What does Box 21 on the CMS-1500 form refer to?

Box 21 on the CMS-1500 form is designated for Diagnosis Codes, which are critical for communicating the reason for the patient's visit or the conditions being treated. This box allows providers to specify the various diagnoses that justify the medical services rendered. Accurately entering diagnosis codes in this section is essential as it influences the processing of the claim by insurance companies, and helps in determining the medical necessity of the services provided.

Including this information helps ensure that the insurance payer understands the clinical justification for the treatment, which is necessary for proper reimbursement. The correct entry in Box 21, therefore, plays a vital role in the billing process and in the overall management of healthcare claims.