Coding Challenge: Understanding Underdosing

While underdosing is no longer a new concept in coding it remains a common area of confusion.  Underdosing is when a patient takes less of a medication than is prescribed. 

Our underdosing codes are found at Z91.1, Patient’s noncompliance with medical treatment and regimen.  That makes sense since that is exactly what we are assigning for – that taking less of a medication than is prescribed. 

There is a Code Also note here that instructs us to code also, if applicable, to identify underdosing of specific drug and it references the code range T36-T50 with a final character of 6.  Our Table of Drugs and Chemicals has a separate column to identify Underdosing.  There is also an Excludes 2 note here.   It instructs that caregiver noncompliance with patient’s medical treatment and regimen would not be coded here and directs us to      Z91.A-.

We have options for intentional and unintentional underdosing as well as other noncompliance.  Z91.12- is for Patient’s intentional underdosing of medication regimen.  Code Z91.120 notes intentional underdosing due to financial hardship.  Z91.128 notes intentional underdosing for other reason. 

You may see documentation noting the patient is cutting their pills in half in order to make their prescription last longer because of the cost.  This would be an intentional underdosing due to financial hardship.

Z91.13- is for Patient’s unintentional underdosing of medication regimen.  Code Z91.130 notes unintentional underdosing due to age-related debility.  Z91.138 notes unintentional underdosing for other reason.  If an Alzheimer patient forgets to take their medication because of their dementia, this would be an unintentional underdosing due to age-related debility.

Both the intentional and unintentional underdosing code sets have a Code First note instructing the coder to code first underdosing of medication with that code range T36-T50 with fifth or sixth character 6.  This is a code first note rather than the code also note at Z91.1 since the underdosing is established. 

If the patient’s intent is not documented or known, we do have Z91.14 for Patient’s other noncompliance with medication regimen.  This includes patient underdosing of medication not otherwise specified.  Codes here break down to that due to financial hardship and that due to other reason.  There is also a Code First note instructing us, if applicable, to code first the adverse effect of underdosing and again notes code range T36-T50.  This adverse effect may be an exacerbation of the condition the medication is prescribed for. 

Be sure to check the Official Guidelines for Coding and Reporting and Coding Clinic for guidance on underdosing.  First quarter 2024 Coding Clinic did instruct that underdosing would not be assigned when a PRN (as needed) medication is not taken.  Coding Clinic, First Quarter 2022 instructs that underdosing should be assigned even in situations where the patient’s condition or status does not change because less medication was taken. 

It notes documentation that the patient had stopped taking the medication on their own was sufficient.  In cases where the underdosing does change the patient’s condition, as that adverse effect, a code for that condition would also be assigned.