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Understanding the ICD-10 Code Structure

We’re breaking down everything you need to know about the ICD-10 code structure but were too confused to ask.

Melissa Hughes
5 min read
June 27, 2023
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ICD-10-CM has been the standard diagnosis coding set for a while now—the implementation of ICD-10 was in 2015, after all—but the tenth revision is still complicated enough to cause the occasional head-scratch for members of the healthcare community. But don’t worry. The ICD-10 head-scratching stops today—at least with respect to the ICD-10 code structure—because the codes are segmented in a standardized fashion, which means they break down into smaller (and less intimidating) bite-sized pieces. So, let’s tear into the ICD-10 coding structure and learn about it piece by piece.

The First Six Characters of the ICD-10 Code Structure

Codes in the ICD-10-CM code set can have anywhere from three to seven characters. The more characters there are, the more specific the diagnosis. The first character is always alpha (i.e., a letter), but characters two through seven can be either alpha or numeric. Let’s take a look at an example. 

icd10 code structure


The first three characters of the ICD-10 code structure designate the category of the diagnosis. In this instance, the letter “S” designates that the diagnosis relates to “Injuries, poisoning and certain other consequences of external causes related to single body regions.”

“S,” used in conjunction with the numerals “8” and “6,” indicates that the diagnosis falls into the category of “Injury of muscle, fascia and tendon at lower leg.” A three-character category that has no further subdivision (i.e., no greater specificity) can stand alone as a code. In this case, however, greater specificity is possible, and you should fill in as many “blanks” as you can.


The next three characters (characters three through six) indicate the related etiology (i.e., the cause, set of causes, or manner of causation of a disease or condition), anatomic site, severity, or other vital clinical details. So, in this case, the numbers “0,” “1,” and “1” indicate a diagnosis of “Strain of the right Achilles tendon.”


The Tricky Seventh Character

Finally, there’s the seventh character: the extension. According to CMS, it “provide[s] information about the characteristic of the encounter.” You can only assign the seventh character to specific codes in certain ICD-10-CM categories, however. To determine which categories are seventh-character eligible, you’ll have to refer to the Tabular List of codes—though most of these codes are found in Chapter 19 (Injury, poisoning and certain other consequences of external causes), Chapter 15 (Pregnancy, childbirth and the puerperium), and sometimes Chapter 13 (Diseases of the musculoskeletal system and connective tissue).  

The extension character must always be in the seventh position. So, if a code has fewer than six characters and requires a seventh character extension, you must fill in all of the empty character spaces with a placeholder “X.” 

Seventh character extensions for injuries (not including fractures) include:

A – Initial encounter. This describes the entire period in which a patient is receiving active treatment for the injury, poisoning, or other consequences of an external cause. So, you can use “A” as the seventh character on more than just the first claim. In fact, you can use it on multiple claims.

D – Subsequent encounter. This describes any encounter after the active phase of treatment, when the patient is receiving routine care for the injury during the period of healing or recovery. (This generally includes rehabilitation therapy.) In the example above, let’s assume a physician referred the patient to a physical therapist for rehabilitation of the patient’s strained Achilles tendon. Rehab therapy would be considered part of the healing and recovery phase, so you would code for “subsequent encounter,” thus assigning the seventh character “D.”

S – Sequela. The seventh character extension “S” indicates a complication or condition that arises as a direct result of an injury. An example of a sequela is a scar resulting from a burn.

Multiple Codes, Single Condition

In some scenarios, you may need to record multiple codes for a single condition. Notes in the Tabular List indicate whether you’re required to report more than one code. These notes use verbiage like “Use additional code” or “Code first.” (“Code first” indicates you should code the underlying condition first.) Remember that ICD-10 also includes several combination codes, which are single codes used to classify two or more conditions that often occur together.

With injury codes, you often will submit external cause codes that further describe the scenario that resulted in the injury. You will find these codes in Chapter 20: External cause codes. These are secondary codes, which means they further describe the cause of an injury or health condition by capturing how it happened (cause), the intent (intentional or accidental), the place the event occurred, the activity the patient was engaged in at the time of the event, and the person’s status (e.g., civilian or military). You can assign as many external cause codes as necessary to explain the patient’s condition to the fullest extent possible.

In this case, let’s say a patient strained an Achilles tendon while running on a treadmill at a gym. To code for this particular set of circumstances, you will need an activity code, a place of occurrence code, and an external cause status code.

As shown above, the appropriate activity code for running on a treadmill is Y93.A1; the place of occurrence code for a gym is Y92.39; and the external cause status code in this case would be Y99.8, recreating or sport not for income or while a student.

So, in this example, you would submit a grand total of four ICD-10 codes to accurately describe that the patient presented with a right Achilles tendon sprain—an injury the patient suffered while recreationally running on a treadmill at a gym.

Important Notes 

Understanding the ICD-10 code structure will help streamline your coding process—but there are a few more random tidbits of info that rehab therapists should know:

  • In addition to codes found in Chapter 19: Injury, poisoning and certain other consequences of external causes, many of the codes applicable to rehab therapists appear in Chapter 13: Diseases of the musculoskeletal system and connective tissue. Most of these codes have site and laterality designations to describe the bone, joint, or muscle related to the patient’s condition.
  • For conditions involving multiple sites, such as osteoarthritis, there is often a “multiple sites” code. If no “multiple sites” code is available, you should report multiple codes to indicate all of the different sites involved.
  • In some treatment scenarios, the bone is affected at the lower end (e.g., Osteoporosis, M80, M81). Even though the affected area may be located at the joint, the site of the condition is still considered the bone, not the joint.
  • Many musculoskeletal conditions are the result of previous injury or trauma, or they are recurrent conditions. Most bone, joint, or muscle conditions resulting from healed injuries appear in Chapter 13 (the “M” chapter). This chapter also includes most recurrent bone, joint, or muscle conditions. So, while you generally should code chronic or recurrent injuries using Chapter 13 codes, you should code current, acute injuries using the appropriate injury code from Chapter 19.
  • Some codes in Chapter 13 (Diseases of the musculoskeletal system and connective tissue) may need external cause codes in addition to the musculoskeletal condition code to help identify the underlying cause for the condition. 

See? When taken apart and examined piece by piece, the ICD-10 code structure isn’t all that scary. And you can appreciate how the specificity provided in the structure can help in medical coding for diseases and injury across all specialties. That said, we understand if you still have questions. Feel free to drop them below, and we’ll do our best to help you out.

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