Units of Measure
Units of Measure
A quantity indicated by a unit of measure is singular, even when the unit is spelled out, except in the case of nonabsolute or informal units.
10 mg was added
5 mL was injected
220 cpm of radioactivity was detected
Twelve milliliters was injected, but 10 drops were added
Change mM to mmol/L.
Change µM to µmol/L
Change micron (outdated term) to µm.
Confidence Intervals
Confidence intervals (CIs) are preferred over confidence limits. Use an en dash in a confidence interval unless the range includes a negative number; for negative confidence intervals, use the word “to.” The abbreviation CI can be used without definition.
(95% CI 0.8–1.6)
(95% CI –1.3 to 4.5)
Currency
Currency may be converted to US dollars or given in local units. The symbols $, £, and € may be used without defining (although specify US $, Can $, or Aus $). Other symbols or abbreviations should be defined; inclusion of a conversion factor should be at the author’s discretion.
The approximate cost per case of disease prevented was US $150.
We analyzed expenses in Japanese yen (¥). The cost of treatment was ≈ ¥10,000.
The unit used in our calculations was the Chinese renminbi (RMB) (RMB 1 ≈ US $0.13).
Dates
Spell out months in text. In tables, figures, and references, abbreviate months with the first 3 letters.
In running text, write the date as Month DD, YYYY. Set the year off in commas only if it is used with both month and day.
We conducted a study in January 2003 on the prevalence…
We began a study on January 3, 2003, on the prevalence…
In tables and figures, avoid the constructions MM/DD/YYYY and DD/MM/YYYY (because usage is not consistent worldwide and they can be confused with each other). In references and in the body of tables, dates should be written as YYYY Mon DD (e.g., 2006 Sep 1).
Digits versus Spelled-out Numbers
Numerals, including values <10, should be used to express numbers in most circumstances. However, do not use numerals for
numbers that begin a sentence, title, subtitle, or heading
common fractions
idiomatic expressions
numbers used as pronouns
other uses of the number “one” in running text
ordinals first through ninth
numbers spelled out in quotations or published titles
The relative risk of exposed participants was 3 times that of the controls.
In the second phase of the study, 3 of the investigators administered the 5 tests to the 7 remaining participants.
Five species were identified. (In this instance, rewording the sentence may be preferable: We identified 5 species.)
The investigators compared a new laboratory method with the standard one (not 1).
In titles, use numerals unless the number is the first word.
Comparison of 2 Methods to Detect Publication Bias in Meta-analyses
If 2 numbers appear adjacent to each other, try to rewrite the sentence. If this occurrence is unavoidable, write out one of them.
We sterilized five 50-mL test tubes.
When a unit of measure follows a number that begins a sentence, it too must be written out, even if the same unit is abbreviated elsewhere in the same sentence.
Two micrograms was administered on day 1, followed by 1 µg on day 2.
Equations
In equations, single-letter abbreviations and variables (except for p values) are italicized; surrounding parentheses are not.
Displayed equations should be used only for complex equations that cannot be clearly run into the text. Displayed equations and text are not separated with punctuation.
The value is calculated as follows:
n = ln 1 – π (S1y 3x + 1)
y
Simpler equations should be run into text and set off with a comma.
Distance is calculated as D = x(y), where D = distance…
Text equations use initial capital letters for each element and no end punctuation.
Fractions
Although decimals are preferred (and required with SI units), for less precise measurements, mixed fractions may be used.
The patient was hospitalized for 5½ days.
Common fractions are expressed with words. Hyphens are used only if the fraction modifies a noun.
approximately one fourth of the population
a two-thirds majority
Mathematical Symbols
Unless the meaning is obscured, symbols are preferred to text in mathematical expressions. Note: Do not automatically substitute < for “up to” or “as high as” because it could obscure author’s emphasis on the highest value. For example, use
>12 persons, not more than 12 persons
<4 days, not no more than 4 days
>80 mm Hg, not at least 80 mm Hg
≈70 geese, not approximately 70 geese
Use words rather than symbols for nonmathematical expressions (disregard this rule for tables and parenthetical expressions), and do not allow symbols to stand for the main verb in a clause.
ribavirin plus interferon, not ribavirin + interferon
where p was <0.05…, not where p<0.05…
Symbols (except for <, >, <, and >) are written with a space on either side when they indicate mathematical equations. In all other contexts, the symbol should be closed up with associated numbers or variables.
