Message type

Laboratory order

Submits a laboratory order with specimen information. The ORC and OBR segments define the order, and SPM carries specimen details. Laboratory information systems consume this to accession and process specimens.

76segments

21required

10groups

v2.9HL7 version

# Zato
from zato.hl7v2 import HL7Message

# An incoming message ..
raw = 'MSH|^~\&|SENDER|FAC|RCV|DEST|20260701||' \
      'OML^O21^OML_O21|MSG001|P|2.9\r' \
      'ORC|NW|ORD001||GRP001|||1^BID\r' \
      'OBR|1||ORD001'

# .. parsed into a Python object ..
msg = HL7Message.parse(raw)

# .. whose fields we can now access.
order_control = msg.orc.order_control
order_number = msg.orc.placer_order_number
order_id = msg.obr.placer_order_number
test = msg.obr.universal_service_identifier

Python examples

Build and parse OML^O21 messages in Python

How to construct, send, receive, and extract fields from real-world OML^O21 messages.

Parse an incoming OML^O21 message

Demonstrates receiving and extracting key fields from a laboratory order transaction

# Zato
from zato.hl7v2 import HL7Message

# An incoming message ..
raw = 'MSH|^~\&|SENDER|FAC|RCV|DEST|20260701||' \
      'OML^O21^OML_O21|MSG001|P|2.9\r' \
      'ORC|NW|ORD001||GRP001|||1^BID\r' \
      'OBR|1||ORD001'

# .. parsed into a Python object ..
msg = HL7Message.parse(raw)

# .. whose fields we can now access.
order_control = msg.orc.order_control
order_number = msg.orc.placer_order_number
order_id = msg.obr.placer_order_number
test = msg.obr.universal_service_identifier

Tutorials

Learn by building

Step-by-step guides for working with HL7 v2 in Zato.

MLLP

Receive and parse HL7 v2 messages

Set up an MLLP channel, parse incoming messages, and respond with acknowledgments.

Start tutorial Build

Build and send HL7 v2 messages

Construct HL7 v2 messages from scratch using typed segment classes and send them over MLLP.

Start tutorial

FAQ

Frequently asked questions

Use the HL7v2 message parser to extract the typed structure. The parsed message gives you direct access to every segment and field by name.

from zato.hl7v2 import HL7Message

raw = 'MSH|^~\&|SENDER|FACILITY|RECEIVER|DEST|20260401120000||OML^O21^OML_O21|MSG00001|P|2.9'
msg = HL7Message.parse(raw)

order_control = msg.orc.order_control
order_number = msg.orc.placer_order_number

Beyond the mandatory MSH header, the required segments are: PID, NK1, PV1, IN1, ORC, TQ1, OBR, OBX. Optional segments provide additional detail when available.

Zato connects to any system that speaks HL7v2 over MLLP or FHIR over REST. This includes Epic, Cerner, Meditech, Allscripts, and other EHR platforms.

Yes, Zato has built-in MLLP support for sending and receiving HL7v2 messages. MLLP channels handle the framing protocol automatically.

Zato supports HL7 v2.9, which is backward compatible with earlier versions including v2.3, v2.5, and v2.7. Standard segments and datatypes are available as typed Python classes.

Yes. Zato handles both HL7v2 and FHIR natively, so you can parse v2 messages and build FHIR resources with IG-specific extensions in the same service. Typed Python classes are available for both protocols, including extensions from US Core, UK Core, Da Vinci, and other Implementation Guides.

Reference

OML^O21 segment composition

Segments that make up the OML^O21 message, in order. Groups are shown with their child segments.

