The ICD-10 code set might possibly be some of the most boring reading material on the planet—unless, of course, you’re into that sort of thing. But I think it’s safe to say that the mainstream media probably isn’t, which is most likely why they’ve spent so much of their time highlighting the more entertaining aspects of the coding manual: specifically, the external cause codes in Chapter 20. Sure, these supplemental codes offer a healthy dose of comic relief to an otherwise dull subject matter, but they’ve spurred quite a few questions and some concerns. So here are the five things you absolutely, positively must know about external cause codes:

1. Most of the time, they’re optional. Per the ICD-10-CM Official Guidelines for Coding and Reporting, “Unless a provider is subject to a state-based external cause code reporting mandate or these codes are required by a particular payer, reporting of ICD-10-CM codes in Chapter 20, External Causes of Morbidity, is not required.” However, even though there’s no national requirement forcing the issue, there are a plenty of good reasons to include external cause codes when you can. After all, they do “provide valuable data for injury research and evaluation of injury prevention strategies.” And according to this article, such data could instigate important preventative education and action. Take motorcycle accidents, for example: If data showed a high frequency of motorcycle accidents in a particular area, local leaders could use that information to demonstrate the need to devote budget dollars to additional motorcycle safety education.

2. You can’t use them all the time—even if you want to. In some cases, adding an external cause code to a diagnosis code simply doesn’t make sense, like in the case of T36OX1 (poisoning by penicillins, accidental), which has the cause built right in. And external cause codes can’t stand alone; they must always be paired with a principal diagnosis code.

Not sure to which codes you can apply an external cause code? Check the tabular list. Above each applicable diagnosis code category, you’ll see instructions to do so.

3. There are four types of external cause codes, each of which answers one of the following questions:

  • How did the injury or condition happen?
  • Where did it happen?
  • What was the patient doing when it happened?
  • What it intentional or unintentional?

You can apply as many external cause codes as necessary to fully explain the patient’s condition. Here’s an example: Let’s say this is the first time you’re treating a patient for a strain of his right Achilles tendon. You’d select the diagnosis code S86.011A (strain of right Achilles tendon, initial encounter). After examining the patient, you find that the patient’s injury occurred as a result of running on the treadmill at a gym he visits for recreation (not work). To code for this, you’d submit the following external cause codes:

  • Y93.A1 (activity, treadmill)
  • Y92.39 (gymnasium as the place of occurrence of the external cause)
  • Y99.8 (other external cause status, recreation or sport not for income or while a student)

4. If you choose to use them, you only need to do so at the initial encounter (for the most part). Typically, you need only to report place of occurrence, activity, and external cause status codes during the patient’s initial evaluation. However, there are a few codes—specifically those that describe how an injury happened—that you can report at other points throughout the patient’s care. These codes usually require a seventh character designating the encounter type.

5. If you’re using multiple external cause codes for a single diagnosis code, report them in order of significance. The first cause code you list should be the one that most closely relates to the principal diagnosis; the last code you list should be the one that least closely relates. However, according to the official coding guidelines, external cause codes for the following events take precedence over all other external cause codes, in the following order of significance (abuse should be listed first no matter what else you code):

  1. Child and adult abuse
  2. Terrorism events
  3. Cataclysmic events
  4. Transport accidents

See, despite all the fuss over external cause codes—and the jokes about patients being struck by an orca—they’re really not that bad. Want a more in-depth look at external cause coding? Check out section 20 of the official coding guidelines. As always, if you have questions, leave them in the comments section