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Physical Therapists’ Guide to ICD-10

We're breaking down everything rehab therapists need to know about using ICD-10 codes in their documentation.

Heidi Jannenga
5 min read
February 21, 2023
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If you’re a HIPAA-covered provider, you’ve been reporting ICD-10 codes for the better part of a decade. In addition to replacing the decades-old ICD-9, ICD-10 offers a far greater level of specificity in coding patient diagnosis. And while that level of specificity is great for clinicians and coders, it does make for a lot more codes to familiarize yourself with — 69,000 compared to ICD-9’s 13,000, to put a finer point on it. 

In addition to a host of new codes, ICD-10 also introduced a new code structure. Whereas ICD-9 codes consist of three to five characters with a decimal point (e.g., 813.15), ICD-10 codes contain three to seven characters in an alpha-numeric combination (e.g., M96.831).

Note: ICD-10 codes are completely separate from CPT codes. The transition to ICD-10 does not affect the use of CPT codes. Additionally, ICD-10 codes do not impact guidelines regarding the KX modifier.

What is the history of ICD-10?

The World Health Organization (WHO)—the public health sector of the United Nations that focuses on international health and outbreaks—started developing the ICD-10 coding system in 1983, but didn’t actually finish it until 1992. Yes, it took almost a decade to create ICD-10, and it has taken more than a decade for the U.S. to actually put the final version of the code set to use.

Australia was one of the first countries to adopt ICD-10. Half of the Australian states implemented ICD-10 in 1998, and the rest of the country followed in 1999. Canada adopted the new code set in 2000, and from there, several European countries as well as Thailand, Korea, China, and South Africa adopted ICD-10 in its original, modified, or translated form. Even Dubai made the switch in 2012.

So, why did it take so long for the US to follow suit? Well, the complexity of our healthcare system, the competing interest groups that hold influence over our healthcare decision-makers, and a lot—a lot—of red tape made it tough for the US to get ICD-10 off the ground. Then, there were the delays: the original deadline for transitioning to the new code set was October 1, 2013. But, 2013 became 2014. And then, just when we thought ICD-10 had finally broken through the bureaucratic red tape and that the implementation extensions were over, 2014 became 2015.

Why ICD-10?

So, what about ICD-10 makes it so much better than ICD-9? Well, the massive number of codes means that medical providers—including rehab therapists—can more accurately document clinical information, including patient diagnoses. Ultimately, that fosters:

  • Greater opportunity for evidence-based practice
  • Better insight for optimizing grouping and reimbursement processes
  • Seamless exchange of data across all healthcare platforms (i.e., interoperability)

ICD-10 offers long-overdue updates to medical terminology and disease classification. It also contains codes that allow for comparison of mortality and morbidity data. And continuing with the data theme, ICD-10’s super-specific code set empowers those in the healthcare field to better:

  • Evaluate patient care
  • Support research initiatives
  • Construct payment systems
  • Process claims
  • Make clinical decisions
  • Observe public health trends
  • Uncover fraud

Furthermore, the US was the last country in the world with modern healthcare to adopt ICD-10 diagnosis codes. Most countries—including Australia and Canada—gave ICD-9 the boot a while ago because it:

  • Produced limited data about patients’ medical conditions
  • Contained outdated terminology that was not consistent with modern medical practice
  • Limited the addition of new codes

Do I have to use ICD-10?

If you’re covered by HIPAA, you must submit ICD-10 codes in order to receive reimbursement from HIPAA-covered entities. (The same holds true for cash-based providers whose patients receive the reimbursement directly from their insurance providers.) However, some non-covered entities such as auto and workers compensation insurance carriers may still require ICD-9 codes, as they were not mandated to make the switch.

What is the ICD-10 code structure?

Codes in the ICD-10-CM code set can have anywhere between three and seven characters. Many three-character codes are used as headings for categories of codes that can further expand to four, five, or six characters. You should only use three-character codes if there’s not a more specific code available.

ICD 10  Code Structure

Let’s take a look at an example: S86.011D, Strain of right Achilles tendon, Subsequent encounter.

The first three characters of any ICD-10 code indicate the category of the diagnosis. In the example above, the letter “S” signifies 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.” As mentioned above, a three-character category can stand alone as a code as long as there is no further specificity available. In this particular example, though, it is possible to achieve greater specificity. And you always want to fill in as many “blanks” as you can.

The next three characters (spaces 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. In this example, the numbers “0,” “1,” and “1” indicate a diagnosis of “Strain of the right Achilles tendon.”

Do you or your staff need an ICD-10 refresh? Watch this free webinar—consider it your ICD-10 boot camp.

What is the 7th character?

Finally, there’s the seventh character. This character represents one of the most significant differences between ICD-9 and ICD-10. Using ICD-9, there’s no way to capture the details that the ICD-10 seventh character provides. However, not all ICD-10 codes require the seventh character—and for some, adding one can invalidate the code altogether.

