Standard Operating Procedure: Widal Agglutination Test

The Widal test is a presumptive serological diagnostic method used to aid in the diagnosis of enteric fever (Typhoid and Paratyphoid fever) by detecting antibodies against Salmonella typhi and Salmonella paratyphi in patient serum. This procedure outlines the slide agglutination technique. As an expert operation manager, I emphasize that this test is semi-quantitative and must be interpreted alongside clinical symptoms, as false positives are common due to cross-reactivity and prior vaccination.

Pre-Analytical Preparation

• Verify Patient Requisition: Ensure the test request is clear and the patient identity is confirmed.
• Sample Collection: Collect 3–5 mL of venous blood in a plain or serum-separator tube (SST).
• Centrifugation: Allow the blood to clot at room temperature, then centrifuge at 3000 rpm for 5–10 minutes to obtain clear serum.
• Reagent Inspection: Ensure Widal antigens (TO, TH, AH, BH) are within the expiration date and have reached room temperature (20–25°C). Gently mix the antigen vials before use.
• Equipment Check: Ensure a clean, dry white agglutination tile, micropipettes, disposable tips, and a mechanical rotator (set to 100 rpm) are available.

Analytical Procedure: Slide Agglutination

• Setup: Label four circles on the agglutination tile: TO, TH, AH, and BH.
• Serum Dispensing: Using a micropipette, place 50 µL of patient serum into each of the four circles.
• Antigen Addition: Add one drop (approximately 50 µL) of the corresponding antigen suspension (TO, TH, AH, BH) to its respective circle.
• Mixing: Use a separate, clean wooden applicator stick for each circle to mix the serum and antigen thoroughly, spreading the mixture to cover the circle area.
• Rotation: Immediately place the tile on the mechanical rotator at 100 rpm for exactly 60 seconds.
• Observation: Turn off the rotator and observe for macroscopic agglutination under a bright light source or against a black background.

Post-Analytical & Interpretation

• Positive Result: Clear, visible clumping (agglutination) within 60 seconds indicates a positive result.
• Negative Result: No clumping or a smooth, homogenous suspension indicates a negative result.
• Reporting: If positive on the slide, proceed to the tube titration method to determine the specific antibody titer (e.g., 1:80, 1:160, 1:320).
• Disposal: Dispose of the tile contents and tips into the biohazard sharps/waste container according to institutional safety protocols.

Pro Tips & Pitfalls

• Pro Tip: Always run a positive and negative control once per batch or per kit change to validate reagent integrity.
• Pitfall - Over-incubation: Never rotate for longer than 60 seconds on the slide, as this may lead to "drying out" of the mixture, resulting in false-positive "pseudo-agglutination."
• Pitfall - Cross-reactivity: Be aware that patients with malaria, brucellosis, or chronic liver disease may yield false-positive results.
• Pro Tip: If the test is negative but clinical suspicion remains high, repeat the test in 7–10 days to observe for a "four-fold rise in titer," which is more diagnostically significant than a single reading.

FAQ

Q1: What does a "TO" antigen versus a "TH" antigen indicate? A: TO antigen detects somatic O-antibodies (indicative of acute, recent infection), while TH antigen detects flagellar H-antibodies (indicative of past infection or vaccination).

Q2: Why should I confirm a positive slide test with a tube test? A: The slide test is a rapid screening tool. The tube test (quantitative) is the gold standard for determining the exact titer, which is necessary to distinguish between baseline population immunity and an active infection.

Q3: Can a patient have Typhoid with a negative Widal test? A: Yes. The Widal test is often negative during the first week of infection as antibody levels have not yet reached detectable thresholds. Blood culture is the definitive test for early-stage diagnosis.