AMA Manual of Style - Stacy L. Christiansen, Cheryl Iverson 2020

Abbreviations
Plurals

For most all-capital abbreviations, the plural is formed by adding a lowercase s. Do not use an apostrophe before the s (see 8.7.5, Using Apostrophes to Form Plurals).

Note: When plural all-capital abbreviations are found in an all-capital setting, such as a heading, the plural s is still lowercase.

REFERRAL PATTERNS IN MIDWESTERN HMOs