The OM1 (General Segment) holds the general definition that is shared by every entry in the observation/test master catalog. It describes a single service, test, or observation that a producer (a laboratory, imaging department, or other diagnostic service) is able to perform and report. OM1 is the anchor of the master-file catalog: it carries identity (OM1-2 Producer's Service/Test/Observation ID), the permitted data types (OM1-3), whether a specimen is required (OM1-4), and the nature of the observation (OM1-18), which in turn signals whether an OM2 (numeric), OM3 (categorical), or OM4 (specimen) segment follows to extend the definition.
Purpose
OM1 defines the attributes that apply to any kind of observation in the master catalog, regardless of how its result is expressed. It is the general record from which type-specific detail hangs. The producing system uses OM1 to publish what a test is, what it is called, who produces it, how it is ordered and reported, how long it takes, where it can be performed, and how its results should be interpreted. Downstream order-entry and result systems load these definitions so that orders and results reference a consistent, agreed-upon catalog.
Because OM1 is the general definition, it is always present for a catalog entry. The more specialized segments are optional and depend on the value of OM1-18 Nature of Service/Test/Observation:
- A numeric or quantitative observation is further described by an OM2 segment.
- A categorical (coded or text) observation is further described by an OM3 segment.
- An observation that requires a specimen is further described by an OM4 segment.
Used in
OM1 appears in MFN master-file notification messages that define the test/observation catalog, most commonly the MFN^M08 (test/observation master file) message. Within that message, the master file identification (MFI) segment names the file being updated, each master file entry (MFE) segment marks one record, and the OM1 segment then provides the general definition for that record. The type-specific segments OM2, OM3, and OM4 follow OM1 to extend it for numeric, categorical, and specimen observations respectively.
See MFN messages for the surrounding structure. OM1 is the general definition shared by all observation types; OM2 (numeric), OM3 (categorical), and OM4 (specimen) extend it.
Field-by-field reference
Source: HAPI HL7v2 v2.5.1 javadocs (OM1 javadoc) for sequence, name, data type, and repetition. Length is not published in the javadocs (—); Required and Table # are filled from the HL7 v2.5.1 standard where well-established.
| Seq | Name | Data Type | Length | Req | Repeat | Table # | Description |
|---|---|---|---|---|---|---|---|
| OM1-1 | Sequence Number - Test/Observation Master File | nm | — | R | — | — | Ordinal position of this entry in the master file |
| OM1-2 | Producer's Service/Test/Observation ID | ce | — | R | — | — | Primary identifier of the test in producer's catalog |
| OM1-3 | Permitted Data Types | id | — | O | Y | HL70125 | Value types allowed for this observation |
| OM1-4 | Specimen Required | id | — | O | — | HL70136 | Whether a specimen is needed for the test |
| OM1-5 | Producer ID | ce | — | O | — | — | Identifier of the producing service or lab |
| OM1-6 | Observation Description | tx | — | O | — | — | Free-text description of the observation |
| OM1-7 | Other Service/Test/Observation IDs for the Observation | ce | — | O | — | — | Alternate identifiers for the same observation |
| OM1-8 | Other Names | st | — | O | Y | — | Synonyms and alternate names for the test |
| OM1-9 | Preferred Report Name for the Observation | st | — | O | — | — | Name used when the result is reported |
| OM1-10 | Preferred Short Name or Mnemonic for Observation | st | — | O | — | — | Short report name or mnemonic |
| OM1-11 | Preferred Long Name for the Observation | st | — | O | — | — | Full descriptive name for the observation |
| OM1-12 | Orderability | id | — | O | — | HL70136 | Whether the observation can be ordered directly |
| OM1-13 | Identity of Instrument Used to Perform this Study | ce | — | O | Y | — | Instruments used to perform the study |
| OM1-14 | Coded Representation of Method | ce | — | O | Y | — | Coded method used for the observation |
| OM1-15 | Portable Device Indicator | id | — | O | — | HL70136 | Whether a portable device performs the test |
| OM1-16 | Observation Producing Department/Section | ce | — | O | Y | — | Department or section producing the observation |
| OM1-17 | Telephone Number of Section | xtn | — | O | — | — | Contact phone for the producing section |
| OM1-18 | Nature of Service/Test/Observation | is | — | O | — | HL70174 | Kind of observation, such as atomic or profile |
| OM1-19 | Report Subheader | ce | — | O | — | — | Subheader grouping for the printed report |
| OM1-20 | Report Display Order | st | — | O | — | — | Ordering key for report display |
| OM1-21 | Date/Time Stamp for any change in Definition for the Observation | ts | — | O | — | — | When the definition last changed |
| OM1-22 | Effective Date/Time of Change | ts | — | O | — | — | When the definition change takes effect |
