To understand the rules about how LOINC creates panels and how to map your local panels to LOINC panels, it is important to distinguish among several kinds of “results”.
7.2.1 Primary measurements or observations
These observations report key measured results, and may indicate the presence/absence or amount of a substance or organism in the sample. For example, in an electrolytes panel, measured amounts of sodium, potassium, chloride, and bicarbonate would be the primary measures. Note that if a single primary result is being reported, the code for that result should be used for the order rather than a panel code. See Section 12.1.11 for more information.
7.2.2 Derived observations
These observations are derived from mathematical or logical operations on the primary measurements. For example, in an electrolytes panel, the anion gap is a derived measure that is calculated by subtracting the concentrations of chloride and bicarbonate (anions) from the concentrations of sodium and potassium (cations).
7.2.3 Ask at order entry (AOE) questions and other associated observations
These observations are obtained from the requester as part of the test order and are generally delivered back to the requester as part of the result package. For example, the concentration of inspired oxygen is always an AOE question for blood gas measurements. Similarly, the date of last menstrual period is an AOE question for pap smears.
Other kinds of observations are operationally treated in a similar manner although they may not be “questions” in the literal sense. For example, the volume and times (start, stop, duration) of urine collection are often sent along with the order and back again with the results. We call these “associated observations” and consider them like AOEs for the purpose of this discussion.
7.2.4 Impressions, interpretations, and comments
LOINC defines an impression/interpretation as a summary statement about multiple observations. This kind of summary observation is often included in collections of results sent back to the requester. (Such summary observation terms in LOINC have the Property of Imp, for impression, in their name).
As an aside, we note that decisions about test results being outside of normal range are best reported with the Interpretation Code (commonly referred to as the “Abnormal flag”) field of the HL7 OBX structure and not as a separate interpretation observation.