Dose syntax implementation guidance for FHIR STU3

This guidance is under active development by NHS Digital and content may be added or updated on a regular basis.

Timing Elements: duration, bounds and count

Adding constraints, bounds or limits to a dosage instruction.

duration and durationMax

A dosage instruction to be administered over a period of time is defined by a duration and / or a durationMax. These relate to the duration of a single administration so would be applicable for an infusion or potentially a transdermal patch.

The FHIR specification states that if there is a durationMax, there must be a duration.

The use of duration does not apply to medicines that are swallowed or otherwise instantly administered.

over 8 hours

<timing>
	<repeat>
		<duration value="8"/>
		<durationUnit value="h" />
	</repeat>
</timing>

over 10 to 15 minutes

<timing>
	<repeat>
		<duration value="10"/>
		<durationMax value="15"/>
		<durationUnit value="min"/>
	</repeat>
</timing>

boundsDuration

An instruction to limit a single course of medication uses the bounds structure. This can be implemented in three ways;

  • boundsDuration - a period of time, for example: for 7 days
  • boundsRange - a low / high quantity, for example: for 2 to 3 weeks
  • boundsPeriod - a date / time start to end period, for example: from 22/02/2021 to 04/03/2021

for 7 days

<timing>
	<repeat>
		<boundsDuration>
			<value value="7"/>
			<unit value="day"/> 
			<system value="http://unitsofmeasure.org"/>
			<code value="d"/>
		</boundsDuration>

	</repeat>
</timing>

for 2 to 3 weeks

<timing> 
	<repeat>
		<boundsRange>
			<low>
				<value value="2"/>
				<unit value="week"/> 
				<system value="http://unitsofmeasure.org"/>
				<code value="wk"/>
			</low>
			<high>
				<value value="3"/>
				<unit value="week"/>
				<system value="http://unitsofmeasure.org"/>
				<code value="wk"/>
			</high>
		</boundsRange>
	</repeat>
</timing>

from 22/02/2021 to 04/03/2021

<timing>  
	<repeat>
        <boundsPeriod>
            <start value="2021-02-22"/>
            <end value="2021-03-04"/>
        </boundsPeriod>
	</repeat>	
</timing>

count and countMax

Note: Use count and countMax with caution as there are other elements within the Dosage structure that will often be more appropriate.

An instruction for a set number of doses, including the use case of “once”, plus also any maximum number of doses, is defined by count and countMax.

Whilst the count is within repeat it does not mean repeat a number of additional times.

  • A count of 1 means adminster a dose once.
  • A count of 3 means administer 3 doses.

The FHIR specification states if there is a countMax, there must be a count.

take once

<timing>
	<repeat>
		<count value="1"/>
	</repeat>
</timing>

take twice

<timing>
	<repeat>
		<count value="2"/>
	</repeat>
</timing>

take three times

Clincal Example: Teicoplanin - 10 mg/kg - every 12 hours - for 3 doses, then 6 to 10 mg/kg - daily

<timing>
	<repeat>
		<count value="3"/>
	</repeat>
</timing>

take three to five times

<timing>
	<repeat>
		<count value="3"/>
		<countMax value="5"/>
	</repeat>
</timing>

Alternatives: Examples where alternatives to using count within a Dosage instruction would be more appropriate.

take once

Instead of using <count value="1"/> use <frequency value="1"/>.

an oral solid - 2 tablets - twice a day - take 14 times

Using a <count value="14"/> is confusing. Is this "take 2 tablets twice a day for 14 days" or "take 2 tablets twice a day and take 14 tablets in total, hence for 7 days"? It would be better to express as a duration.

<duration value="7"/>
<durationUnit value="d" />
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