Here is what a structured `referenceRange` looks like:
https://github.com/chb/sample_ccdas/blob/master/NIST%20Samples/CCDA_CCD_b1_Ambulatory_v2.xml#L3231

```
<referenceRange>
  <observationRange>
    <value xsi:type="IVL_PQ">
      <low value="4.3" unit="10+3/ul"/>
      <high value="10.8" unit="10+3/ul"/>
    </value>
  </observationRange>
</referenceRange>
```