Box 21 on the CMS-1500 form

Box 21 on the CMS-1500 form is used to provide the diagnosis or nature of illness or injury. Specifically, it is where the ICD-10-CM codes (International Classification of Diseases, 10th edition, Clinical Modification) are entered to describe the patient’s condition or diagnosis that necessitated the medical service.

Key Details for Box 21:

1. Purpose:
   • To list the diagnosis codes that explain the medical condition being treated.
   • The diagnosis codes directly support the medical necessity of the services being billed on the CMS-1500 form.
2. Format:
   • ICD-10-CM Codes are entered here. Each code is typically alphanumeric and can have up to 7 characters.
   • You can list up to 12 diagnosis codes in Box 21. The number of codes you use depends on the complexity of the diagnosis and the payer’s specific requirements.
3. How to Complete Box 21:
   • Enter the ICD-10-CM code(s) (e.g., J03.90 for acute tonsillitis).
   • If there are multiple diagnoses, list them in descending order of significance, with the primary diagnosis (or the most relevant) in the first position.
   • Ensure that the codes match the services provided and are consistent with the clinical records.
4. Linking Diagnoses to Services (Diagnosis Pointers):
   • In Box 24E, use diagnosis pointers (letters A-L) to link the diagnosis codes in Box 21 to specific procedures or services in Box 24D. For example:
     • If Diagnosis Code J03.90 (acute tonsillitis) was linked to a procedure for a throat culture, you would enter the corresponding letter in Box 24E (e.g., “A” if this was the first diagnosis).
5. Important Notes:
   • Make sure the diagnosis codes are current and accurate based on the patient’s condition.
   • Double-check that the codes align with the payer’s coverage criteria and medical necessity requirements.
   • Incorrect or outdated diagnosis codes can result in claim denials or delays.

Example of Box 21:

In this example, three diagnosis codes are provided, with each linked to a specific service or procedure as indicated in Box 24E.