The Collaborative International Dictionary of English v.0.48:

Widal's test \Wi*dal's" test\, or Widal test \Wi*dal" test\ or
Widal reaction \Wi*dal" reaction\ . [After Fernand Widal (b.
   1862), French physician.] (Med.)
   A test for typhoid fever based on the fact that blood serum
   of one affected, in a bouillon culture of typhoid bacilli,
   causes the bacilli to agglutinate and lose their motility.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Widal test
    n 1: a test for detecting typhoid fever and other salmonella
         infections [syn: Widal test, Widal's test]