So, how do you know which codes require a seventh character—and which don’t? You check the instructions that precede each chapter. If you don’t see a reference to a seventh character, leave the slot empty. As a side note, seventh characters are required for codes within Chapter 19 (Injury, poisoning, and certain other consequences of external causes) and Chapter 15 (Pregnancy, childbirth, and the puerperium).

While rehab therapists probably won’t use many codes in the pregnancy and childbirth chapter, they most likely will have to code for conditions related to injuries. For injuries, poisonings, and other external causes, the seventh character provides information about the episode of care, and there are seventh-character extensions for many of these conditions (with the exception of fractures, which have their own seventh-character system).

The seventh character extensions for injuries are:

  • A – Initial encounter: The entire period in which a patient is receiving active treatment for the injury, poisoning, or other consequences of an external cause. Please note that you can use “A” as the seventh character on more than one claim (i.e., it doesn’t necessarily have to be the patient’s initial visit).
  • D – Subsequent encounter: Any encounter that follows the active phase of treatment. This is the period during which the patient receives routine care for the injury while he or she heals or recovers (this often includes rehabilitation therapy). For instance, 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. In this case, rehab therapy is a component of the healing and recovery period, so you would assign the seventh character “D” to code for a subsequent encounter.
  • S – Sequela: Indicates a complication or condition that arises as a direct result of an injury. A scar resulting from a burn would be an example of a sequela.

“A” vs. “D”

Since ICD-10’s inception, there’s been a lot of debate as to the difference between A (initial encounter) and D (subsequent encounter) with respect to rehab therapy encounters. In fact, on a CMS national provider call, one attendee asked for clarification, and here’s the CMS representative’s response: “There is no specific hard set definition of what active treatment is. There are some examples that are given in the official guidelines, such as surgical treatment, emergency department encounter, and that type of situation. So there—it’s not an all-exhaustive list. But what I think is probably clearer is that for the subsequent encounters, usually those are where there’s routine healing or a problem with the healing.”


How do I code for surgical aftercare?

According to this presentation, providers shouldn’t use aftercare Z codes for aftercare of injuries or fractures when seventh characters are necessary to identify subsequent care. However, if you’re providing surgical aftercare, Z-codes are perfectly appropriate. For example, you could use Z51.89, encounter for other specified aftercare, or Z47.1, aftercare following joint replacement surgery. However, as this article notes, “you should not submit Z51.89 as a patient’s sole diagnosis—if you can help it—because on its own, this code might not adequately support the medical necessity of therapy treatment. Thus, using it as a primary diagnosis code could lead to claim denials.” In fact, whenever you use an aftercare code, you also should code for the underlying conditions/effects. For chronic or recurrent bone, muscle, or joint conditions, check out Chapter 13.

For more on using Z codes in ICD-10, check out this post.

How do I use multiple ICD-10 codes for a single condition?

In some cases, you might need to submit multiple codes to describe a single condition. To find out whether more than one code is required for a particular scenario, check the notes section in the Tabular List. There, you’ll find directives such as “Use additional code” or “Code first” (“Code first” indicates you should code the underlying condition first). Also, keep in mind that there are single combination codes (i.e., one code that indicates multiple diagnoses) you can use to classify conditions that often occur simultaneously.

External Cause Codes

With injury codes, you often will submit external cause codes that further describe the scenario that resulted in the injury. These codes are listed in Chapter 20: External cause codes. They’re secondary codes, which means they expand upon the description of the cause of an injury or health condition by indicating how it happened (i.e., the cause), the intent (i.e., intentional or accidental), the location, what the patient was doing at the time of the event, and the patient’s status (e.g., civilian or military). You should use as many external cause codes as necessary to explain the patient’s condition as completely as possible. However, external cause codes need only be used once, usually at the initial encounter.

Going back to our example above, let’s say the patient strained his or her Achilles tendon while running on a treadmill at a gym. To code for this set of circumstances, you would need an activity external cause code, a place of occurrence code, and an external cause status code.

In this case, the activity code would be Y93.A1 (running on a treadmill); the place of occurrence code would be Y92.39 (gym); and the external cause status code 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 diagnosis codes to accurately describe that the patient presented with an Achilles tendon sprain in his or her right foot, an injury the patient suffered while recreationally running on a treadmill at a gym. Here are the four codes:

  • S86.011D = Strain of right Achilles tendon, Subsequent encounter
  • Y93.A1 = Activity, exercise machines primarily for cardiorespiratory conditioning, treadmill
  • Y92.39 = Gymnasium as a place of occurrence of the external cause
  • Y99.8 = Recreation or sport not for income or while a student

Click here to watch a short video about finding and using ICD-10 external cause codes.

What are unspecified codes?

