How to get the "extern value" of enumerated attribute?

Apologies if this has been asked and answered many time before, but I have not been able to find the answer.

I have an enumerated type with external value
<enumeration attributeTypeId="enum_deadline_from_today" name="Deadline From Today">
                    <literal default="true" externalValue="1" icon="processattachment:/enumeration/minor.gif" id="enum_deadline_from_today.literal.l2" name="N/A" null="true"/>
                    <literal externalValue="2" icon="processattachment:/enumeration/low.gif" id="enum_deadline_from_today.literal.l4" name="12+ months"/>
                    <literal externalValue="3" icon="processattachment:/enumeration/medium.gif" id="enum_deadline_from_today.literal.l6" name="6-12 months"/>
                    <literal externalValue="4" icon="processattachment:/enumeration/high.gif" id="enum_deadline_from_today.literal.l8" name="3-6 months"/>
                    <literal externalValue="5" icon="processattachment:/enumeration/critical.gif" id="enum_deadline_from_today.literal.l10" name="0-3 months"/>
                </enumeration>

I need to get the externalValue when calculating some values (score_deadline_from_today is of type enum_....)
workItem.getValue(WorkItemAttributes.score_deadline_from_today)
doesn't seem to return the external value. How can I get it?

Many thanks