Box 1 and 1a: Insurance type and insured ID number
By Dr. Daniel Turner, DC · Reviewed September 2, 2026
What goes in Box 1 and 1a of the CMS-1500?
Box 1 marks which kind of coverage the claim bills: Medicare, Medicaid, Tricare, group health, or other. Box 1a carries the insured person’s ID number exactly as the payer issued it, which for Medicare means the MBI from the red white and blue card.
What does Box 1 and 1a mean for chiropractic billing?
The chiropractic trap lives in 1a formatting. Member IDs must match the payer’s records character for character, including or excluding dashes and prefixes exactly as the card shows. A transposed or reformatted ID is a fast route to a claim that denies as patient not found.
The mistakes that turn this box into denials
- ✗Typing the ID with dashes the payer does not store, or without ones it does
- ✗Using an old plan’s ID after a coverage change
- ✗Using the SSN based HICN where Medicare expects the MBI
The denials this box causes
How Cervica fills Box 1 and 1a
Cervica generates the CMS-1500 and its electronic 837P equivalent from the chart and the payer's own rules: identifiers from the verified insurance record, modifiers and codes per payer, and pre submission scrubbing that flags exactly the mistakes listed above before a claim leaves the building.
Related boxes
- →Box 4 · Insured name
- →Box 11 and 11a to 11d · Insured policy group number and related details
General information for billing teams, not billing advice for any individual claim. The NUCC's 1500 Claim Form instructions and each payer's companion guides control.