As a medical transcriptionist and medical editor, the AAMT Book of Style is my reference, and it states: Use 's to form the plural of lowercase abbreviations, but no apostrophe following all-capital abbreviations.
rbc's not rbcs or RBC's WBCs EEGs
Use 's to form the plural of single letters and symbols, i.e., serial 7's.
For numbers, add s without an apostrophe. Exception: With single numerals, add 's:
Do’s and Don’t's
As a medical transcriptionist and medical editor, the AAMT Book of Style is my reference, and it states: Use 's to form the plural of lowercase abbreviations, but no apostrophe following all-capital abbreviations.
rbc's not rbcs or RBC's
WBCs
EEGs
Use 's to form the plural of single letters and symbols, i.e., serial 7's.
For numbers, add s without an apostrophe. Exception: With single numerals, add 's:
500s
She is in her 20s.
6's and 7's