# 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Change micron (outdated term) to µm.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 3*x* + 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.

14*x* + 27*y* = *z*

45% ± 2%

p<0.01

–70°C

G+C content

densities of <103 cells per cm^{2}

magnification ×200, 4 × 10^{6}

the MIC was <8.0 g/mL

260,000 × 10^{9} 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 |
mm |

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 H_{2}O), 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)

*The conclusions, findings, and opinions expressed by authors contributing to this journal do not necessarily reflect the official position of the U.S. Department of Health and Human Services, the Public Health Service, the Centers for Disease Control and Prevention, or the authors' affiliated institutions. Use of trade names is for identification only and does not imply endorsement by any of the groups named above.*