Implementation guide for interoperable medicines

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>

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>

Monthly dosing

The FHIR Dosage standard supports monthly dosing instructions, for example, "once a month".

once a month

{
    "timing" : {
        "repeat": {
            "frequency": 1,
            "period": 1,
            "periodUnit": "mo"
        }
    }
}

How should the above instruction be interpreted? Is this once a month, at any time in the month? Is this once a month, at regular intervals but can flex a day here or there? Is this once a month on a strict schedule that should not be deviated from?

The answer is the consumer will never know as the instruction is ambiguous, especially when considering months have different numbers of days.

Our recommended implementation guidance is that if "once a month" is received, the consumer interprets on a case by case basis using their professional judgement.

Where the prescriber wants to be more specific, we recommend the following.

For strict/regular timing express as a number of weeks, e.g. "every 4 weeks".

every 4 weeks

{
    "timing" : {
        "repeat": {
            "frequency": 1,
            "period": 4,
            "periodUnit": "wk"
        }
    }
}

For a specific day on each month, e.g. "on the 4th day of each month", there is a FHIR extension within R4 for dayOfMonth. This extension is defined as a positive integer but should be constrained to a value between 1 and 28.

on the 4th day of each month

{
    "timing" : {
        "repeat": {
            "frequency": 1,
            "period": 1,
            "periodUnit": "mo",
            "dayOfMonth": 4
        }
    }
}

NOT YET SUPPORTED... on the 2nd Monday of each month

// Cannot yet express on the 2nd Monday. The below would mean on any Monday every month
{
    "timing" : {
        "repeat": {
            "frequency": 1,
            "period": 1,
            "periodUnit": "mo",
            "dayOfWeek": "mon"
        }
    }
}

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>

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

<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.