Segment cheat-sheet

The 30-or-so segments that account for roughly 90% of what you will read on any inbound or outbound transaction. Memorize the ones in Bold; refer back here for the rest.

Envelope

Segment	Position	Meaning	Key elements
ISA	First segment of every interchange	Interchange Control Header	ISA01-04 qualifiers, ISA06/ISA08 IDs, ISA13 control #, ISA15 P/T, ISA16 sub-element separator
GS	After ISA, before ST	Functional Group Header	GS01 functional ID code, GS02/GS03 app sender/receiver, GS06 group control #, GS08 impl convention
ST	First segment of each transaction	Transaction Set Header	ST01 transaction set ID (837, 835, 270, 278…), ST02 control #, ST03 impl convention (in newer transactions)
SE	Last segment of each transaction	Transaction Set Trailer	SE01 segment count (inclusive of ST and SE)
GE	After all STs in a group	Functional Group Trailer	GE01 # of STs, GE02 group control # (must match GS06)
IEA	Last segment of every interchange	Interchange Control Trailer	IEA01 # of GSs, IEA02 control # (must match ISA13)

Header / metadata (mostly 837 / 270 / 278)

Segment	Where	Meaning
BHT	After ST	Beginning of Hierarchical Transaction. Carries purpose code, originator ref, date/time. BHT02 is 00 (original), 08 for 277CA, etc.
REF	Throughout	Reference identification — many qualifiers (87 transaction-set policy, D9 claim ID, EA member ID alt, LU location).
DTP	Throughout	Date/time period. DTP01 qualifier (472 service date, 434 statement period, 573 line adjustment date).
PER	Submitter / contact	Administrative contact.
HL	837 only	Hierarchical Level. Establishes parent/child relationships for billing/subscriber/patient loops.

Names and parties

Segment	Where	Meaning
NM1	1000A/B, 2010s, 2310s, throughout	Name. NM101 qualifier (41 submitter, 40 receiver, 85 billing provider, IL insured, QC patient, PR payer, PE payee, 82 rendering, DN referring).
N3	Following NM1	Address line 1/2.
N4	Following N3	City / state / ZIP / country.
DMG	After NM1	Demographics — DMG01 D8, DMG02 DOB, DMG03 gender.
PER	After NM1	Contact info.

837 claim segments

Segment	Loop	Meaning
CLM	2300	Claim Information. CLM01 patient control #, CLM02 total amount, CLM05 POS+claim freq+claim type.
HI	2300	Health Care Information — diagnoses (HI*BK principal, HI*BF other), procedure codes, DRG (HI*DR), occurrence codes (HI*BH/HI*BI), value codes.
K3	2300	Free-form file information (rare; payer-specific).
PWK	2300	Paperwork — attachment indicators.
SV1	2400 (P)	Professional Service Line. CPT/HCPCS + modifiers + amount + units + diagnosis pointer.
SV2	2400 (I)	Institutional Service Line. Revenue code + HCPCS + amount + units + non-covered.
SV3 / SV5	(other types)	Dental / DME service lines.
LX	2400	Service line counter (1-based).
AMT	Various	Amount segments (AMT*F5 patient amount paid, AMT*EAF amount owed, AMT*F2 allowed amount).

Authorization (278) segments

Segment	Loop	Meaning
UM	2000E	Health Care Services Review. Carries request type, certification type, service type.
HSD	2000E	Health Care Services Delivery. Quantity, frequency, unit-of-measure.
HCR	2000E (response)	Health Care Services Review. HCR01 action code (A1 certified, A2 certified partial, A3 not certified, A4 pended, CT contact payer).
HI	2000E	Diagnoses for the auth (same HI*BK/HI*BF shape as 837).

835 remittance segments

Segment	Loop	Meaning
BPR	Header	Beginning Payment & Remittance. Carries payment method (CHK/ACH), amount, effective date, DFI info.
TRN	After BPR	Trace Number. The unique-per-payer tracking ID.
N1	Header	Payer / payee name. N1*PR payer, N1*PE payee.
CLP	2100	Claim Payment. CLP01 patient control # (echoes our 837 CLM01), CLP02 claim status, CLP03 total billed, CLP04 total paid, CLP07 payer claim control #.
CAS	2100 / 2110	Claim Adjustment. CAS01 group code (CO/PR/OA/PI), then triplets of (reason, amount, qty).
NM1*QC	2100	Patient.
SVC	2110	Service Payment Info. Procedure code (composite), billed, paid, units, etc.
AMT	2110	Service-line amounts (AMT*B6 allowed, AMT*KH deductible amount).
PLB	Trailer	Provider-Level Adjustment — provider-wide adjustments outside any single claim.

Eligibility (270 / 271) segments

Segment	Where	Meaning
BHT	After ST	Begin Hierarchical — BHT06 is RU realtime / RT request.
HL	Throughout	Hierarchical level (info source → info receiver → subscriber → dependent).
NM1	Multiple	Names — 2B info source, 1P info receiver, IL subscriber, 03 dependent.
EQ	270 / 2110C	Eligibility / Benefit Inquiry. Service type code (single, e.g. 30 health benefit plan, or R for full set).
EB	271 / 2110C	Eligibility or Benefit Information. The response payload. EB01 benefit info code (1 active, 6 inactive, A co-insurance, C deductible, B copay).
MSG	271 / 2110C	Free-form text — payer notes.
AAA	270 or 271	Request Validation. Used both for accept / reject. AAA01 Y valid / N invalid.
DTP	Throughout	Dates — eligibility window, plan begin/end.

Acknowledgment segments

See 9.3 — Acknowledgment codes for the full code lists.

Segment	Where	Meaning
TA1	Inside the ISA	Interchange acknowledgment.
AK1	999	Functional Group Response Header. AK101 HC echoes GS01.
AK9	999	Functional Group Response Trailer. AK901 A/P/E/R.
IK3	999	Implementation Data Segment Note — segment-level error.
IK4	999	Implementation Data Element Note — element-level error.
STC	277CA / 277	Status Information. Carries the per-claim acknowledgment / status decision.
CTX	277CA / 999	Context — the location in the original transaction the status / error refers to.

See also

• Glossary — for higher-level terminology.
• Acknowledgment codes — the codes inside STC / AK9 / IK3 / TA1.