Q&A: Sequencing unintentional poisoning with manifestations
Q: A patient with dementia has not taken prescribed Lasix for one week due to forgetting and presents with worsening acute heart failure. What is going to be sequenced first: the underdose or the acute heart failure?
A: According to the 2026 ICD-10-CM Official Guidelines for Coding and Reporting, “codes for underdosing should never be assigned as principal or first-listed codes.” The condition should be listed first followed by the underdosing T code as a secondary diagnosis code. With limited information provided, coders would report ICD-10-CM I50.9 for unspecified heart failure.
The underdose would be reported next. Looking up Lasix in the Table of Drugs and Chemicals and finding the underdosing column leads to ICD-10-CM subcategory T50.1X6- (underdosing of loop [high-ceiling] diuretics). The sixth character will always be “6” as that value is reserved for underdosing when it comes to poisoning codes. It is best practice to verify codes in the Tabular List before assigning them, as more characters may be required than what is listed in the table. In this case, a seventh character of “A” for initial encounter is required for this code. Therefore, the final code reported is T50.1X6A.
An additional code is needed to explain who is at fault for the underdose. Either a Z91.- or Y63.- code would be reported. Z91.- codes are for situations where the patient made the choice to underdose or mistakenly did so, while Y63.- codes are for situations that happened during clinical care. The patient did not mean to miss or reduce doses—it happened due to circumstances outside their control or understanding. Looking at the Tabular List, one of the Z91.13- codes would then be listed for the patient’s unintentional underdosing of a medication regimen. The diagnosis says the patient has dementia and forgot, so the final code would be Z91.130 for unintentional underdosing due to age-related debility.
Editor’s note: This Q&A was originally published in JustCoding. The information was provided by James F. Salter IV, CCS, a coding and regulatory education specialist for HCPro, and the answer was provided based on limited information submitted to JustCoding.