CMS-1500 Form Box by Box: What Each Field Means and Which Ones Cause Denials
The CMS-1500 form is the standard paper claim form that physicians, non-physician practitioners and other non-institutional suppliers use to bill Medicare and most other payers. The current version is 02/12, maintained by the National Uniform Claim Committee (NUCC), and its electronic equivalent is the 837P.
Few claims go out on the red paper form anymore, but claim screens, clearinghouse rejections and remittances still refer to its box numbers.
Where you see it
| Where | What it is |
|---|---|
| CMS Forms List | Form CMS-1500, Health Insurance Claim Form, revision date 02/01/2012, OMB 0938-1197; printed in red by forms vendors, since the NUCC’s PDF is for reference only |
| The instruction manual | the NUCC 1500 Health Insurance Claim Form Reference Instruction Manual, version 13.0, July 2025 |
| Medicare’s instructions | Claims Processing Manual, chapter 26, Completing and Processing Form CMS-1500 Data Set |
Medicare accepts paper only under an Administrative Simplification Compliance Act (ASCA) exception, most commonly fewer than 10 full-time equivalent employees.
The boxes, grouped
Payer and patient (boxes 1 to 13)
| Box | Title | Notes |
|---|---|---|
| 1, 1a | Payer type; insured’s ID number | for Medicare, the 11-character MBI |
| 2, 3, 5, 6 | Patient’s name, birth date and sex, address, relationship to insured | as on the card |
| 4, 7, 11 to 11d | Insured’s name, address, policy or group number, birth date, plan name, other plan? | the subscriber, when not the patient |
| 9, 9a, 9d | Other insured’s name, policy number, plan name | the secondary payer |
| 10a to 10d | Condition related to employment, auto or other accident; claim codes | liability and workers’ compensation coordination |
| 12, 13 | Patient’s and insured’s signatures | “Signature on File” is standard; 8, 9b, 9c and 30 are reserved and stay blank |
The encounter (boxes 14 to 23)
| Box | Title | Notes |
|---|---|---|
| 14, 15, 16 | Date of current illness; other date; dates unable to work | each with a qualifier where the form asks |
| 17, 17a, 17b | Referring provider or other source; other ID; NPI | qualifier DN, DK or DQ left of the name; see rendering vs billing provider |
| 18, 19, 20, 23 | Hospitalization dates; additional claim information; outside lab charges; prior authorization number | |
| 21 | Diagnosis or nature of illness or injury | ICD indicator (0 for ICD-10) and up to 12 codes, A to L |
| 22 | Resubmission code and original reference number | frequency code 7 (replacement) or 8 (void) with the payer’s original claim number |
Service lines (box 24, lines 1 to 6)
| Column | Title | Notes |
|---|---|---|
| 24A, 24B | Dates of service; place of service | from and to; the two-digit POS code, see POS 22 |
| 24D, 24E | Procedures, services or supplies; diagnosis pointer | CPT or HCPCS with up to four modifiers; letters from box 21 |
| 24F, 24G, 24C, 24H | Charges; days or units; EMG; EPSDT / family plan | the last two are rarely used outside Medicaid |
| 24I, 24J | ID qualifier (shaded); rendering provider ID | NPI in the unshaded part of 24J; a non-NPI ID such as taxonomy (ZZ) in the shaded part, which also carries an NDC where required |
Provider and totals (boxes 25 to 33)
| Box | Title | Notes |
|---|---|---|
| 25 | Federal tax ID number | EIN or SSN, with the box checked |
| 26 | Patient’s account number | your internal number, echoed on the remittance |
| 27, 28, 29 | Accept assignment; total charge; amount paid | amount paid is from the patient or another payer |
| 31 | Signature of physician or supplier | for incident-to services under Medicare, the supervising physician |
| 32, 32a, 32b | Service facility location, NPI, other ID | when the service was not at the billing address; Medicare does not use 32b |
| 33, 33a, 33b | Billing provider info and phone, NPI, other ID | the entity being paid; 33b for a payer ID or taxonomy when the payer asks |
The boxes that cause denials
| Box | Common denial | What went wrong |
|---|---|---|
| 1a | CARC 16 with RARC N382 | patient identifier invalid, for example an old Medicare HICN |
| 17, 17b | CARC 16 with RARC N285 or N286; CARC 183 | referring provider missing, invalid or not eligible to refer |
| 21, 24E | CARC 16 | pointer to a blank diagnosis letter, or a code not valid for the date |
| 24B, 24D | payer edits, often CARC 16 | POS does not fit the procedure; invalid code or missing modifier |
| 24J | CARC 16 with RARC N290; CARC 185 | rendering NPI missing, a Type 2 in the rendering position, or a provider not enrolled |
| 25, 33a | CARC 16 with RARC N257 | billing NPI or tax ID does not match the payer’s file |
| 22 | rejected as duplicate | corrected claim sent without frequency code 7 and the original claim number |
Every row is a correction and resubmission, not an appeal.
