In the “per” construction, use a forward slash only when (1) the construction involves units of measure (including time) and (2) at least 1 element includes a specific numerical quantity and (3) the element immediately adjacent on each side is either a specific numerical quantity or a unit of measure. In such cases, the units of measure should be abbreviated in accordance with 14.12, Abbreviations, Units of Measure. (See also 19.7.3, Numbers and Percentages, Forms of Numbers, Reporting Proportions and Percentages.)
The hemoglobin level was 14 g/dL.
The CD4+ cell count was 200/μL.
Blood volume was 80 mL/kg of body weight.
Respirations were 60/min; pulse rate was 98/min.
The drug dosage was 30 mg/d.
Do not use the forward slash in a “per” construction (1) when a prepositional phrase intervenes between the 2 units of measure, (2) when no specific numerical quantity is expressed, or (3) in nontechnical expressions.
4.5 mEq of potassium per liter
(Avoid: 4.5 mEq/L of potassium; instead reword: a potassium concentration of 4.5 mEq/L.)
expressed in milliliters per minute
2 days per year