AMA Manual of Style - Stacy L. Christiansen, Cheryl Iverson 2020
Parentheses and Brackets
Parentheses and brackets are internal punctuation marks used to set off material that is nonrestrictive or, as in the case of mathematical and chemical expressions, to alert the reader to the special functions occurring within. Note: Excessive use of parentheses and brackets can make text challenging to read and understand.
220.127.116.11 Supplementary Expressions.
Use parentheses to indicate supplementary explanations, identification, direction to the reader, or translation (see 8.3.2, Dashes, and 8.5.2, Brackets).
A known volume of fluid (100 mL) was injected.
The differences were not significant (P = .06).
One of us (B.O.G.) saw the patient in 2006.
Asymmetry of the upper part of the rib cage (patient 5) and pseudarthrosis of the first and second ribs (patient 8) were incidental anomalies (Table 3).
Of the 761 hospitalized patients, 171 (22.5%) were infants (younger than 1 year).
In this issue of the same journal (p 1037), a successful transplant is reported.
The 3 cusps of the aortic valve (the Mercedes-Benz sign) were clearly shown on the echocardiogram.
If there is a close relationship between the parenthetical material and the rest of the sentence, commas are preferred to parentheses.
The hemoglobin level, although in the reference range, was lower than expected.
If the relationship in thought after the expressions namely, that is (ie), and for example (eg) is incidental, use parentheses instead of commas.
He weighed the advice of several committee members (namely, Jones, Burke, and Easton) before making his proposal.
18.104.22.168 Back-to-Back Parentheses.
Back-to-back parentheses tend to be awkward. Sometimes they can be avoided by using a single set of parentheses and separating the items inside the parentheses with a semicolon.
The effect size was particularly high for respiratory tract infections (adjusted hazard ratio, 2.27; 95% CI, 1.32-3.91).
In other cases, however, combining back-to-back parentheses, although more streamlined, muddies the meaning.
Significantly more patients withdrew from the placebo group (n = 174) than from the chelation group (n = 115) (hazard ratio, 0.66; 95% CI, 0.41-0.85; P = .001). [Here, the back-to-back parentheses make it clear that the hazard ratio and P value apply to both groups, not just the chelation group.]
22.214.171.124 Punctuation Marks With Parentheses.
Use no punctuation before the opening parenthesis except in enumerations (see 126.96.36.199, Introducing Quotations or Enumerations).
Any punctuation mark can follow a closing parenthesis, but only the 3 end marks (the period, the question mark, and the exclamation point) may precede it when the parenthetical material interrupts the sentence. If a complete sentence is contained within parentheses, it is not necessary to have punctuation within the parentheses if it would noticeably interrupt the flow of the sentence. Note that with complete sentences (ie, if closing punctuation is used within the parentheses), the initial letter of the first word is capitalized.
The discussion on informed consent lasted 2 hours. (A final draft has yet to be written.) The discussion failed to resolve the question.
The discussion on informed consent lasted 2 hours (a final draft has yet to be written) and did not resolve the question.
After what seemed an eternity (It took 2 hours!), the discussion on informed consent ended.
When the parenthetical material includes special punctuation, such as an exclamation point or a question mark, or several statements, terminal punctuation is placed inside the closing parenthesis.
Oscar Wilde once said (When? Where? Who knows? But I read it in a book once upon a time; hence, it must be true.) that “anyone who has never written a book is very learned.”4
188.8.131.52 Identifying Numbers or Letters.
When an item identified by letter or number is referred to later by that letter or number only, enclose the letter or number in parentheses.
You then follow (3), (5), and (6) to solve the puzzle.
If the category name is used instead, parentheses are not needed.
Steps 1, 2, and 3 must be done slowly.
For division of a short enumeration that is run in and indicated by numerals or lowercase italic letters, enclose the numerals or letters in parentheses (see 18.5, Enumerations.)
The patient is to bring (1) all pill bottles, (2) past medical records, and (3) our questionnaire to the first office visit.
184.108.40.206 References in Text.
Use parentheses to enclose a full or partial reference given in the text (see 3.3, References Given in Text).
A systematic review published in JAMA Pediatrics noted that, on any given day, 60% of young persons play video games (2013;167:574-580).
220.127.116.11 In Legends or Captions.
In legends or captions, use parentheses to identify a case or patient and parts of a composite figure when appropriate (see 4.2.7, Titles, Legends, and Labels).
Facial paralysis on the right side (patient 3).
Long-term survival after early surgery vs initial medical management in the overall population (A) and in the propensity score—matched cohort (B).
A, Axial T1 postcontrast image shows avid enhancement of nerve roots (patient 1). B, Sagittal T1 fat-saturated, postcontrast images of the lumbar spine show a rim-enhancing fluid collection in the dorsal epidural space (patient 2).
The date, if given, is similarly enclosed.
Fracture of the left femur (patient 7, September 3, 2013).
For photomicrographs, give the magnification and the stain, if relevant, in parentheses (see 4.2.7, Titles, Legends, and Labels).