#	Segment	Usage	Repeats	Description	Group
1	`MSH`	Required	No	Message header	-
2	`ARV`	Optional	Yes	Access restriction	-
3	`SFT`	Optional	Yes	Software segment	-
4	`UAC`	Optional	No	User authentication credential	-
5	`NTE`	Optional	Yes	Notes and comments	-
6	`PID`	Required	No	Patient identification	PATIENT
7	`PD1`	Optional	No	Patient additional demographic	PATIENT
8	`PRT`	Optional	Yes	Participation information	PATIENT
9	`OH1`	Optional	Yes	Employment status	PATIENT
10	`OH2`	Optional	Yes	Past or present job	PATIENT
11	`OH3`	Optional	No	Usual work	PATIENT
12	`OH4`	Optional	Yes	Combat zone work	PATIENT
13	`NTE`	Optional	Yes	Notes and comments	PATIENT
14	`NK1`	Required	No	Next of kin / associated parties	PATIENT
15	`OH2`	Optional	Yes	Past or present job	PATIENT
16	`OH3`	Optional	No	Usual work	PATIENT
17	`ARV`	Optional	Yes	Access restriction	PATIENT
18	`PV1`	Required	No	Patient visit	PATIENT
19	`PV2`	Optional	No	Patient visit - additional info	PATIENT
20	`PRT`	Optional	Yes	Participation information	PATIENT
21	`IN1`	Required	No	Insurance	PATIENT
22	`IN2`	Optional	No	Insurance additional info	PATIENT
23	`IN3`	Optional	No	Insurance additional info - certification	PATIENT
24	`GT1`	Optional	No	Guarantor	PATIENT
25	`AL1`	Optional	Yes	Patient allergy information	PATIENT
26	`ORC`	Required	No	Common order	ORDER
27	`NTE`	Optional	Yes	Notes and comments	ORDER
28	`PRT`	Optional	Yes	Participation information	ORDER
29	`TQ1`	Required	No	Timing / quantity	ORDER
30	`TQ2`	Optional	Yes	Timing / quantity relationship	ORDER
31	`OBR`	Required	No	Observation request	ORDER
32	`TCD`	Optional	No	Test code detail	ORDER
33	`NTE`	Optional	Yes	Notes and comments	ORDER
34	`PRT`	Optional	Yes	Participation information	ORDER
35	`CTD`	Optional	No	Contact data	ORDER
36	`DG1`	Optional	Yes	Diagnosis	ORDER
37	`REL`	Optional	No	Clinical relationship segment	ORDER
38	`OBX`	Required	No	Observation result	OBSERVATION
39	`PRT`	Optional	Yes	Participation information	OBSERVATION
40	`TCD`	Optional	No	Test code detail	OBSERVATION
41	`NTE`	Optional	Yes	Notes and comments	OBSERVATION
42	`SPM`	Required	No	Specimen	SPECIMEN
43	`NTE`	Optional	Yes	Notes and comments	SPECIMEN
44	`OBX`	Required	No	Observation result	SPECIMEN
45	`PRT`	Optional	Yes	Participation information	SPECIMEN
46	`SAC`	Required	No	Specimen container detail	SPECIMEN
47	`NTE`	Optional	Yes	Notes and comments	SPECIMEN
48	`OBX`	Required	No	Observation result	CONTAINER_OBSERVATION
49	`PRT`	Optional	Yes	Participation information	CONTAINER_OBSERVATION
50	`IPC`	Optional	No	Imaging procedure control segment	ORDER
51	`SGH`	Optional	No	Segment group header	ORDER
52	`PID`	Required	No	Patient identification	PRIOR_RESULT
53	`PD1`	Optional	No	Patient additional demographic	PRIOR_RESULT
54	`PRT`	Optional	Yes	Participation information	PRIOR_RESULT
55	`ARV`	Optional	Yes	Access restriction	PRIOR_RESULT
56	`PV1`	Required	No	Patient visit	PRIOR_RESULT
57	`PV2`	Optional	No	Patient visit - additional info	PRIOR_RESULT
58	`PRT`	Optional	Yes	Participation information	PRIOR_RESULT
59	`AL1`	Optional	Yes	Patient allergy information	PRIOR_RESULT
60	`ORC`	Required	No	Common order	PRIOR_RESULT
61	`PRT`	Optional	Yes	Participation information	PRIOR_RESULT
62	`OBR`	Required	No	Observation request	PRIOR_RESULT
63	`NTE`	Optional	Yes	Notes and comments	PRIOR_RESULT
64	`PRT`	Required	No	Participation information	OBSERVATION_PARTICIPATION_PRIOR
65	`DEV`	Optional	Yes	Device	OBSERVATION_PARTICIPATION_PRIOR
66	`TQ1`	Required	No	Timing / quantity	TIMING_PRIOR
67	`TQ2`	Optional	Yes	Timing / quantity relationship	TIMING_PRIOR
68	`OBX`	Required	No	Observation result	OBSERVATION_PRIOR
69	`PRT`	Optional	Yes	Participation information	OBSERVATION_PRIOR
70	`NTE`	Optional	Yes	Notes and comments	OBSERVATION_PRIOR
71	`SGT`	Optional	No	Segment group trailer	ORDER
72	`FT1`	Optional	Yes	Financial transaction	ORDER
73	`CTI`	Optional	Yes	Clinical trial identification	ORDER
74	`BLG`	Optional	No	Billing	ORDER
75	`DEV`	Required	No	Device	DEVICE
76	`OBX`	Optional	Yes	Observation result	DEVICE

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