Accessing Variables while sorting

Hi,

I have an XML that reads as 
<PatientList>
	<SortDetails>
		<SortField>FirstName</SortField>
	</SortDetails>
	<Patient>
		<LastName>AAndrew</LastName>
		<FirstName>ZBenjamin</FirstName>
		<Gender>M</Gender>
		<Age>56</Age>
		<DOB>04-20-1945 </DOB>
		<PatientID>234-562-7890</PatientID>
	</Patient>
</PatientList>

In the code below, how do i assign the value of SortField to $Variable ?
<xsl:for-each select="PatientList/Patient">
	<xsl:sort select="$Variable" order="ascending"></xsl:sort>
	   <tr> 
	       <td width="17%" height="30"><xsl:value-of select="LastName"/></td>
		-----

Thanks in advance,
Seema.

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