Unspecified codes are available for the rare cases in which there is absolutely no other, more specific option. If a more specific option is available, you should use it.

What was the CMS grace period?

During the ICD-10 grace period (October 1, 2015–September 30, 2016), “Medicare review contractors will not deny physician or other practitioner claims billed under the Part B physician fee schedule through either automated medical review or complex medical record review based solely on the specificity of the ICD-10 diagnosis code as long as the physician/ practitioner used a valid code from the right family.” During this time, providers received reimbursement from Medicare—even if they didn’t use the most specific ICD-10 code available. If your code was in the right family—or category—of codes, you got paid.

As we mentioned above, the first three characters of an ICD-10 code usually denote the code family. The remaining characters account for the specific clinical details relevant to each patient’s condition. During the grace period, submitting any code with the correct first three digits was enough to get you paid. However, since the grace period has now concluded, all providers must code to the highest level of specificity possible—or risk claim denials and even documentation audits.

To learn more about how to choose the right ICD-10 code, check out this FAQ.

How do I prevent claim denials?

Speaking of claim denials, here are four ways you can prevent them, so you get properly reimbursed for your services, no matter the payer:

1. Be specific.

ICD-10 is meant to enhance diagnostic specificity for patient data. No matter what payers considered “acceptable” to process payments previously, today they’re demanding you use ICD-10 as it was intended. Good thing you’ve had some time to practice locating the right family of codes as well as the most specific codes available—seventh characters and all—to convey your patients’ conditions to the fullest possible extent. And don’t forget to ensure your documentation fully supports your claim.

2. Audit your codes.

Regardless of whether you’ve implemented an EMR—or not (eek)—you’re ultimately responsible for ensuring that you submit the proper codes for your patients. So, double—or triple—check that the codes you should submit are the ones that are actually going through. To do so, schedule an internal audit to make sure your documentation matches your claims. This is especially important if you use an EMR that doesn’t have an intelligent ICD-10 tool.

3. Continue learning.

Sure, the deadline to transition to ICD-10 has come and gone, but that doesn’t mean the education stops. There’s still plenty to learn in terms of navigating the new coding system—especially those tricky situations. So, devote some time to staying up to date on all things ICD-10, including each of your payer’s individual specifications.

4. Choose the best EMR.

Your EMR should—at the very least—do the following (and if it doesn’t, well, it’s time to switch to one that does):

  • Prompt you to document patient condition details
  • Suggest that you select more specific codes when ones with greater specificity exist
  • Bases its ICD-10 code library on more than general equivalence mappings (GEMs)
  • Offer free system upgrades and lifetime access to ICD-10 support and training resources

Does ICD-10 change my documentation?

The short answer is “no.” Sure, ICD-10 helps healthcare providers better communicate detailed diagnostic information through codes. However, codes aren’t enough by themselves; providers must also continue to complete detailed documentation to support their code selection. According to CMS, “Quality clinical documentation is essential for communicating the intent of an encounter, confirming medical necessity, and providing detail to support ICD-10 code selection.”

What else should I know about ICD-10?

Here are five more things physical therapists should know about ICD-10

  1. As mentioned above, many codes applicable to rehab therapists appear in Chapter 19: Injury, poisoning, and certain other consequences of external causes. Additionally, there are several therapy-related codes 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.
  2. For diagnoses affecting multiple sites—osteoarthritis, for example—there often are “multiple sites” codes available. If you cannot find a “multiple sites” code for the diagnosis in question, you should report multiple codes to cover all of the sites.
  3. Bone vs. Joint: Some conditions affect the bone at the lower end (e.g., osteoporosis, M80, M81). However, even if the affected area is located at the joint, the site of the condition is still considered the bone, not the joint.
  4. Acute traumatic vs. Chronic or Recurrent: Many musculoskeletal conditions result from previous injury or trauma, or they are recurrent conditions. Within Chapter 13 (the “M” chapter), you’ll find most bone, joint, or muscle conditions resulting from healed injuries. Chapter 13 also contains most recurrent bone, joint, or muscle conditions. So, while you should code chronic or recurrent injuries using Chapter 13 codes, you should use an injury code from Chapter 19 to designate current, acute injuries.
  5. There are frequent updates to IDC-10-CM codes, so be sure to check the CMS page for the 2024 ICD-10-CM for added and updated ICD-10 codes.

Overall, how did the transition to ICD-10 go?

In the days leading up to the transition from ICD-9 to ICD-10, tensions ran high. The threat of delayed—and denied—reimbursements felt all too real. After all, no one really knew how payers were going to process the new codes—not to mention how providers and billers were going to be able to deal with such a huge code set. However, things went relatively smoothly—thanks in part to Medicare’s one-year grace period. According to this RevCycle Intelligence article, RelayHealth Financial successfully processed more than 13 million ICD-10 claims—worth more than $25 billion—in the first three weeks of the transition alone. Sure, there were a few challenges in terms of productivity and workflow; however, most providers said the implementation process went smoother than expected.