| OM1-23 | Typical Turn-Around Time | nm | — | O | — | — | Usual time from order to result |
| OM1-24 | Processing Time | nm | — | O | — | — | Time the test itself takes to process |
| OM1-25 | Processing Priority | id | — | O | Y | HL70168 | Priorities at which the test can be processed |
| OM1-26 | Reporting Priority | id | — | O | — | HL70169 | Priorities at which results can be reported |
| OM1-27 | Outside Site(s) Where Observation may be Performed | ce | — | O | Y | — | External sites able to perform the observation |
| OM1-28 | Address of Outside Site(s) | xad | — | O | Y | — | Addresses of the outside performing sites |
| OM1-29 | Phone Number of Outside Site | xtn | — | O | — | — | Phone number for the outside site |
| OM1-30 | Confidentiality Code | cwe | — | O | — | HL70177 | Confidentiality classification of the observation |
| OM1-31 | Observations Required to Interpret the Observation | ce | — | O | — | — | Other observations needed for interpretation |
| OM1-32 | Interpretation of Observations | tx | — | O | — | — | Guidance on interpreting the result |
| OM1-33 | Contraindications to Observations | ce | — | O | — | — | Conditions that contraindicate the test |
| OM1-34 | Reflex Tests/Observations | ce | — | O | Y | — | Tests triggered reflexively by this one |
| OM1-35 | Rules that Trigger Reflex Testing | tx | — | O | — | — | Conditions that trigger reflex testing |
| OM1-36 | Fixed Canned Message | ce | — | O | — | — | Standard message appended to results |
| OM1-37 | Patient Preparation | tx | — | O | — | — | Patient preparation required before the test |
| OM1-38 | Procedure Medication | ce | — | O | — | — | Medication given as part of the procedure |
| OM1-39 | Factors that may Affect the Observation | tx | — | O | — | — | Factors that can influence the observation |
| OM1-40 | Service/Test/Observation Performance Schedule | st | — | O | Y | — | When the test is performed or batched |
| OM1-41 | Description of Test Methods | tx | — | O | — | — | Narrative description of the test methods |
| OM1-42 | Kind of Quantity Observed | ce | — | O | — | — | Kind of quantity that is measured |
| OM1-43 | Point Versus Interval | ce | — | O | — | — | Whether result is a point or interval value |
| OM1-44 | Challenge Information | tx | — | O | — | — | Challenge or provocation details |
| OM1-45 | Relationship Modifier | ce | — | O | — | — | Modifier of the observation relationship |
| OM1-46 | Target Anatomic Site Of Test | ce | — | O | — | — | Anatomic site the test targets |
| OM1-47 | Modality Of Imaging Measurement | ce | — | O | — | — | Imaging modality used for the measurement |
Most-used fields
- OM1-1 Sequence Number - Test/Observation Master File: the ordinal key that ties a record to its MFE segment and orders the catalog.
- OM1-2 Producer's Service/Test/Observation ID: the primary coded identifier of the test, usually carrying a LOINC or local code.
- OM1-3 Permitted Data Types: the result value types allowed, which signals how results will be expressed.
- OM1-4 Specimen Required: indicates whether an OM4 specimen definition is expected.
- OM1-9 / OM1-10 / OM1-11 Report names: the names used when displaying the observation on reports.
- OM1-18 Nature of Service/Test/Observation: classifies the entry and signals whether a numeric, categorical, or specimen extension follows.
Version differences (2.3 to 2.8.2)
OM1 has grown steadily across versions. In v2.3 the segment ended at a smaller field set focused on identity, naming, and basic reporting attributes. Later releases added imaging and method-related fields toward the end of the segment: OM1-46 Target Anatomic Site Of Test and OM1-47 Modality Of Imaging Measurement reflect the expanded imaging coverage seen from v2.5 onward. Data types were also refined over time; for example confidentiality codes migrated toward the CWE data type (OM1-30) as coded-with-exceptions types were introduced. Through v2.8.2 the core identity fields (OM1-1, OM1-2) and the nature field (OM1-18) have remained stable, so catalogs built against v2.5.1 generally interoperate with neighboring versions for the most-used fields. Always confirm trailing field positions against the exact version in use, since additions accumulate at the end of the segment.
Common mistakes
- Omitting OM1-1 or OM1-2. Both are required; without the sequence number and producer ID the record cannot be tied to its MFE entry or referenced by orders and results.
- Treating OM1 as type-specific. OM1 is the general definition. The numeric, categorical, and specimen detail belongs in OM2, OM3, and OM4.
- Mismatching OM1-1 with the MFE sequence. The sequence number must align with the master file entry it describes.
- Setting OM1-4 Specimen Required inconsistently with the presence of an OM4 segment, which confuses loaders about whether a specimen definition exists.
- Putting result value types in the wrong field. Permitted data types belong in OM1-3, not in a free-text description.
- Assuming a fixed field count. Trailing imaging and method fields exist only in later versions; do not rely on their position when targeting older receivers.
