ICD 10 Coding Alert

Nail down which codes qualify only as secondary diagnoses.

Every coder knows that assigning the most accurate ICD-10-CM code allows you to demonstrate why a service was performed, thereby helping prove medical necessity for all of your services. But one thing that some coders don’t know is that not every diagnosis code can be used as primary.

Why? Some ICD-10-CM codes were specifically created to serve as secondary or tertiary diagnoses. In these cases, such codes exist only to provide additional information to the payer, and these codes must be reported in addition to a primary diagnosis code.

Check out several examples of ICD-10-CM codes that shouldn’t be used as primary diagnoses:

External Causes of Morbidity (V00-Y99)

“The external causes of morbidity codes should never be sequenced as the first-listed or principal diagnosis,” the 2025 ICD-10-CM Official Guidelines for Coding and Reporting FY 2025 note in chapter 20. Instead, these codes were created to provide data for evaluation of illness/injury prevention strategies, or for illness/injury research. They tell payers how a particular injury or condition occurred, whether it was accidental or intentional, where it occurred, and the patient’s status (such as military, civilian, etc.).

Most payers don’t require you to use external cause codes, but providers are encouraged to use them for data collection, and to provide more details to insurers. You can use just one external cause code or several if you need additional ways to share information about your patient.

Example: Suppose a female patient presents complaining of bleeding with urination following an extended ride on a bicycle on a bumpy bicycle path. The physician diagnoses her with a minor contusion of the urethra.

In this situation, you’d report S37.32XA (Contusion of urethra, initial encounter) followed by V18.0XXA (Pedal cycle driver injured in noncollision transport accident in nontraffic accident, initial encounter) and Y92.482 (Bike path as the place of occurrence of the external cause).

Codes With Etiology/Manifestation Convention Situations

Some medical conditions include both an underlying etiology and manifestations in multiple areas of the body that stem from the underlying etiology. In these cases, the ICD-10-CM system requires that you sequence the underlying condition first and report the manifestation second.

“Wherever such a combination exists, there is a ‘use additional code’ note at the etiology code, and a ‘code first’ note at the manifestation code. These instructional notes indicate the proper sequencing order of the codes, etiology followed by manifestation,” the ICD-10-CM guidelines indicate in Section A.13.

Example: If you look up dementia with Parkinson’s disease, the ICD-10-CM guidelines advise you to report a code from the G20 (Parkinson’s disease) series first to indicate the etiology, followed by a code from the F02.80-F02.81 (Dementia in other diseases) series to represent the manifestation of dementia in diseases classified elsewhere.

You may also see “code first” and “use additional code” notations throughout your ICD-10-CM code book to share the sequencing rules for codes that have specific guidelines but aren’t part of an etiology/manifestation combination.

Sequela Codes

When an ICD-10-CM code has the 7th character “S,” that means the acute phase of the illness has passed and the provider is addressing the sequala (residual effect) of the illness or injury. “Coding of sequela generally requires two codes sequenced in the following order,” the ICD-10-CM guidelines note. “The condition or nature of the sequela is sequenced first. The sequela code is sequenced second.” The sequela code, with the 7^th character of “S” shouldn’t typically be the only diagnosis on a claim.

For example: Suppose a patient says a scar on the back of their left hand has recently become painful. The provider notes that five years ago, the patient suffered a third-degree burn to that location and notes that the scar is a sequela to the burn. In this situation, the injury (the burn) has healed, and the patient is experiencing no further issues from the injury itself. However, the injury has created a scar, which is a sequela of the injury and the reason for the encounter.

In this case, you would code both the scar and the injury that caused it, using the 7th character “S” for the injury code like this:

• L90.5 (Scar conditions and fibrosis of skin)
• T23.362S (Burn of third degree of back of left hand, sequela)

There’s one exception to this rule: When the sequela code is followed by a manifestation code or the sequela code has been expanded to include the manifestation, then it may be reportable as primary. The code that describes the acute phase of the illness or injury that led to the sequala is never used with a code for the late effect, the guidelines state.

Torrey Kim, Contributing Writer, Raleigh, North Carolina