2.4 Time Aspect

One can either measure a Property at a moment (point) in time or measure it over a time interval and integrate, in the mathematical sense, over time. In the latter case, we aggregate a “series” of physiologic states into a single scalar value that reflects some “average” Property measured over the specified time interval. Intervals also have relevance for rate measurements such as excretion (substance rate or mass rate) or clearances (volume rates). The amount over an interval is often expressed as a mass rate (MRat, e.g., g/24h) or a substance rate (SRat, e.g., mol/24h). Interval measurements often apply to urine and stool (e.g., collection over 24 hours and calculation of a concentration, total amount, or clearance). They also apply to clinical measurements, such as urine outputs where we have shift totals and 24-hour totals. Event counts on physiologic monitors, such as the number of premature ventricular contractions (PVCs) over 24 hours on a Holter monitor, are also of this type, as are look back periods for survey instruments.

The allowed values for non-point Time Aspect are defined as a syntax exactly like the syntax for the times in challenge tests, for example:

<numeric value><S|M|H|W>

The most common one is 24H. Table 9 gives some other examples. The complete set of active LOINC Time Aspects is available in the LOINC Part File and can be identified because they contain a PartTypeName of TIME.

For urine collection, 24H is the “standard” integrated measure and these are almost always reported as mass rates (MRat), substance rates (SRat), or catalytic (CRat) rates. These would contrast with spot or random urine tests that are represented as point (Pt) measures in our nomenclature and usually reported as concentrations – MCnc, CCnc, or SCnc for mass, catalytic, and substance concentrations respectively. However, we can also report the average concentration on a 24-hour specimen – in this case the Time Aspect value would be 24H but the Property would be MCnc/SCnc/CCnc instead of MRat/SRat/CRat.

The designation of 24H collection is maintained for tests that traditionally have reference ranges based on amount of substance of a Component cleared or excreted in 24 hours. However, a given specimen could have a 23-hour collection time and would still be called a 24H study. Depending upon the policies and procedures of the lab, they might extrapolate the reported value to what it would have been if the collection continued for the full 24 hours and report it as moles per day.

We also allow indirect specifications of a time window. Stdy identifies the duration of the study (without specifying an exact time); Enctr identifies the Encounter (ER visit, hospital stay, etc.).

Sample volumes reported for timed measurements are carried in other fields or as separate “test” results in other OBX segments.

Table 9: Example Duration Categories

2.4.1 Time Aspect Modifier

The second and optional subpart of the Time component allows an indication of some sub-selection or integration of the measures taken over the defined period of time: 8H^max heart rate would be the highest heart rate observed over 8H (Shift). Min, max, first, last, mean are the other possible values for this subpart. When nothing is stored in this subpart, we assume a mean value over the time period in questions. Valid values for this subpart are listed in table below.

Table 10: Time Aspect Modifier Codes