Field 11a of the CMS 1500 claim form, the “Insured’s Date of Birth, Sex” is the birth date and gender of the insured as indicated in Item Number 1a.

NUCC INSTRUCTIONS: Enter the 8-digit date of birth (MM│DD│YYYY) of the insured and an X to indicate the sex (gender) of the insured. Only one box can be marked. If gender is unknown, leave blank.

MEDICARE INSTRUCTIONS - Enter the insured's 8-digit birth date (MM | DD | CCYY) and sex if different from item 3.