Understand the structure of the ICD-10 code

ICD-10-CM has been the standard diagnostic coding for some time now – since 2015, in fact – but it’s complicated enough to cause occasional scratches to members of the healthcare community. But do not worry. The scratching of the head of the ICD-10 ceases today – at least as far as the code structure goes – because the codes are segmented in a standardized way, which means that they break down into smaller pieces (and less intimidating). So, let’s find out the coding structure of ICD-10 and learn it piece by piece.

The first six characters

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

Explanation of ICD-10 CM code set


The first three characters of an ICD-10 code designate the diagnostic category. In this case, the letter “S” indicates that the diagnosis relates to “injuries, poisoning and certain other consequences of external causes linked to regions of the single body”.

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


The next three characters (characters three to six) indicate the associated etiology (that is, the cause, all of the causes or the mode of causation of a disease or condition), the anatomical site , severity or other vital clinical details. So in this case, the numbers “0”, “1” and “1” indicate a diagnosis of “right Achilles tendon strain”.

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The seventh delicate character

Finally, there is the seventh character: the extension. According to CMS, he gives[s] information on the characteristics of the encounter. However, you can only assign the seventh character to specific codes in certain categories of ICD-10-CM. To determine which categories are eligible for the seventh character, you must refer to the Tabular list of codes – although most of these codes can be 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. Thus, if a code is less than six characters long and requires a seventh character extension, you must fill all empty character spaces with an “X” placeholder.

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

A – First meeting. It describes the entire period during which a patient receives active treatment for the injury, poisoning or other consequences of an external cause. So you can use “A” as the seventh character over more than the first claim. In fact, you can use it on multiple claims.

D – Later meeting. 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 usually includes rehabilitation therapy.) In the example above, suppose a doctor referred the patient to a physical therapist for rehabilitation of the patient’s strained Achilles tendon. Rehabilitation therapy would be considered part of the healing and recovery phase, so you would code for “later encounter”, thus assigning the seventh character “D”.

S – Sequela. The seventh character extension “S” indicates a complication or condition that results directly from an injury. An example of a sequel is a scar resulting from a burn.

Multiple codes, single condition

In some scenarios, you may need to register multiple codes for a single condition. The tabular list notes indicate whether you are required to report multiple codes. These notes use verbiage such as “Use additional code” or “Code first”. (“Code first” indicates that you must first code the underlying condition.) Remember that ICD-10 also includes several combination codes, which are unique codes used to classify two or more conditions that match. often produce together.

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

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

As noted above, the appropriate activity code for running on a treadmill is Y93.A1; the gymnasium’s place of occurrence code is Y92.39; and the external cause status code in this case would be Y99.8, recreating or not playing sports 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 sprained right Achilles tendon – an injury that they suffered during recreational running on a treadmill. rolling in a gym.


Understanding the ICD-10 coding structure will help you streamline your coding process, but there is a few more random bits of information rehabilitation therapists should know:

  • In addition to the codes in Chapter 19: Injury, Poisoning and Certain Other Consequences of External Causes, many codes applicable to rehabilitation therapists are found in Chapter 13: Musculoskeletal and Connective Tissue Diseases. 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 “multi-site” code. If no “multiple sites” code is available, you must report several codes to indicate all the different sites concerned.
  • In some treatment scenarios, the bone is affected at the lower end (eg, osteoporosis, M80, M81). Even though the affected area may be located at the level of the joint, the site of the disease is still considered the bone, not the joint.
  • Many musculoskeletal conditions are the result of a previous injury or trauma, or are recurrent conditions. Most bone, joint, or muscle conditions resulting from healed wounds appear in Chapter 13 (Chapter “M”). This chapter also includes most of the recurrent bone, joint or muscle conditions. So while you should generally code chronic or recurring injuries using codes from Chapter 13, you should code current acute injuries using the appropriate injury code from Chapter 19.
  • Some codes in chapter 13 (Musculoskeletal system and connective tissue diseases) may require external cause codes in addition to the musculoskeletal condition code to help identify the underlying cause of the disease.

See? When taken apart and examined piece by piece, the ICD-10’s coding structure isn’t all that scary. That said, we understand if you still have questions. Please feel free to drop them below and we’ll do our best to help you.


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