HL7 OML_O21 (Laboratory Order) | Integration Soup HL7 Reference Guide

HL7 OML_O21 Laboratory Order

A lab order message that gives specimens and containers a proper home.

HL7 message structure OML_O21 groups and segments from HL7 v2.5.1 Hide structure

These are the generated groups and segments for the version selected at the top of the page. The article explains the workflow, and this panel follows the chosen HL7 version.

Message Structure

Segment	Name	Required	Repeatable
MSH	Message Header	Yes	No
SFT	Software Segment	No	Yes
NTE	Notes and Comments	No	Yes
OML_O21.PATIENT	Patient group	No	No
PID	Patient Identification	Yes	No
PD1	Patient Additional Demographic	No	No
NTE	Notes and Comments	No	Yes
NK1	Next of Kin / Associated Parties	No	Yes
OML_O21.PATIENT_VISIT	Patient Visit group	No	No
PV1	Patient Visit	Yes	No
PV2	Patient Visit - Additional Information	No	No
OML_O21.INSURANCE	Insurance group	No	Yes
IN1	Insurance	Yes	No
IN2	Insurance Additional Information	No	No
IN3	Insurance Additional Information, Certification	No	No
GT1	Guarantor	No	No
AL1	Patient Allergy Information	No	Yes
OML_O21.ORDER	Order group	Yes	Yes
ORC	Common Order	Yes	No
OML_O21.TIMING	Timing group	No	Yes
TQ1	Timing/Quantity	Yes	No
TQ2	Timing/Quantity Relationship	No	Yes
OML_O21.OBSERVATION_REQUEST	Observation Request group	No	No
OBR	Observation Request	Yes	No
TCD	Test Code Detail	No	No
NTE	Notes and Comments	No	Yes
CTD	Contact Data	No	No
DG1	Diagnosis	No	Yes
OML_O21.OBSERVATION	Observation group	No	Yes
OBX	Observation/Result	Yes	No
TCD	Test Code Detail	No	No
NTE	Notes and Comments	No	Yes
OML_O21.SPECIMEN	Specimen group	No	Yes
SPM	Specimen	Yes	No
OBX	Observation/Result	No	Yes
OML_O21.CONTAINER	Container group	No	Yes
SAC	Specimen Container detail	Yes	No
OBX	Observation/Result	No	Yes
OML_O21.PRIOR_RESULT	Prior Result group	No	Yes
OML_O21.PATIENT_PRIOR	Patient Prior group	No	No
PID	Patient Identification	Yes	No
PD1	Patient Additional Demographic	No	No
OML_O21.PATIENT_VISIT_PRIOR	Patient Visit Prior group	No	No
PV1	Patient Visit	Yes	No
PV2	Patient Visit - Additional Information	No	No
AL1	Patient Allergy Information	No	Yes
OML_O21.ORDER_PRIOR	Order Prior group	Yes	Yes
ORC	Common Order	No	No
OBR	Observation Request	Yes	No
NTE	Notes and Comments	No	Yes
OML_O21.TIMING_PRIOR	Timing Prior group	No	Yes
TQ1	Timing/Quantity	Yes	No
TQ2	Timing/Quantity Relationship	No	Yes
OML_O21.OBSERVATION_PRIOR	Observation Prior group	Yes	Yes
OBX	Observation/Result	Yes	No
NTE	Notes and Comments	No	Yes
FT1	Financial Transaction	No	Yes
CTI	Clinical Trial Identification	No	Yes
BLG	Billing	No	No

OML_O21 is the laboratory order message introduced for more explicit lab and automation workflows. It still has the familiar order pieces, but it gives specimen and container detail first-class structure through groups such as specimen, container, prior result, and observation request.

If ORM_O01 is the broad old workhorse, OML_O21 is the lab-aware version you reach for when specimen identity, specimen type, collection details, container IDs, and lab automation matter. Many modern lab interfaces prefer OML because it avoids overloading OBR fields with specimen facts that deserve their own segments.

A small OML lab order example

MSH|^~\&|EHR|CITYHOSP|LAB|CITYLAB|20260715110500||OML^O21^OML_O21|OML00001|P|2.5.1 PID|1||123456^^^CITYHOSP^MR||Smith^Jane^Anne^^Ms^^L||19800314|F PV1|1|O|CLINIC^12^^CITYHOSP ORC|NW|ORD448812^EHR|||||^^^20260715114500|||12345^Careful^Clara OBR|1|ORD448812^EHR||24323-8^Comprehensive metabolic panel^LN|||20260715114500 SPM|1|SPEC20260715001^CITYLAB||BLD^Blood^HL70487|||||||||||||20260715114000 SAC|||TUBE778899^CITYLAB||||SER^Serum tube^L

What workflow it represents

The sender is normally an ordering system, EHR, or middleware. The receiver is a LIS, lab automation system, specimen processing system, or integration engine in front of those systems. The receiver creates the order and ties it to one or more specimens, containers, and tests.

The practical distinction from ORM is that the specimen is not an afterthought. SPM identifies the specimen and its type. SAC can identify the container. That becomes critical when the same order has multiple specimens, aliquots, tubes, or automation steps.

How to read the structure

The patient group is optional in the abstract structure but common in real interfaces. PID identifies the patient, PV1 ties the order to a visit when needed, and insurance or guarantor groups appear when the lab needs billing context.

The required order group starts with ORC. The observation request group usually contains OBR, notes, diagnosis, order-level OBX observations, then specimen groups. Prior result groups can carry history that helps the lab interpret or process the order.

Specimens and containers

Use SPM for specimen identity and specimen characteristics. Do not bury specimen type and collection details in a local OBR note if both systems can handle SPM. Container identifiers belong in SAC when the workflow tracks tubes, plates, trays, or automation containers.

Implementation guides sometimes map older OBR specimen fields into SPM fields during migrations from ORM to OML. That is fine as a transition plan, but the long-term model should make specimen and container information explicit.

Reference notes

Caristix describes OML_O21 as a laboratory and other order message that is required for lab automation use cases in older HL7 versions. Public lab-order implementation guides also show practical mappings from ORM-style OBR specimen details into OML/SPM structures.