Ace the CMS-1500 Claim Form Test 2026 – Master the Art of Medical Billing Magic!

On a CMS-1500 claim, the diagnosis codes that explain the reason for the visit are placed in the block labeled Diagnosis or nature of illness or injury. This is Block 21. Enter the ICD-10-CM codes here, typically with the primary diagnosis first and any secondary diagnoses following, up to four codes. When billing line items, each service line includes a diagnosis pointer that refers back to the relevant code(s) in Block 21, linking the service to the correct condition. The other blocks mentioned are used for different information and not for listing diagnosis codes, so Block 21 is the correct place for diagnoses.