The Best Practices page says:

Reporting results: The result of a specimen analysis is required to be reported as text and may also be represented by a code. This allows exchange of the result meaning among systems that do not share code systems or do not use standardized code systems. The value of the result may be reported in several text formats:

• Word or phrase indicating presence or absences with no quantity (e.g., "Detected", "Not detected")
• Mathematical expression of quantity with units (e.g., "= 0.160 g/dL")
• Mathematical expression of quantity range with units (e.g., "< 2.5 ng/mL")

But the the Toxicology Lab Result profile on Observation doesn't put any constraints on Observation.value to enforce reporting as text. Is the Best Practice intended to be a requirement? 

As well, the Best Practice mentions text as a required alternative to a coded value, but several of the examples you gave (the mathematical expression) would be a quantity rather than a coded value. Does the requirement to send results as text apply to non-coded data types as well? Please clarify what data types can be used to convey results in Observation.value.