D84.821 vs T50.91

D84.821 (Immunodeficiency due to drugs) compared with T50.91 (Poisoning by, adverse effect of and underdosing of multiple unspecified drugs, medicaments and biological substances), from the official CMS tabular data. Both may be reported when both conditions are documented (Excludes2).

Relationship

Sequencing

Additional-code relationship

D84.821

Immunodeficiency due to drugs

→

T50.91

Poisoning by, adverse effect of and underdosing of multiple unspecified drugs, medicaments and biological substances

Add a secondary code when documented.

“code for adverse effect if applicable, to identify adverse effect of drug (T36-T50 with fifth or six character 5)”

A "Use additional code" note means a secondary code should follow this one to fully describe the condition when the documentation supports it.

Source: CMS ICD-10-CM tabular instructional notes

Informational

Excludes2 relationship

T50.91

Poisoning by, adverse effect of and underdosing of multiple unspecified drugs, medicaments and biological substances

↔

D84.821

Immunodeficiency due to drugs

Distinct conditions — both may be reported.

“immunodeficiency due to drugs (D84.821)”

An Excludes2 note means the excluded condition is not part of the code above it, but a patient can have both. Report both when both are documented.

Source: CMS ICD-10-CM tabular instructional notes

Can these codes be reported together?

Yes, when both are documented. T50.91 carries an Excludes2 note covering D84.821: “immunodeficiency due to drugs (D84.821)”

Excludes2 marks distinct conditions — both may be reported when both are documented.

Sequencing on the claim

1stD84.821 then T50.91

D84.821 carries a Use Additional Code instruction covering T50.91: “code for adverse effect if applicable, to identify adverse effect of drug (T36-T50 with fifth or six character 5)”

Side by side

	D84.821Immunodeficiency due to drugs	T50.91Poisoning by, adverse effect of and underdosing of multiple unspecified drugs, medicaments and biological substances
Billing status	Billable	Non-billable header
Classification	D50-D89 — Diseases of the Blood and Blood-forming Organs and Certain Disorders Involving the Immune Mechanism (D50-D89)	S00-T88 — Injury, Poisoning and Certain Other Consequences of External Causes (S00-T88)
Definition	Immunodeficiency due to drugs is a billable ICD-10-CM diagnosis code (D84.821).	Poisoning by, adverse effect of and underdosing of multiple unspecified drugs, medicaments and biological substances is a non-billable ICD-10-CM category code (T50.91).
7th character	—	The appropriate 7th character is to be added to each code from category T50
Includes	defects in the complement system immunodeficiency disorders, except human immunodeficiency virus [HIV] disease sarcoidosis Immunodeficiency due to (current or past) medication	adverse effect of correct substance properly administered poisoning by overdose of substance poisoning by wrong substance given or taken in error underdosing by (inadvertently) (deliberately) taking less substance than prescribed or instructed
Use additional code	code for adverse effect if applicable, to identify adverse effect of drug (T36-T50 with fifth or six character 5) code, if applicable, for associated long term (current) drug therapy drug or medication such as: long term (current) drug therapy systemic steroids (Z79.52)	code(s) to specify: manifestations of poisoning underdosing or failure in dosage during medical and surgical care (Y63.6, Y63.8-Y63.9)
Code first	—	, for adverse effects, the nature of the adverse effect, such as: adverse effect NOS (T88.7)

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Derived from the official CMS ICD-10-CM tabular data (FY2026). Not billing advice; verify sequencing rules for the encounter. All data sources