That being said, some providers did experience challenges with their EHRs. In this Healthcare Dive article, Richard Bruno—AAFP board member and resident in a joint family and preventive medicine program in Baltimore—said: “The challenge has been with the transition, especially within the medical records system, using the electronic health record and making sure that it’s searchable and that the right codes are associated with the right people, as these are tied to payment.” To handle the greater level of specificity, Munger’s practice actually had to upgrade its EHR.

And EHRs weren’t the only challenge; causation coding proved tough as well. Although the challenges inherent to causation coding aren’t anything new, many providers found it difficult to reach the necessary level of specificity because they weren’t able to obtain enough information. It seems that the Centers for Disease Control and Prevention (CDC) and the ICD-10 governing committee are aware of opportunities to provide more guidance around the use of causation codes. However, it doesn’t appear to be a top priority—at least not yet.

Even with these challenges, the American Medical Association believes that the transition was a success overall. The same Healthcare Dive article cited above noted that there was “no major uptick in Medicare claims rejections.”

What are the most commonly used ICD-10 codes? 

Here are the most frequently used ICD-10 codes by WebPT Members from September 2022 through February 2023:

What are the new ICD-10 codes implemented in response to COVID-19?

On January 1, 2021, several new COVID-19 ICD-10 codes went into effect, including:

  • Z20.822 Contact with and (suspected) exposure to COVID-19
  • Z11.52 Encounter for screening for COVID-19
  • Z86.16 Personal history of COVID-19
  • M35.81 Multisystem inflammatory syndrome (MIS)
  • M35.89 Other specified systemic involvement of connective tissue 
  • J12.82 Pneumonia due to coronavirus disease 2019

What about ICD-11?

In the summer of 2018, The World Health Organization (WHO) released the 11th edition of the International Classification of Diseases (ICD-11); however, the earliest that ICD-11 could have been  implemented was January 1, 2022—and according to this article, it’s unlikely that we’ll see ICD-11 adoption before 2025—or possibly 2027, according to this article from the AAPC. And given how long it took for the US to implement ICD-10, it’s unlikely that it will be a quick transition—although, in this 3M article, author Rebecca Caux-Harry writes, “Let’s hope the delay won’t be as long or as intimidating this time around.”

According to Caux-Harry, “There are some major differences between ICD-10 and ICD-11, including the coding scheme, meaning how the codes are structured. ICD-11 changes the three-character category codes (the characters to the left of the decimal) to four with an alpha character in the second position and a number always in the third position. The first character of any code will correlate to the chapter number. For chapters 1-9, the first character of the code will mirror the chapter number, for chapters 10-27, the first character is a letter. So all codes in a single chapter will always begin with the same character.”

In the article “Preparing for the International Classification of Diseases, 11th Revision (ICD-11) in the US Health Care System” James A. Feinstein. MD, MPH, Peter J. Gill, MD, DPhil, MSc, and Brett R. Anderson, MD, MBA, MSc, cite some of the other advantages ICD-11 offers, including: 

  • Specific diagnoses that were previously left out;
  • A “novel clustered code structure comprised of a stem code joined to optional postcoordination codes” that allows for greater flexibility and usefulness without the need for local customization; and

The potential to support automated or AI-assisted coding.   

To familiarize yourself with the new code set now, check out this WHO resource page.

Where can I go to learn more about ICD-10?


While the ICD-10 Ombudsman and ICD-10 Coordination Center (ICC) are no longer available to answer provider questions directly, CMS does offer links to additional resources for ICD-10 questions on this resource page, including this ICD-10 resource guide and contact list for providers.

The American Hospital Association (AHA) Portal

You can submit specific clinical coding questions to the AHA at this site. However, please do not ask the service to code your entire superbill; send an entire patient record and ask for proper coding; ask for the appropriate code for a certain disease or procedure; or ask about payments, coverage issues, or general equivalence maps (GEMs).

WebPT’s ICD-10 Bootcamp

WebPT offers free ICD-10 training courses as part of our ICD-10 Bootcamp

The Code Set

You can access the code set in its entirety here. However, if you’d like a PT-specific ICD-10 code book for educational purposes, you can purchase one at a discount within the WebPT Marketplace here (only available for WebPT Members). If you’re not a WebPT Member, you can read this helpful guide from the APTA.

Find the right ICD-10 codes for your patients—the first time around—with WebPT.

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Download your copy of the One ICD-10 FAQ to Rule Them All.

Still wondering when to use the initial encounter designation and when to use the subsequent encounter one? Confused about how to code for aftercare? Enter your email below, and we’ll send you the definitive ICD-10 resource for physical therapists.


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