HCFA-1500 Box 21 - Diagnosis or Nature of Illness or Injury

On the HCFA-1500 form, Box 21 is used to report the patient’s diagnosis or condition using ICD-10-CM (or ICD-9-CM for older claims) codes. This box supports up to 12 diagnosis codes, which help payers determine the medical necessity of the services billed.

Tip: Enter each diagnosis code without decimals when submitting electronically, but include the decimal when entering on the printed HCFA-1500 form for clarity. Codes must be valid for the date of service and match the coding guidelines for the specific payer.

In our example below, the provider reported two diagnosis codes: J09.X1 (Influenza due to identified novel influenza A virus with pneumonia) and J09.X9 (Influenza due to identified novel influenza A virus with other manifestations).

In the X12 837P 5010 transaction, Box 21 maps to the HI segment in the 2300 Claim Information Loop. The HI segment is used to transmit diagnosis codes and related information in a standardized format.

Sample 837P (5010) – 2300 HI Segment

Segment Breakdown:

• HI01-1: Diagnosis Code Qualifier (ABK = ICD-10-CM Principal Diagnosis)
• HI01-2: First diagnosis code (J09X1) without the decimal
• HI02-1: Diagnosis Code Qualifier (ABF = ICD-10-CM Additional Diagnosis)
• HI02-2: Second diagnosis code (J09X9) without the decimal

Note: The decimal is omitted in the EDI transaction, but should be included on the printed claim form for human readability. Diagnosis codes must be valid for the date of service and must comply with payer requirements.

ICD-10-CM Resources

Our website provides two helpful ICD-10-CM tools for finding and reviewing diagnosis codes:

• ICD-10-CM Code Lookup – Search for diagnosis codes quickly by keyword or code.
• ICD-10-CM Resources – Access guidelines, updates, and full code listings.

For additional information on the X12 837 standard, please refer to the official implementation guides published by X12.

HCFA-1500 Form Box Locations

• HCFA-1500 Box 0 - Carrier Block
• HCFA-1500 Box 1 - Insurance Type
• HCFA-1500 Box 1a - Insured's ID Number
• HCFA-1500 Box 2 - Patient's Name
• HCFA-1500 Box 3 - Patient's Birth Date and Sex
• HCFA-1500 Box 4 - Insured's Name
• HCFA-1500 Box 5 - Patient's Address
• HCFA-1500 Box 6 - Patient Relationship to Insured
• HCFA-1500 Box 7 - Insured's Address
• HCFA-1500 Box 8 - Reserved For NUCC Use
• HCFA-1500 Box 9 - Other Insured's Name
• HCFA-1500 Box 9a - Other Insured's Policy or Group Number
• HCFA-1500 Box 9b - Reserved For NUCC Use
• HCFA-1500 Box 9c - Reserved For NUCC Use
• HCFA-1500 Box 9d - Insurance Plan Name or Program Name
• HCFA-1500 Box 10 - Is Patient's Condition Related To
• HCFA-1500 Box 10d - Claim Codes
• HCFA-1500 Box 11 - Insured's Policy Group or FECA Number
• HCFA-1500 Box 11a - Insured's Date of Birth and Sex
• HCFA-1500 Box 11b - Other Claim ID
• HCFA-1500 Box 11c - Insurance Plan Name or Program Name
• HCFA-1500 Box 11d - Is There Another Health Plan
• HCFA-1500 Box 12 - Patient's or Authorized Person's Signature
• HCFA-1500 Box 13 - Insured's or Authorized Person's Signature
• HCFA-1500 Box 14 - Date of Current Illness, Injury, Pregnancy (LMP)
• HCFA-1500 Box 15 - Other Date
• HCFA-1500 Box 16 - Dates Patient Unable to Work in Current Occupation
• HCFA-1500 Box 17 - Name of Referring Provider or Other Source
• HCFA-1500 Box 17a - Other ID
• HCFA-1500 Box 17b - NPI
• HCFA-1500 Box 18 - Hospitalization Dates Related to Current Services
• HCFA-1500 Box 19 - Additional Claim Information
• HCFA-1500 Box 20 - Outside Lab Charges
• HCFA-1500 Box 21 - Diagnosis or Nature of Illness or Injury [You are Here]
• HCFA-1500 Box 22 - Resubmission Code and Original Reference Number
• HCFA-1500 Box 23 - Prior Authorization Number
• HCFA-1500 Box 24a - Date(s) of Service
• HCFA-1500 Box 24b - Place of Service
• HCFA-1500 Box 24c - EMG
• HCFA-1500 Box 24d - Procedures, Services, or Supplies
• HCFA-1500 Box 24e - Diagnosis Pointer
• HCFA-1500 Box 24f - Charges
• HCFA-1500 Box 24g - Days or Units
• HCFA-1500 Box 24h - EPSDT/Family Plan
• HCFA-1500 Box 24i - Rendering Provider ID Qualifier
• HCFA-1500 Box 24j - Rendering Provider ID
• HCFA-1500 Box 25 - Federal Tax ID Number
• HCFA-1500 Box 26 - Patient's Account No.
• HCFA-1500 Box 27 - Accept Assignment
• HCFA-1500 Box 28 - Total Charge
• HCFA-1500 Box 29 - Amount Paid
• HCFA-1500 Box 30 - Rsvd for NUCC Use
• HCFA-1500 Box 31 - Signature of Physician or Supplier Including Degrees or Credentials
• HCFA-1500 Box 32 - Service Facility Location Information

Grab a sample of the HCFA-1500 claim form here - HCFA 02/12 Claim Form