The 837P equivalent
| CMS-1500 box | 837P location |
|---|---|
| 1a | loop 2010BA, subscriber NM109 |
| 21 | loop 2300, HI segment |
| 22 | CLM05-3 frequency code and loop 2300 REF*F8 payer claim control number |
| 24A to 24G | loop 2400, SV1 and DTP segments |
| 24J | loop 2310B (claim level) or 2420A (line level) |
| 26, 32, 33 | CLM01; loops 2310C and 2010AA |
Worked example
Sample Radiation Oncology, PC bills one week of treatment management for a Medicare patient. The boxes that matter most:
| Box | Entry |
|---|---|
| 1a | the patient’s MBI, 11 characters, no dashes |
| 21 | 0 as the ICD indicator; A: C61 (malignant neoplasm of prostate) |
| 24A to 24G | 10/13/2026 to 10/17/2026, POS 11, 77427, pointer A, 240.00, 1 unit |
| 24J | Dr. Patel’s Type 1 NPI |
| 25, 26 | the group’s EIN; the account number |
| 33, 33a | Sample Radiation Oncology, PC and its Type 2 NPI |
Illustrative payment: allowed 180.00, CO-45 60.00, PR-2 36.00, paid 144.00. Box 26 is what matches that payment back to the account at posting.
What to do
- Use the NUCC manual as the reference, and follow the payer’s guide where it differs. The manual describes itself exactly that way.
- Send claims electronically. Paper claims to Medicare need an ASCA exception, and paper claims to any payer take longer to be paid.
- Correct, do not duplicate. A corrected claim carries frequency code 7 and the payer’s original claim number in box 22 (or CLM05-3 and REF*F8). A plain resend comes back as a duplicate.
- Iridium Suite builds the 837P from the same fields that fill these boxes and runs every claim through the claim scrubber before it goes to the clearinghouse.
Related terms
- NPI numbers: Type 1 vs Type 2
- Rendering provider vs billing provider
- Taxonomy codes
- Place of service codes
- HIC number, MBI and ICN
- Modifier Q6 and locum tenens billing
Further reading: NUCC 1500 Claim Form Reference Instruction Manual; CMS Forms List: Form CMS-1500; MLN006976 Medicare Billing: CMS-1500 and 837P; Medicare Claims Processing Manual, chapter 26.
Frequently asked
What is the CMS-1500 form used for?
The CMS-1500 is the standard paper claim form that physicians, non-physician practitioners and other non-institutional suppliers use to bill Medicare and most other payers. Hospitals and other facilities use the UB-04 instead. The current version is 02/12, maintained by the National Uniform Claim Committee.
What is the difference between the CMS-1500 and the 837P?
They carry the same claim. The CMS-1500 is the paper form; the 837P is the HIPAA electronic transaction (version 005010X222A1) that most claims are actually sent in. Each box on the form maps to a loop and segment in the 837P, and payers edit both against the same rules.
Can I still send paper CMS-1500 claims to Medicare?
Only if you meet an exception under the Administrative Simplification Compliance Act, most commonly having fewer than 10 full-time equivalent employees. Otherwise Medicare requires initial claims to be submitted electronically. Commercial payers set their own rules, and most prefer electronic claims.
Which CMS-1500 boxes cause the most denials?
Box 1a (patient ID), box 17 and 17b (referring provider), box 21 and 24E (diagnosis and pointer), box 24B (place of service), box 24D (procedure and modifiers), box 24J (rendering NPI) and box 33a (billing NPI). Most of these deny as CARC 16 with a remark code that names the field.