14x + 27y = z
45% ± 2%
p<0.01
–70°C
G+C content
densities of <103 cells per cm2
magnification ×200, 4 × 106
the MIC was <8.0 g/mL
260,000 × 109 cells/L
If a symbol begins a heading (e.g., column heading in table), capitalize the next word
% Infected
Numbers
Use commas in all numbers with >4 digits, except in dates and in numbers that represent a position (as with a page number or amino acid)
2,000 people by 2002
We found a Leu→Phe substitution at amino acid 1242.
Use a combination of numerals and words to express rounded large numbers and consecutive numerical expressions.
The disease affects 5 million to 6 million persons.
Study participants were given twenty 5-mL syringes.
For all decimals between –1 and 1 (≠0), use a leading zero to the left of the decimal.
p<0.05 not p<.05
Ordinals
Ordinals generally express order or rank rather than quantity. Ordinals first through ninth are spelled out. Ordinals greater than ninth are expressed as numerals, except at the beginning of sentence, title, or heading.
In a series that includes an ordinal greater than ninth, all ordinals are expressed as numerals.
Samples were taken during the 2nd, 4th, 8th, and 16th weeks after illness onset.
Percentages
Always use a digit and the % symbol with percentages, unless it is the first word in a sentence.
Forty-three percent of our sample tested positive.
Percentages should immediately follow the number, not the phrase.
The disease was found in 12 (50%) of the 24 children.
Ranges
Limit to specific statistical meaning. Connect ranges with an en-dash, unless introduced by the preposition “from,” in which case use a joining conjunction is needed (e.g., “from… to” or “from… through”). Be careful with “between… and” constructions because they exclude both endpoints.
Participants were 18–20 years of age.
from 5% to 10%
from 1984 through 1990 (not between 1984 and 1990, unless 1984 and 1990 are not included)
The “from… to” construction may also be used for clarity if one or both of the limits in a range are negative.
from –70° to –40°
In ranges of time, particularly years, prepositions and conjunctions are critical in determining the actual amount of time included.
Between 1999 and 2001 = 1 year
From 1999 to 2001 = 2 years
From 1999 through 2001 = 3 years
When the range of years is not necessarily a calendar year and indicates only 1 year (e.g., influenza season, fiscal year, school year), it is ok to truncate the second year.
2005–06 influenza season
2004–05 school year
Repeat units in ranges if the unit is set closed up with the digit, but do not repeat units that are separated with a space.
from 40ºC to 70ºC
25%–50%
50–100 mL
from 80 to 89 mm Hg
Ratios
Use a colon to express a ratio or dilution. Titers are expressed as the reciprocal of the dilution.
(length:width, 2:1)
The end IgG titer was 256 after a serial 1:2 dilution.
A ratio may also be expressed as a percentage. Note that a ratio becomes a rate only when an element of time is involved (such as number of deaths per population per year).
Seven of the 12 patients died; death ratio was 58.3%.
During 2000–2004, the average death rate was 12.7% per year.
SI Units
Use SI units for units of measure, for example:
|
Unit name |
SI symbol |
|
centimeter |
cm |
|
cubic millimeter |
mm3 |
|
dalton |
Da |
|
deciliter |
dL |
|
gram |
g |
|
kilodalton |
kDa |
|
kilogram |
kg |
|
liter |
L |
|
meter |
m |
|
microgram |
µg |
|
microliter |
µL |
|
milligram |
mg |
|
milliliter |
mL |
|
millimeter |
mm |
|
mole |
mol |
|
molar |
mol/L |
However, some medical measurements follow other conventions [e.g., blood pressure (mm Hg), oxygen pressure (cm H2O), temperature (°C)]. To report both SI and conventional units, repeat the number in parentheses along with the unit of measure.
20 mmol/L (20 mEq/L)
