Periods

Periods:  Do not use periods within or at the end of most abbreviations, including acronyms, abbreviated units of measure, and brief forms. Use a period at the end of abbreviated English units of measure if they may be misread without the period. Better still, write out most English units of measure, thereby avoiding this use of a period at the end of an abbreviation.
wbc
WBC
mg
exam
prep
inch preferred to in. (Do not use in meaning inch without a period.)
Do not use periods with abbreviated academic degrees and professional credentials.
BA PA-C
CMT PhD
MD RN
However, use periods in lowercase drug-related abbreviations.
b.i.d.
q.4 h.
p.o.
p.r.n.
Periods may be used with courtesy titles (e.g., Mr., Mrs.) and following Jr. and Sr., although there is a trend toward dropping them; either remains acceptable, but be consistent.