In medical coding, how do CPT and ICD-10-CM codes work together for payment?

• A. CPT codes describe diagnoses; ICD-10-CM describes services.
• B. CPT describes service; ICD-10-CM describes diagnoses.
• C. CPT and ICD-10-CM both describe procedures.
• D. CPT describes payer requirements.

Answer: (B)

The key idea is that billing relies on two pieces of information: what was done and why it was done. CPT codes describe the service or procedure performed—the actual work the clinician carried out. ICD-10-CM codes describe the patient’s diagnosis or the reason for the encounter—why the service was needed. For payment, payers look at the CPT code to determine the service’s reimbursement level and at the ICD-10-CM diagnosis to confirm medical necessity and the reason the service was appropriate. Both codes are reported together to justify the claim: the service (CPT) is linked to the condition (ICD-10-CM) that prompted it. For example, billing an MRI uses a CPT code for the imaging study, paired with an ICD-10-CM code for the diagnosed condition (such as knee pain due to osteoarthritis) to support payment. This pairing ensures the service is accurately described and medically justified.