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Element: sequence

Multi-sequence dosage instructions.

A multi-sequence would be required where a dosage instruction changes over a given time schedule, where a dosage instruction includes optional follow-up doses or where a concurrent dosage instruction is required for the same medication at a different dose.

Examples below include Prednisolone to treat Bell’s palsy where a dose starts at 60 mg per day for a period of time then reduces by 10 mg per day. This would be described as a sequential dosage instruction. A concurrent dosage example is provided to describe a different dose in the morning to that at midday.

Sequential Instructions

Where the value of sequence is an incremental integer it defines a sequential instruction.

Example use of the FHIR STU3 Dosage using multiple sequential sequences

Concurrent Instructions

Where the same integer value of sequence is defines it means a concurrent instruction.

Element: text

Free text dosage instructions.

In the future, a nationally available Dose to Text Service (similiar to the one in our Demonstrator) will be available, and will be recommended to be used.

In the interim - if the text element is used:

Information to clinicians about administration or preparation of the medication (e.g. “infuse as rapidly as possibly via intraperitoneal port” or “immediately following drug x”) should be populated in dosage.text. It may also be be used (rarely) for cases where the instructions to patient are too complex to code.

The use of the Dosage.text element is discouraged to maximise dosage instructions defined using the coded structures. This position may be subject to change as a result of initial clinical and implementation experience or feedback from the FHIR community.

The Dosage.text element would also be populated in an implementation which does not support the coded structures.

In some cases, where the same instruction is relevant to both a clinician and patient, then the instructions may be repeated using several dosage structures - additionalInstruction or patientInstruction toward patient, text toward the clinician. This will ensure the receiving system makes the information available to the appropriate person.

Element: additionalInstruction

Additional dosage instructions that can be either SNOMED-CT coded terms or free-text instructions.

Where suitable SNOMED-CT coded terms exist, use them instead of free-text instructions. See the SNOMED-CT hierarchy descendants of 419492006 Additional dosage instructions (qualifier value). These terms are also available from the example FHIR valueset binding.

Coded instruction

<additionalInstruction>
    <coding> 
        <system value="http://snomed.info/sct"/> 
        <code value="419529008"/> 
        <display value="Dissolved under the tongue "/> 
    </coding>
</additionalInstruction>

Multiple coded instructions

<additionalInstruction>
    <coding> 
        <system value="http://snomed.info/sct"/> 
        <code value="417995008"/> 
        <display value="Dissolve or mix with water before taking"/> 
    </coding>
</additionalInstruction>
<additionalInstruction>
    <coding> 
        <system value="http://snomed.info/sct"/> 
        <code value="417980006"/> 
        <display value="Contains aspirin"/> 
    </coding>
</additionalInstruction>

Free-text instruction (try to avoid using free-text where possible)

<additionalInstruction>
    <text value="with dialysis"/>
</additionalInstruction>

additionalInstruction / when overlap

For example, they can both describe “with or after food”.

In such cases, use Timing.when instead of a coded Dosage.additionalInstruction.

continue / stop instructions

There may be the need for a clinician to give explicit instructions to either stop or continue taking medication. This could be applicable for discharge medication where one medicine may be for short term pain relief and should be stopped after the prescribed quantity has been taken, while another medicine must be continued until further notice.

The patient’s GP would be informed of this as part of the discharge letter but having these instructions explicitly coded provides a more robust solution.

Three SNOMED-CT codes exist that can be used for this purpose.

• “Then stop” (SNOMED concept identifier = 422327006)
• “Then discontinue” (SNOMED concept identifier = 421484000)
• “Do not stop taking this medicine except on your doctor’s advice” (SNOMED concept identifier = 419444006)

as directed

The use of a dosage instruction akin to “as directed” should be avoided wherever possible; however, if required then this can be SNOMED coded using 421769005 Follow directions (qualifier value) or conveyed as text as a patient instruction.

Element: patientInstruction

Free-text instructions for the patient. Patient instructions can only be defined as one text string per dosage instruction.

<patientInstruction value="Some relevant instructions for the patient"/>

Timing Elements: frequency and period

A large proportion of cases, especially those where the medication are described using a VMP or AMP concept, can have a dosage instruction defined with a combination of frequency and period elements within the Timing structure.

Simple dosage timing instructions can be described using frequency and period.

The combination of frequency and period allows for the two commonly used expressions of:

• x times a period
• every x period

The unit of the period must be one of the UCUM units:

A frequencyMax and/or periodMax can also be used to define ranges. The FHIR specification states if there's a periodMax, there must be a period.