Examples
Minimal valid segment (only the required fields):
OM1|1|GLU^Glucose^L
Fully-populated segment:
OM1|1|GLU^Glucose^L|NM~SN|Y|LAB001^Vorro Reference Lab^L|Fasting plasma glucose|2345-7^Glucose^LN|Blood Sugar~Glu|Glucose, Plasma|GLU|Glucose, Plasma (Fasting)|Y|INST7^Analyzer 7^L|HEX^Hexokinase^L|N|CHEM^Chemistry^L|(555)555-0100|A|Chemistry Panel|10|20260601120000|20260605000000|60|30|S~R|R|OUT9^Partner Lab^L|123 Lab Way^^Metropolis^NY^10001|(555)555-0199|N^Normal^HL70177|||Fasting required||GTT reflex if elevated|GTT^Glucose Tolerance^L|Elevated above 200|||Patient must fast 8 hours|||Daily|Hexokinase enzymatic assay|Mass concentration|Point||||
Annotated breakdown:
OM1|1|GLU^Glucose^L|NM~SN|Y|LAB001^Vorro Reference Lab^L|...
| | | | |
| | | | └─> OM1-5 Producer ID (the producing lab)
| | | └────> OM1-4 Specimen Required (Y = a specimen is needed)
| | └──────────> OM1-3 Permitted Data Types (NM and SN, repeating)
| └────────────────────────> OM1-2 Producer's Service/Test/Observation ID (required)
└───────────────────────────> OM1-1 Sequence Number (required, ties to MFE)
In-context excerpt, OM1 carrying a numeric observation inside an MFN^M08:
MSH|^~&|LIS|VORRO|EMR|HOSP|20260610081500||MFN^M08^MFN_M08|MSG00041|P|2.5.1
MFI|OMA^Observation Master^L|VORRO|UPD|20260610081500||AL
MFE|MAD|MFE0001|20260610081500|GLU^Glucose^L|CE
OM1|1|GLU^Glucose^L|NM|Y|LAB001^Vorro Reference Lab^L|Fasting plasma glucose|||Glucose, Plasma||||||CHEM^Chemistry^L||A
OM2|1|mg/dL^milligram per deciliter^UCUM|2|mmol/L^millimole per liter^UCUM|0.0555|70-99^^^F||60-110|
In-context excerpt, OM1 carrying a categorical observation inside an MFN^M08:
MSH|^~&|LIS|VORRO|EMR|HOSP|20260610082000||MFN^M08^MFN_M08|MSG00042|P|2.5.1
MFI|OMA^Observation Master^L|VORRO|UPD|20260610082000||AL
MFE|MAD|MFE0002|20260610082000|BLDABO^ABO Group^L|CE
OM1|2|BLDABO^ABO Group^L|CE|Y|LAB001^Vorro Reference Lab^L|ABO blood group|||ABO Group||||||BLDBNK^Blood Bank^L||A
OM3|2|HL70112^ABO^L|A^A^L~B^B^L~O^O^L~AB^AB^L|A^A^L~B^B^L~O^O^L~AB^AB^L|||CE
FHIR mapping
There is no segment-level ConceptMap published in the v2-to-FHIR Implementation Guide for OM1. The observation/test master catalog represented by OM1 maps conceptually to ObservationDefinition in FHIR, with orderable/performable aspects aligning with ActivityDefinition. Identity fields (OM1-2) correspond to ObservationDefinition.code, report names (OM1-9 through OM1-11) to display names, and the nature field (OM1-18) helps determine which FHIR resource and structure best represent the entry. Treat any field-level alignment as conceptual rather than a published transform.
Engine considerations
- OM1 is the general definition; loaders should read it first, then branch on OM1-18 and OM1-4 to decide whether to expect OM2, OM3, or OM4.
- Many OM1 fields repeat (for example OM1-3, OM1-8, OM1-13, OM1-14, OM1-16, OM1-25, OM1-27, OM1-28, OM1-34, OM1-40). Engines must preserve repetition rather than collapse to a single value.
- The segment is long and trailing fields vary by version. Parse by field index against the negotiated version, and tolerate missing trailing fields from older senders.
- Use OM1-1 as the join key to the MFE segment; do not assume positional adjacency alone guarantees alignment.
How Vorro parses and produces OM1
When Vorro parses OM1, it reads OM1-1 and OM1-2 as the primary keys and normalizes the producer's identifier in OM1-2 against the configured code system so that orders and results resolve to the same catalog entry. Repeating fields are retained as ordered lists. Vorro inspects OM1-4 Specimen Required and OM1-18 Nature of Service/Test/Observation to anticipate and validate any following OM2, OM3, or OM4 segment, raising a warning when the extension is missing or inconsistent.
When Vorro produces OM1, it always emits the required OM1-1 and OM1-2, populates report names from the catalog model, and sets OM1-3 and OM1-18 so that downstream loaders can route the entry correctly. Trailing imaging and method fields are emitted only when the negotiated version supports them, keeping output compatible with older receivers.
Related pages
- HL7 OM2 Segment: Numeric Observation
- HL7 OM3 Segment: Categorical Service/Test/Observation
- HL7 MFN Master File Notification messages
