Recommendations on interpretation of laboratory results: Positive SARS diagnostic test findings

1. Confirmed positive PCR for SARS virus
   • at least 2 different clinical specimens (e.g., nasopharyngeal and stool) OR
   • the same clinical specimen collected on 2 or more days during the course of the illness (e.g., 2 or more nasopharyngeal aspirates) OR
   • 2 different assays or repeat PCR using the original clinical sample on each occasion of testing
2. Seroconversion by ELISA or IFA
   • negative antibody test on acute serum followed by positive antibody test on convalescent serum OR
   • four-fold or greater rise in antibody titre between acute and convalescent phase sera tested in parallel
3. Virus isolation
   • Isolation in cell culture of SARS -CoV from any specimen; plus PCR confirmation using a validated method.

(Reference: World Heath Organization )

Confirmation of positive PCR

• The PCR procedure should include appropriate negative and positive controls in each run, which should yield the expected results:
• 1 negative control for the extraction procedure and 1 water control for the PCR run
• 1 positive control for extraction and PCR run
• the patient sample spiked with a weak positive control to detect PCR inhibitory substances (inhibition control)
• If a positive PCR result has been obtained, it should be confirmed by:
• repeating the PCR using the original sample OR
• having the same sample tested in a second laboratory.

Amplifying a second genome region could further increase test specificity

Recommendations for laboratories testing for SARS

Reference laboratories should be identified at national level.

Antibody testing

ELISA and IFA tests are being developed by research laboratories. Because SARS a new disease in humans, SARS -CoV antibodies are not found in populations that have not been exposed to the virus. An antibody rise between acute and convalescent phase sera tested in parallel is very specific.