If you work in health care or medical billing, you’ve probably come across ICD-10 codes — those seemingly random strings of letters and numbers attached to every diagnosis. But what do they really mean? Learn more in the first installment of the Coding Corner, a new feature on the Science Speaks blog that updates IDSA members about coding and billing issues.
A quick ICD-10 breakdown
The International Statistical Classification of Diseases and Related Health Problems, 10th Revision, also known as ICD-10, is maintained by the World Health Organization and has been the standard in the U.S. health care system since 2015. These codes are used to standardize diagnoses, ensuring that terms like “left lower extremity cellulitis” and “left leg cellulitis” are coded the same way.
So, how is a code chosen? Whether it’s a health care provider or a medical biller selecting the correct ICD-10 code, the key is accurate medical documentation. The more specific the diagnosis, the more precise the code. For example:
- If the medical record simply states “cellulitis,” the general code L03.90 (Cellulitis, unspecified) would be used.
- If the record specifies “left leg cellulitis,” the more precise L03.116 (Cellulitis of left lower limb) is applied.
Why does specificity matter? Insurance companies are more likely to request documentation when an unspecified code is used, which can lead to delays in payments. Details like laterality (left, right or bilateral) and whether a condition is acute or chronic help ensure the correct ICD-10 code is assigned.
What’s with the “A”?
ICD-10 codes range from three to seven alphanumeric characters:
- The first character is always a letter.
- The second character is always a number.
- The seventh character (when required) is often used for obstetrics, injuries and poisoning.
For infectious diseases specialists, three common seventh characters are:
- “A” (Initial Encounter): Used when the patient is receiving active treatment for the condition.
- “D” (Subsequent Encounter): Used for follow-ups when the active treatment phase is over.
- “S” (Sequela): Used for long-term complications resulting from the original condition.
Example: Prosthetic joint infections
Let’s say a patient is diagnosed with an infection of a left prosthetic hip. The base ICD-10 code is:
T84.52 (Infection and inflammatory reaction due to internal left hip prosthesis)
However, because this code only has four alphanumeric characters, an “X” is used as a placeholder for the fifth and sixth characters.
- If the patient is actively being treated with antibiotics, the code would be: T84.52XA (Initial Encounter).
- If the patient is in a follow-up phase with no active treatment, the code would be: T84.52XD (Subsequent Encounter).
- If the patient has developed a long-term complication (e.g., chronic pain or scarring), the code would be: T84.52XS (Sequela).
Key takeaway
The seventh character in ICD-10 codes is not about how many times a patient has been seen, but rather about the stage of care they’re in. “A” is for active treatment, “D” is for follow-ups, and “S” is for complications.
The next time you see an ICD-10 code ending in “A,” you’ll know it’s more than just a random letter — it tells the story of the patient’s treatment journey.
The Coding Corner is a new blog feature that updates IDSA members about coding and billing issues. If you would like to ask a question for the Coding Corner, or if you have a general coding and billing question, please visit the IDSA Coding and Payment webpage and click on the yellow “Ask the Coder” button (member login required to submit a question).