HL7 AL1 Segment: Patient Allergy Information

The HL7 AL1 segment (Patient Allergy Information) carries allergy and adverse reaction data for a patient. It repeats for each known allergy and appears primarily in ADT messages. Accurate allergy communication is a patient safety requirement — allergies must propagate to every downstream system that participates in medication ordering, lab testing, or surgical planning.

AL1 Field Reference

AL1 6 fields

All Fields Required & Critical With Notes

Seq	Name	Type	Opt	Description
AL1-1	Set ID	SI	R	Sequence number (1, 2, 3...)
★ AL1-2	Allergen Type Code	CE	O	DA (Drug), FA (Food), MA (Misc), MC (Contrast), EA (Environment)
★ AL1-3	Allergen Code/Description	CE	R	Code and name of the allergen
★ AL1-4	Allergy Severity Code	CE	O	SV (Severe), MO (Moderate), MI (Mild), U (Unknown)
★ AL1-5	Allergy Reaction Code	ST	O	Description of the reaction (e.g., Anaphylaxis, Rash)
AL1-6	Identification Date	DT	O	When the allergy was first documented

AL1-1 R

Set ID SI

Sequence number (1, 2, 3...)

★ AL1-2 O

Allergen Type Code CE

DA (Drug), FA (Food), MA (Misc), MC (Contrast), EA (Environment)

DA=Drug Allergy (most common), FA=Food, MA=Miscellaneous (latex), MC=Contrast dye/MRI contraindications, EA=Environmental (pollen, dust).

★ AL1-3 R

Allergen Code/Description CE

Code and name of the allergen

CE data type: code^name^coding system. Some systems use RxNorm, NDF-RT, or local codes. Integration engines should map between coding systems.

★ AL1-4 O

Allergy Severity Code CE

SV (Severe), MO (Moderate), MI (Mild), U (Unknown)

Drives CDS response: SV=Hard stop (pharmacist must override), MO=Soft stop (provider must acknowledge), MI=Informational alert.

★ AL1-5 O

Allergy Reaction Code ST

Description of the reaction (e.g., Anaphylaxis, Rash)

AL1-6 O

Identification Date DT

When the allergy was first documented

R = Required, O = Optional, C = Conditional, W = Withdrawn (backward compatibility only)

AL1-2: Allergen Type Codes (Table 0127)

Code	Type	Description
DA	Drug Allergy	Medication allergy (most common)
FA	Food Allergy	Food or dietary allergy
MA	Miscellaneous Allergy	Latex, adhesives, other materials
MC	Miscellaneous Contraindication	Contrast dye, MRI contraindications
EA	Environmental Allergy	Pollen, dust, animal dander

AL1-3: Allergen Code

AL1-3 uses CE (Coded Element) to identify the specific allergen:

AL1|1|DA^Drug Allergy^HL70127|PCN^Penicillin^HL70127|SV^Severe^HL70128|Anaphylaxis|20150601

Component	Value	Meaning
CE.1	PCN	Allergen code
CE.2	Penicillin	Allergen name
CE.3	HL70127	Coding system

Some systems use RxNorm, NDF-RT, or local codes instead of HL7 Table 0127. Integration engines should map between coding systems when allergen codes differ between sending and receiving systems.

AL1-4: Allergy Severity Codes (Table 0128)

Code	Severity	Clinical Significance
SV	Severe	Life-threatening reaction — hard stop in ordering systems
MO	Moderate	Significant reaction — warning in ordering systems
MI	Mild	Minor reaction — informational alert
U	Unknown	Severity not determined

Severity drives the clinical decision support (CDS) response:

• SV (Severe): Hard stop — pharmacist must override to dispense a contraindicated medication
• MO (Moderate): Soft stop — provider must acknowledge the alert before proceeding
• MI (Mild): Informational — displayed but does not block ordering

Sample AL1 Segments

Multiple allergies for one patient:

AL1|1|DA^Drug Allergy^HL70127|PCN^Penicillin^HL70127|SV^Severe^HL70128|Anaphylaxis|20150601
AL1|2|DA^Drug Allergy^HL70127|ASA^Aspirin^HL70127|MO^Moderate^HL70128|GI Bleeding|20180315
AL1|3|FA^Food Allergy^HL70127|PEANUT^Peanuts^LOCAL|SV^Severe^HL70128|Anaphylaxis|20100101
AL1|4|EA^Environmental^HL70127|LATEX^Latex^LOCAL|MI^Mild^HL70128|Contact Dermatitis|20200601

Key Implementation Considerations

No Known Allergies (NKA)

When a patient has no known allergies, the system should communicate this explicitly rather than omitting AL1:

AL1|1|DA|NKA^No Known Allergies^LOCAL|U

An absent AL1 is ambiguous — it could mean “no allergies” or “allergies not assessed.” Explicitly stating NKA confirms the assessment was performed. Some systems use NKDA (No Known Drug Allergies) to distinguish between drug-specific and comprehensive allergy reviews.

Patient Safety Implications

Allergy data has direct patient safety consequences:

• Drug-allergy checking: Pharmacy systems cross-reference AL1-3 against ordered medications for class-based alerts (e.g., Penicillin allergy → alert for Amoxicillin)
• Cross-reactivity: Drug allergy classes (beta-lactams, sulfonamides) have cross-reactivity rules that CDS systems enforce
• Pre-procedure screening: Contrast dye allergies (AL1-2 = MC) trigger pre-medication protocols before CT scans
• Surgical planning: Latex allergies (AL1-2 = MA) require latex-free OR setup

Full Replacement on Update

When ADT^A08 (Update Patient) includes AL1 segments, receiving systems should:

• Replace all allergies: The incoming AL1 set represents the complete current allergy list
• Do not merge: Adding new AL1 segments to existing allergies can create duplicates
• Audit trail: Maintain a history of allergy changes for patient safety reviews

Allergy Reconciliation

Different clinical systems may store allergies in different formats:

• EHR: Coded allergies with RxNorm or NDF-RT identifiers
• Pharmacy: Drug-class allergies mapped to formulary
• Lab: Specimen handling alerts (latex sensitivity)

Integration engines should map allergen codes between systems and flag discrepancies for clinical review. A patient documented as “Penicillin allergy” in one system and “Amoxicillin allergy” in another may need reconciliation.

Related Resources

HL7 ADT Messages ADT message structure — where AL1 segments communicate patient allergies during registration and updates.

PID Segment Reference Patient demographics — AL1 extends PID with allergy and adverse reaction data.

HL7 Workbench Parse and validate HL7 messages with segment highlighting and field lookup.

HL7 Resource Center Message types, segments, reference guides, and integration tools.