Intended Use
The S. Typhi Antigen Rapid Test Cassette is designed for the qualitative detection of Salmonella Typhi (S. Typhi) antigen in human fecal specimens, aiding in the diagnosis of typhoid fever.
Summary
Typhoid fever, caused by the bacterium Salmonella typhi, is a life-threatening illness often observed in developing countries like Nepal, affecting approximately 12.5 million people annually all over the world. Infection typically occurs through ingestion, leading to the bacteria’s attachment to intestinal epithelial cells, followed by multiplication in the lymph nodes and bloodstream, resulting in severe clinical symptoms.
Principle
The S. Typhi Antigen Rapid Test employs a qualitative membrane strip immunoassay. These kits contains key components which include:
a. Conjugate Pad: Contains anti-S. Typhi antibody conjugated with colloidal gold
b. Nitrocellulose Membrane: Contains a test band (T band) coated with anti-typhoid antibody and a control band (C band) coated with goat anti-mouse IgG.
When a specimen is applied, if S. Typhi antigen is present, it binds to the antibodies and forms a colored T band, indicating a positive result. A control line will always appear to verify that the test was performed correctly.
Warnings and Precautions
- For in vitro diagnostic use only.
- Intended for healthcare professionals and point-of-care sites.
- Do not use after the expiration date.
- Read all instructions carefully before use.
- Treat all specimens as potentially hazardous and dispose of according to regulations.
Materials Provided:
- Test cassette
- Buffer
Materials Required But Not Provided:
- Specimen collection container
Storage and Stability
- Store in a sealed pouch at 4-30°C (40-86°F).
- Use before the expiration date indicated on the packaging.
- After opening the pouch, use the test within one hour.
- Avoid prolonged exposure to hot and humid environments.
Specimen Collection
- Use fecal samples (1-2 mL or 1-2 g) in a clean, dry container for optimal antigen detection.
- Best results obtained if assays are performed within 6 hours of collection. Specimens can be stored at 2-8°C for 3 days or below -20°C for long-term storage.
- To collect, use a specimen collection applicator to sample from various sites in the feces, then mix with the provided dilution buffer.
Test Procedure
- Allow the test device and specimens to reach room temperature (15-30°C or 59-86°F).
- Remove the test cassette from the sealed pouch.
- Transfer 3 drops (approximately 100 µL) of the specimen to the specimen well (S) and start the timer.
- Read results at 15 minutes; do not interpret after 20 minutes.
Interpretation of Results
- Positive: Two colored lines (C and T) indicate the presence of S. Typhi antigen.
- Negative: One colored line (C) indicates the absence of detectable S. Typhi antigen.
- Invalid: Control line (C) does not appear; retest with a new cassette if this occurs.
Quality Control
The control line (C) serves as an internal procedural control, confirming sufficient specimen volume and proper technique. It is good practice to test known positive and negative controls periodically to ensure proper test performance.
Limitations
- This test is qualitative; it cannot provide quantitative values or determine antigen concentration.
- All results should be interpreted in conjunction with clinical information.
Performance Characteristics
- Agreement with Commercial Tests:
- 97.30% for positive specimens, 99.21% for negative specimens.
- Clinical Sensitivity: 95.50%
- Clinical Specificity: 98.68%
- Overall Accuracy: 97.96%
No cross-reactivity observed with specimens from patients with various other infectious diseases or common biological analytes.
Bibliography
- Ivanoff BN et al. “Vaccination against typhoid fever: present status.” Bulletin of the World Health Organization 1994; 72: 957-71.
- Gotuzzo E et al. “Association between AIDS and infection with Salmonella typhi in an endemic area.” Archives of Internal Medicine 1991; 151: 381-389.
- Wain, J. et al. “Typhoid fever 2015.” Lancet 385 (9973): 1136-45.
- Rabsch, W. et al. “Salmonella enterica Serotype Typhimurium and Its Host-Adapted Variants.” Infection and Immunity 2002; 70 (5): 2249-2255.