What Is HL7 v2 OBX-6? Units, Explained for Testers
A numeric result without its unit is close to meaningless — a potassium of 4.2 is normal, but the same number in the wrong unit could look like a critical value — which makes OBX-6 one of the quieter fields that can cause real clinical confusion if it is dropped or mismatched.
OBX-6 is the sixth field of the HL7 v2 OBX segment — it carries the unit of measure for the value in OBX-5, commonly expressed as a UCUM code alongside a human-readable label.
What this field contains
Example
mmol/L in OBX-6 gives the unit for the numeric potassium value 4.2. Synthetic result — not a real patient.
Where this trips people up
Some labs report certain analytes in different units depending on the instrument or panel — for example a value in mg/dL from one feed and mmol/L from another for the same analyte. Systems that store or display OBX-5 without checking OBX-6 against an expected unit can end up comparing values on different scales, which silently corrupts trending and can make a normal result look abnormal or vice versa.