T78.40XA (Allergy, unspecified, initial encounter) compared with T78.40XS (Allergy, unspecified, sequela), from the official CMS tabular data. T78.40XA and T78.40XS code the same injury at a different phase of treatment. MedCoder-derived
Same underlying diagnosis, different encounter phase.
The 7th character records the phase of care for the encounter being coded, so a single encounter takes one of these characters, not several. Review whether both phases belong on the same episode of care.
Source: ICD-10-CM Official Guidelines, Section I.C.19.a
Same injury, other encounter phase. T78.40XA and T78.40XS code the same injury at a different phase of treatment, set by the 7th character.
The 7th character records the type of care given at the encounter being coded (ICD-10-CM Official Guidelines, Section I.C.19.a), so one encounter takes one of these, not both.
| T78.40XAAllergy, unspecified, initial encounter | T78.40XSAllergy, unspecified, sequela | |
|---|---|---|
| Billing status | Billable | Billable |
| Classification | S00-T88 — Injury, Poisoning and Certain Other Consequences of External Causes (S00-T88) | S00-T88 — Injury, Poisoning and Certain Other Consequences of External Causes (S00-T88) |
| Definition | Allergy, unspecified, initial encounter is a billable ICD-10-CM diagnosis code (T78.40XA). | Allergy, unspecified, sequela is a billable ICD-10-CM diagnosis code (T78.40XS). |
| 7th character | The appropriate 7th character is to be added to each code from category T78 | The appropriate 7th character is to be added to each code from category T78 |
| Includes | Allergic reaction NOS Hypersensitivity NOS | Allergic reaction NOS Hypersensitivity NOS |
Derived from the official CMS ICD-10-CM tabular data (FY2026). Not billing advice; verify sequencing rules for the encounter. All data sources