Widal Test (Tube Agglutination) is a laboratory test used to diagnose typhoid fever and other infections caused by *Salmonella* species. In this method, a patient’s serum is mixed with known antigens (O and H antigens) of *Salmonella* in test tubes. If antibodies against the bacteria are present, they will cause agglutination (clumping) of the antigens.

The tube agglutination method is more sensitive and specific than the slide agglutination version and is typically performed in a laboratory setting. It is used to detect O (somatic) and H (flagellar) antibodies, helping to confirm active or past infection with *Salmonella* typhi or *Salmonella* paratyphi.