Units of measure, compounds, concentrations, and other common scientific expressions are typically abbreviated. In scientific writing, abbreviations are usually represented without periods.
kg (kilogram)
mA (milliampere)
NaCl (sodium chloride)
1% wt/vol (one percent weight-to-volume ratio)
15 mg/kg iv (fifteen milligrams per kilogram of body weight administered intravenously)
Whether an abbreviation is capitalized or not depends on convention, so check with your instructor or consult a style guide for the discipline in which you are writing.
Formal scientific reports sometimes present a glossary of all technical terms and abbreviations used in the report, either at the beginning or the end of the report. Frequently, however, readers would be expected to know such abbreviations. Spell out terms on first mention if nontechnical readers are part of your audience.
Exercise: Abbreviations
Related topic:
Capitalizing scientific and technical terms