What is icd-10-cm in medical billing?

Coding

What is ICD-10-CM?

Also called: diagnosis code · ICD-10 coding

ICD-10-CM is the diagnosis code set used in the United States to report the clinical reason for a service. Codes run three to seven characters, and the later characters carry specificity — laterality, encounter type, episode — that payers increasingly require before they will accept medical necessity.

The practical failure mode is unspecified codes. A code ending in a placeholder for "unspecified" is valid, but many payer policies will not accept it as supporting medical necessity for the procedure billed, and the claim denies for a reason that reads as clinical when it is really documentation.

ICD-10-CM updates annually on October 1, six months offset from the CPT cycle — which means a practice has two separate annual code maintenance obligations, not one.

Where Vizora handles this

Medical coding

Primary sources

Where "ICD-10-CM" is defined by the bodies that set the rules, rather than by us.

CPT code set (opens in a new tab)
American Medical Association — Maintainer of CPT. Annual changes to CPT are the most common cause of a sudden, unexplained rise in denials each January.
ICD-10-CM official guidelines and files (opens in a new tab)
CDC / National Center for Health Statistics — The official ICD-10-CM code files and coding guidelines, updated annually. Specificity requirements here drive a large share of medical necessity denials.
HCPCS Level II code set (opens in a new tab)
Centers for Medicare & Medicaid Services — Codes for supplies, drugs, DME and services outside CPT — and the modifier definitions that go with them.

Last reviewed August 20, 2026

Related terms

CPT Code

A CPT code is a five-character code maintained by the American Medical Association that identifies the procedure or service a provider performed. CPT answers what was done; ICD-10-CM answers why. Together they establish medical necessity, and a mismatch between them is a leading denial cause.

Medical Necessity

Medical necessity is a payer's determination that a service was appropriate for the patient's condition under its coverage policy. It is established by the pairing of diagnosis and procedure codes and supported by the documentation — which means a medically necessary service can still be denied if the coding does not demonstrate it.

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