Every 8 hours

<timing>
    <repeat>
        <frequency value="1"/>
        <period value="8"/>
        <periodUnit value="h"/>
    </repeat>
</timing>

4 times a day

<timing>
    <repeat>
        <frequency value="4"/>
        <period value="1"/>
        <periodUnit value="d"/>
    </repeat>
</timing>

2 to 4 times a day

<timing>
    <repeat>
        <frequency value="2"/>
        <frequencyMax value="4"/>
        <period value="1"/>
        <periodUnit value="d"/>
    </repeat>
</timing>

every 3 to 4 weeks

<timing>
    <repeat>
        <frequency value="1"/>
        <period value="3"/>
        <periodMax value="4"/>
        <periodUnit value="wk"/>
    </repeat>
</timing>

3 to 4 times every 1 to 2 weeks

<timing>
    <repeat>
        <frequency value="3"/>
        <frequencyMax value="4"/>
        <period value="1"/>
        <periodMax value="2"/>
        <periodUnit value="wk"/>
    </repeat>
</timing>

daily

<timing>
    <repeat>
        <frequencyvalue="1"/>
        <periodvalue="1"/>
        <periodUnitvalue="d"/>
    </repeat>
</timing>

twice a week

<timing>
    <repeat>
        <frequencyvalue="2"/>
        <periodvalue="1"/>
        <periodUnitvalue="wk"/>
    </repeat>
</timing>

Examples of potentially confusing timing instructions

When converting the information into a FHIR message to a human readable version the frequency could be misconstrued.

For example:

take twice a week on a Monday and Thursday

Is the patient being instructed to take the medication twice per day on a Monday and Thursday (4 individual doses), or twice per week on a Monday and Thursday (2 individual doses)?

<timing>
    <repeat>
        <count value="2" />
        <dayOfWeek value="mon" />
        <dayOfWeek value="thu" />
    </repeat>
</timing>

Using the dose-to-text guidance within this documentation the above would be displayed as:

on Monday and Thursday - take twice

<timing>
    <repeat>
        <frequency value="2"/>
        <period value="1" />
        <periodUnit value="wk"/>
        <dayOfWeek value="mon" />
        <dayOfWeek value="thu" />
    </repeat>
</timing>

Using the dose-to-text guidance within this documentation the above would be displayed as:

twice a week - on Monday and Thursday

<timing>
    <repeat>
        <dayOfWeek value="mon" />
        <dayOfWeek value="thu" />
    </repeat>
</timing>

Using the dose-to-text guidance within this documentation the above would be displayed as:

on Monday and Thursday

Real-World Example

Co-trimoxazole 80mg/400mg tablets (Actavis UK Ltd) - 1 tablet - twice a day - on Monday, Wednesday and Friday.
This prescription is used for immunocompromised patients to provide prophylaxis against a particular strain of pneumonia.

Timing Elements: when, offset, dayOfWeek, timeOfDay, event and code

A dosage instruction can be tied to the specific days, dates and times, plus life events related to eating or sleeping.

when and offset

The when element ties to the regular life events of sleeping and eating, for example:

• C = event occurs at a meal
• WAKE = event occurs [offset] after waking

An offset allows the event to be tied x minutes before or after.

The offset is an unsigned integer value so different codes within the when value-set are used to define before or after.

Event occurs at breakfast

<timing>
    <repeat>
        <when value="CM"/>
    </repeat>
</timing>

Event occurs at 1 hour before breakfast

<timing>
    <repeat>
        <when value="ACM"/>
        <offset value="60" />
    </repeat>
</timing>

Event occurs at 1 hour after breakfast

<timing>
    <repeat>
        <when value="PCM"/>
        <offset value="60" />
    </repeat>
</timing>

dayOfWeek and timeOfDay

A dosage instruction can specify days of a week and/or specific times within a day for administration.

on Monday and Thursday at 09:00 and 15:00

<timing>
    <repeat>
        <dayOfWeek value="mon" />
        <dayOfWeek value="thu" />
        <timeOfDay value="09:00:00" />
        <timeOfDay value="15:00:00" />
    </repeat>
</timing>

Note the FHIR standard states that the elements frequency and period cannot be used as well as dayOfWeek, and the elements when, frequency and period cannot be used as well as timeOfDay.

event

A dosage instruction can specify specific dates and times for administration.

on 1st Nov 2019

<timing>
    <event value="2019-11-01" />
</timing>

on 1st Nov 2019 at 10:30 and again on 1st Dec 2019 at 22:30

<timing>
    <event value="2019-11-01T10:30" />
    <event value="2019-12-01T22:30" />
</timing>

code

Allows a code, often a Latin abbreviation, for a timing schedule to be specified, for example: BID = twice a day

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

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>

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" />

Element: asNeeded

Dosing 'asNeeded' and 'asNeededCodeableConcept' instructions.

Dosage.asNeeded[x]

The asNeeded structure can be used to define a simple “as required” (or “pro re nata” expressed as “PRN” for those who still prefer to use Latin abbreviations) instructions.

In this case use the asNeededBoolean option. The absence of asNeededBoolean is equivalent to where asNeededBoolean has a value of false.

The asNeededCodeableConcept option is used to bound the dosage instruction to a coded term. Any number of coded terms can be used from the SNOMED-CT hierarchy as a descendant of 404684003 Clinical Finding (finding). An extensive medication-as-needed-reason value-set is defined within FHIR which is a subset of the SNOMED-CT clinical findings hierarchy.

Example #1: Dosage.asNeeded

Zopiclone 3.75mg tablets, 1 tablet, at BEDTIME, PRN, oral

Example #2: Dosage.asNeededCodeableConcept