literacy was associated with worse mental health, whereas another concluded that literacy was not associated with depression.
If such conjunctions appear at the beginning of a sentence, however, the comma following the conjunction may not be necessary.
I have seen many cases of vertigo. Yet this one was particularly troubling.
Setting Off Parenthetical Expressions.
Use commas to set off parenthetical words, phrases, questions, and other expressions that interrupt the continuity of a sentence, eg, therefore, moreover, on the other hand, of course, nevertheless, after all, consequently, however. (See 8.8.1, Ellipses, Omission Within a Sentence.)
The real issue, after all, was how to fund the next study.
Therefore, we were disappointed that the article did not include consideration of medical schools and their influence on the culture of medicine.
Note: In some cases, removal of the commas around parenthetical expressions changes the meaning of the sentence. In the example immediately above, then suggests a summing-up. Without these commas, then suggests time, ie, what comes next.
Setting Off Degrees and Titles.
Academic degrees and titles are set off by commas when they follow the name of a person. Although it is not incorrect to set Jr and Sr off by commas when they follow the name of a person, JAMA and the Archives Journals are now deleting these commas.
Berton Smith Jr, MD, and Priscilla Armstrong, MD, PhD, interpreted the radiographic findings in this study.
Joyce Fredrickson-Smith, MD, PhD, vice-chancellor, attended the conference on health system reform.
In running text and in affiliation footnotes, use commas to separate the elements in an address. Use commas after the city and before and after the state or country name. (Note: In US and Canadian addresses, commas are not used before the zip or the postal code.)
This year, the editorial board meeting will be held in conjunction with the Academy meeting at the Westin Bonaventure Hotel and Suites, 404 S Figueroa St, Los Angeles, CA 90071.
The study was conducted at The Wilmer Institute, Baltimore, Maryland, in 2004.
In dates and similar expressions of time, use commas according to the following examples. Commas are not used when the month and year are given without the day, or between a holiday and its year.
The first issue of JAMA was published on Saturday, July 14, 1883.
The patient’s rhinoplasty was scheduled for August 19, 2002, at Strong Memorial Hospital, with postoperative evaluation on August 30.
The terrorist attack in London, England, in July 2005 led to further examination of major disaster preparedness.
The publication offices were closed on New Year’s Day 2005.
In accordance with SI convention, separate digits with a thin space, not a comma, to indicate place values beyond thousands. (See 18.4.3, Units of Measure, Use of Numerals With Units, Number Spacing.)
5034 12 345 615 478 9 473 209
A comma may be used to separate adjacent unrelated numerals if neither can be expressed easily in words, but it is preferable to reword the sentence or spell out 1 of the numbers.
By December 2003, a total of 929 985 cases of AIDS had been reported in the United States.
Units of Measure.
Do not use a comma between 2 or more measures whose units are the same dimension.
3 years 4 months 2 days old
3 lb 4 oz
The comma is placed inside quotation marks (see 8.6.5, Quotation Marks, Placement) and before superscript citation of references and footnote symbols.
As a result of the “back-to-sleep campaigns,” a call has been issued for a “back-to-the-bench” campaign.
These missed opportunities have been shown to occur during office visits,6-9 health department appointments,10-13 and hospitalizations.16
To Indicate Omission.
The comma is used to indicate omission or to avoid repeating a word when the sense is clear. (See 7.8.7, Grammar, Subject-Verb Agreement, Shift in Number of Subject and Resultant Subject-Verb Disagreement.)
Three patients could not be studied: in 1, duration of treatment was too short; in 2, too long.
A plus indicates present; a minus, absent.
Commas are often used before direct dialogue or conversation is introduced. (See also 8.2.3, Colon, Introducing Quotations or Enumerations.)
In the middle of the laboratory examination, a student asked, “Would it be OK to take a break?”