Implementation guide for interoperable medicines

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

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:

UCUM Unit Definition
s second
min minute
h hour
d day
wk week
mo month
a year

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>

up to 4 times a day

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

twice a week

<timing>
    <repeat>
        <frequency value="2"/>
        <period value="1"/>
        <periodUnit value="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)?

Bad ambiguious example:
take twice - on Monday and Thursday 
<timing>
    <repeat>
        <count value="2" />
        <dayOfWeek value="mon" />
        <dayOfWeek value="thu" />
    </repeat>
</timing>

The above could be read as both 2 individual doses or 4 individual doses hence is potentially confusing.

Better specific example:
twice a week - on Monday and Thursday 
<timing>
    <repeat>
        <frequency value="2"/>
        <period value="1" />
        <periodUnit value="wk"/>
        <dayOfWeek value="mon" />
        <dayOfWeek value="thu" />
    </repeat>
</timing>

Real-World Example

This prescription is used for immunocompromised patients to provide prophylaxis against a particular strain of pneumonia.

Co-trimoxazole 80mg/400mg tablets (Actavis UK Ltd) - 1 tablet - twice a day - on Monday, Wednesday and Friday.

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