Yellow arrowheads identify the prominent goblet cells in the intestinal metaplasia of Barrett esophagus (hematoxylin-eosin, original magnification ×20).
18.104.22.168 Trade Names.
If there is a reason to provide a trade name for a drug or for equipment, enclose the trade name in parentheses immediately after the first use of the nonproprietary name in the text and in the abstract (see 14.4.3, Drugs, Proprietary Names; 14.5, Equipment, Devices, and Reagents; and 5.6.15, Trademark).
Treatment included oral administration of indomethacin (Indocin), 25 mg 3 times a day.
If used in the text, specialized abbreviations (as specified in 13.11, Clinical, Technical, and Other Common Terms) are enclosed in parentheses immediately after first mention of the term, which is spelled out in full.
A data and safety monitoring board (DSMB) of investigators outside the university provided oversight.
Abusive head trauma (ABT) of infants and young children has considerable morbidity and economic costs.
22.214.171.124 Explanatory Notes.
Explanatory notes, when incorporated into the text, are placed within parentheses. In such instances, terminal punctuation is used before the closing parenthesis, the sentence(s) within the parentheses being a complete thought but only parenthetical to the text.
126.96.36.199 Parenthetical Expressions Within a Parenthetical Expression.
Whenever a parenthetical expression is embedded within another, use brackets within parentheses.
Antirejection therapy included parenteral antithymocyte globulin ([ATGAM], at a daily dosage of 15 mg/kg).
But: In mathematical expressions, parentheses are placed inside brackets (see 188.8.131.52, Brackets, Within Parentheses).
184.108.40.206 Parenthetical Plurals.
Parentheses are sometimes used around the letters s and/or es to express the possibility of a plural when singular or plural could be meant (see 7.5.4, Subject-Verb Agreement, Parenthetical Plurals).
The name(s) of the editor(s) of the book in reference 2 is unknown.
Note: If this construction is used, the verb should be singular, because the s is parenthetical. In general, try to avoid this construction or rephrase the sentence:
We do not know the name(s) of the editor(s) of the book in reference 2.
220.127.116.11 Insertions in Quotations.
Brackets are used to indicate editorial interpolation within a quotation and to enclose corrections, explanations, or comments in material that is quoted (see 8.6.1, Quotations; 8.8.6, Ellipses, Change in Capitalization; and 8.8.7, Ellipses, Omission of Ellipses).
“The following year  was a turning point.”
“Enough questions had arisen [these are not described] to warrant medical consultation.”
Thompson stated, “Because of the patient’s preferences, surgery was absolutely contraindicated [italics added].”
Note: Use sic (Latin for “thus” or “so”) in brackets to indicate an error or peculiarity in the spelling or grammar of the preceding word in the original source of the quotation. As with apologetic quotation marks (see 8.6.8, Apologetic Quotation Marks), use sic with discretion.
“The plural [sic] cavity was filled with fluid.”
“Breathing of the gas is often followed by extraordinary fits of extacy [sic].”
18.104.22.168 Within Parentheses.
Use brackets to indicate parenthetical expressions within parenthetical expressions.
A nitrogen mustard (mechlorethamine hydrochloride [Mustargen]) was one of the drugs used.
In scientific text, one sometimes encounters complex parenthetical constructions, such as consecutive parentheses and brackets within parentheses.
Her platelet count was 100 000/mm3 (100 × 109/L) (reference range, 150 000-450 000/mm3 [150-450 × 109/L]).
Among the 3578 patients (2841 men [79.4%]) and 11 775 control group participants (91 845 men [78%]), all were diagnosed as having depression.
22.214.171.124 In Formulas.
In mathematical formulas, parentheses are generally used for the innermost units. Parentheses are changed to brackets when the formula itself is parenthetical.
The equation suggested by this phenomenon (t = d[r1 − r2 ]) can be applied in a variety of circumstances.
(See 20.2, Stacked vs Unstacked Fractions or Formulas.)
Make sure that every parenthetical or bracketed expression has an opening and closing parenthesis or bracket symbol. For chemicals, consult the most recent edition of USP Dictionary of USAN and International Drug Names for drug formulas and The Merck Index for chemical compounds to verify the correct use of parentheses and brackets.
An experimental drug (9-[(2-hydroxy-1-(hydroxymethyl)ethoxy methyl)]guanine) was used to treat the cytomegalovirus retinopathy in patients with AIDS.
If the older style of parentheses, braces, and brackets has been used by the author, retain it. The notation will be readily understood by the author’s intended audience.
When a parenthetical or bracketed insertion in the text contains a mathematical formula in which parentheses or brackets appear, the characters within the formula should be left as given unless that would place 2 identical punctuation symbols (eg, 2 open parentheses) immediately adjacent to each other. To avoid adjacent identical characters, change parentheses to brackets or brackets to parentheses in the formula as needed, working from inside out, starting with parentheses, to brackets, to braces.
In some instances, a better option might be to restructure the passage.