Poisoning, Adverse Effect, or Underdosing: Which T36–T50 Column, and What Goes First

Section I.C.19.e opens by saying what kind of code these are:

Official source

“Codes in categories T36-T65 are combination codes that include the substance that was taken as well as the intent. No additional external cause code is required for poisonings, toxic effects, adverse effects and underdosing codes.”

Every drug subcategory in T36–T50 is laid out the same way. Take T39.01, aspirin: the 6th character is the column.

Each then takes a 7th character — A, D or S — for the encounter. The column is a coding decision about how the drug came to cause harm, and the Guidelines define each column by that.

Adverse effect (character 5): the reaction goes first

Official source

“When coding an adverse effect of a drug that has been correctly prescribed and properly administered, assign the appropriate code for the nature of the adverse effect followed by the appropriate code for the adverse effect of the drug (T36-T50). The code for the drug should have a 5th or 6th character ‘5’ (for example T36.0X5-) Examples of the nature of an adverse effect are tachycardia, delirium, gastrointestinal hemorrhaging, vomiting, hypokalemia, hepatitis, renal failure, or respiratory failure.”

Two conditions have to hold: correctly prescribed and properly administered. The tabular writes the sequencing into the T36–T50 section as a Code First note “for adverse effects, the nature of the adverse effect,” naming aspirin gastritis (K29.-) and adverse effect NOS (T88.7) among its examples. The drug code is the second code.

Poisoning (characters 1–4): the drug goes first, intent is in the code

Official source

“When coding a poisoning or reaction to the improper use of a medication (e.g., overdose, wrong substance given or taken in error, wrong route of administration), first assign the appropriate code from categories T36-T50. The poisoning codes have an associated intent as their 5th or 6th character (accidental, intentional self-harm, assault and undetermined). If the intent of the poisoning is unknown or unspecified, code the intent as accidental intent. The undetermined intent is only for use if the documentation in the record specifies that the intent cannot be determined. Use additional code(s) for all manifestations of poisonings.”

The Guidelines list what counts as poisoning: an error in prescription or administration by anyone; an intentional overdose; a nonprescribed drug taken with a correctly prescribed one, where the interaction causes the reaction; and a drug–alcohol interaction. The third of these is the one that surprises people — the prescribed drug was taken correctly, but adding something else turns the whole event into a poisoning. Abuse or dependence of the substance, when also diagnosed, is an additional code.

Note the default and the exception: unknown intent is coded accidental; character 4, undetermined, is reserved for a record that says intent could not be determined. “Not documented” and “documented as undeterminable” are different columns.

Underdosing (character 6): never first

Official source

“Underdosing refers to taking less of a medication than is prescribed by a provider or a manufacturer’s instruction. Discontinuing the use of a prescribed medication on the patient's own initiative (not directed by the patient's provider) is also classified as an underdosing. For underdosing, assign the code from categories T36-T50 (fifth or sixth character ‘6’). Documentation of a change in the patient’s condition is not required in order to assign an underdosing code. Documentation that the patient is taking less of a medication than is prescribed or discontinued the prescribed medication is sufficient for code assignment.”

Official source

“Codes for underdosing should never be assigned as principal or first-listed codes. If a patient has a relapse or exacerbation of the medical condition for which the drug is prescribed because of the reduction in dose, then the medical condition itself should be coded.”

Underdosing is the only column that codes an absence of drug. What leads is whatever the missing drug was treating, when it has relapsed; the underdosing code follows, and the Guidelines add that noncompliance codes (Z91.12-, Z91.13-, Z91.14-, Z91.A4-) or complication-of-care codes (Y63.6–Y63.9) “are to be used with an underdosing code to indicate intent, if known.”

The same drug, aspirin, in three different charts. Each is an initial encounter.

1. Prescribed low-dose aspirin, taken as directed, develops gastritis. Correctly prescribed and properly administered, so this is an adverse effect: the gastritis code from K29.- first, then T39.015A. The T36–T50 Code First note names aspirin gastritis as its own example.
2. Took a large quantity of aspirin; record does not state intent. Improper use, so poisoning; unknown intent defaults to accidental: T39.011A first, then a code for each manifestation. If the record instead says the overdose was intentional self-harm, the column changes to T39.012A; if it says intent could not be determined, T39.014A.
3. Stopped prescribed aspirin on their own; the condition it was prescribed for has flared. Underdosing: the flared condition is coded first — it is the reason for the encounter and underdosing can never lead — then T39.016A, then a Z91.12- or Z91.14- code for the intent behind the noncompliance if documented.

Two housekeeping rules from I.C.19.e apply across all three: “Do not code directly from the Table of Drugs and Chemicals. Always refer back to the Tabular List,” and when several drugs are involved, “code each individually unless a combination code is listed in the Table of Drugs and Chemicals” — with T50.91- for multiple unspecified substances. The same code is assigned only once even if it describes the agent for more than one reaction.

The sequencing in each column has a payer echo. The Medicare Code Editor’s edit 09 (unacceptable principal diagnosis) lists T39.015A — the adverse-effect column — and T39.016A, D and S — the underdosing column — but not T39.011A or T39.012A, the poisoning columns. Across T36–T50, 163 initial-encounter underdosing codes are on that list, along with the Z91.12- and Z91.A4- noncompliance codes. That is the Guidelines’ own logic restated as an edit: an adverse effect follows the reaction it caused, underdosing never leads, and a poisoning can. Sequence per Section I.C.19.e and the edit is satisfied as a consequence; the rule does not come from the edit.

1. Determine how the drug caused harm: correctly prescribed and properly administered (adverse effect), improper use or interaction (poisoning), or less than prescribed (underdosing).
2. Adverse effect: code the reaction first, then the T36–T50 code with 5th/6th character 5.
3. Poisoning: T36–T50 code first with the intent character (unknown intent → accidental; undetermined only when the record says so), then manifestations; add abuse/dependence if diagnosed.
4. Underdosing: never first; the relapsed condition leads, then the …6 code, then Z91.12-/.13-/.14-/.A4- or Y63.6–Y63.9 for intent if known.
5. Verify in the Tabular — never code from the Table of Drugs alone — and code each drug separately unless a combination code exists; T50.91- for multiple unspecified.

• Adverse-effect column for a drug taken with alcohol or a nonprescribed drug — that is a poisoning.
• Undetermined intent for an undocumented intent — the default is accidental.
• Underdosing code as principal.
• Requiring a worsened condition before coding underdosing — not required.
• Coding from the Table of Drugs without verifying the Tabular.

• Whether the drug was correctly prescribed and properly administered.
• The intent of a poisoning, and whether the record states that intent cannot be determined.
• Interaction with alcohol or a nonprescribed drug.
• Taking less than prescribed, or self-discontinuation, and any stated reason.
• The manifestation(s) of the poisoning or reaction.