Have you been gnawed on lately by Bertha or her sister? These girls can be brutal! And if you had to see a doctor about it, a medical coder may have assigned W55.21XA, Bitten by cow, intitial encounter, as an external cause code.

A cow that bites and external cause code W55.21XA Bitten by cow, initial encounter

External Cause Code for Bitten by Cow

Yes, it sounds kind of silly. I didn’t even know cows could bite. In fact, I have been told by friends who farmed in my home state of Nebraska that cows don’t bite, at least not as a form of aggression. If someone puts a finger in one of their mouths, however, anything is possible I suppose. And you may need treatment for it.

At any rate, we have a code for it. It’s W55.21XA to indicate the patient was bitten by a cow and it is an initial encounter. The patient is receiving active treatment.

If you are new to the coding world or haven’t had to report it, this may be something you weren’t aware of. This code is considered an external cause code and can be found in Chapter 20 of the ICD-10-CM coding manual. External cause codes start with V, W, X, or Y.

Code Structure in ICD-10-CM

According to CMS (Centers for Medicare & Medicaid Services), codes in ICD-10-CM are built as follows:

  • 3 to 7 characters in each code, with 3 characters before the decimal, and 4 characters after the decimal
  • The first character is always alpha
  • The second character is numeric
  • Characters 3 through 7 are either alpha or numerical
  • Alpha characters are not case-sensitive, e.g., S93.611A, s93.611A, S93.611a, s93.611a

Let’s take a look at the specifics of how W55.21XA was built: 

W55: these first three characters indicate the category. W55 is the category for “Contact with other mammals.” 

The first character “W” is alpha, character 2 is numeric, and character 3 is numeric in this case. 

A decimal point is used after the first three characters. 

W55.2: the 4th character “2” indicates contact with, specifically, a cow (this code also pertains to a bull). 

The 4th character can be changed to “0” to mean cat (W55.0, “1” to mean horse (W55.1), “3” to mean other hoof stock (W55.3), and so on. 

W55.21: the 5th character “1” indicates the injury was a bite. A “2” would mean the patient was struck by a cow, and a “9” would mean there was “other” contact with a cow.

W55.21X: the 6th character “X” is a placeholder which is needed because this code requires seven characters. Without the placeholder “X,” the code would be invalid.

W55.21XA: the 7th character “A” indicates this was an initial encounter. If we were to change it to a “D,” it would mean it was a subsequent visit, and if we were to change it to an “S,” it would mean the patient has a residual or late effect of the original cow (or bull) bite.  

Locating the External Cause Code

To locate this code in the ICD-10-CM coding manual, we need to go to the External Cause of Injuries Index and look under Bite, bitten by. Looking at the subcategory cow, it refers us to W55.21. In the Tabular, we can verify W55.21 as Bitten by cow. However, we are not done. There is a note at this code to assign an “X” placeholder (A = initial encounter, D = subsequent encounter, S = sequela). In this case, “A” has been added to the code to indicate it is an initial encounter and the patient is receiving active treatment. Our code is W55.21XA.

According to the 2019 Official Coding Guidelines, external cause codes should never be sequenced as the first-listed or principal diagnosis. These codes explain how an injury or medical condition occurred, the intent, where it occurred, the activity of the patient at the time of the event, and the status of the patient.


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