XSLT Sorting Conditionally

Hi

I have the following xml

<TestRequests>

 <TestRequest CollectionStatus="Resulted">
  <Category>Hem</Category>
  <Text>Complete Blood Count</Text>
  <SortKey>A<SortKey/>
 </TestRequest>

 <TestRequest CollectionStatus="Resulted">
  <Category>CHem</Category>
  <Text>RBC Count</Text>
  <SortKey>B<SortKey/>
 </TestRequest>

</TestRequests>

I want to achieve sorting based on some conditions

In my case if any of the SortKey has value then the TestRequest should be sorted on SortKey ,
if all the sortkey elements are blank in that casethe sorting has to on say Category element

Could someone tell me how to solve this using XSLT

Regards
Gururaja.K.S