Widal Test (Slide Test)

The Widal slide test is a rapid agglutination assay that mixes patient serum with Salmonella O and H antigen suspensions on a slide. Visible clumping (agglutination) suggests presence of anti‑Salmonella antibodies, providing a presumptive diagnosis of typhoid or paratyphoid fever. Results have limited sensitivity and specificity, may cross‑react, and should be confirmed by quantitative testing and clinical correlation.

Slide Widal shows agglutination with O (somatic) and H (flagellar) antigens. Read as negative/positive degrees (+ to +++) or approximate titre; O antibodies suggest recent/active typhoid, H may reflect current or past exposure. A single positive slide is not diagnostic—confirm with quantitative tube titres and a paired sample showing a fourfold rise, and interpret with clinical findings because cross‑reactivity and vaccination can cause false positives.

Widal detects agglutinating antibodies to Salmonella Typhi (O and H). A positive result is indicated by a high antibody titre or a fourfold rise between paired sera collected 7–10 days apart; commonly used cut-offs vary by region (for example, ≥1:160 in non‑endemic areas). Results must be interpreted with clinical signs and confirmed by blood culture or molecular testing due to false positives/negatives.

The Widal test is a serological agglutination assay detecting patient serum antibodies (agglutinins) against Salmonella Typhi antigens—O (somatic) and H (flagellar). Patient serum is mixed with standardized killed bacterial antigen suspensions; visible agglutination indicates presence and titre of specific antibodies. Rising O titres suggest recent/acute typhoid infection, while H antibodies may reflect later or past exposure.

The Widal slide (rapid) test has limited accuracy: sensitivity and specificity vary widely and false positives occur after prior infection, vaccination, or cross‑reacting organisms. It’s less quantitative than tube titration and can miss early cases. A single positive slide result is unreliable—paired sera showing a rising titre or, ideally, blood culture are needed to confirm typhoid fever.

Slide agglutination is a rapid serologic test: a specimen (cells, bacteria, or serum) is mixed with specific antiserum on a glass slide. If matching antigens and antibodies are present, antibodies cross-link particles, producing visible clumping within seconds to minutes. Controls check for autoagglutination; results are read as positive (clumping) or negative (smooth suspension). It’s used for blood typing and quick organism identification.