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about 50 % of patients had seroconverted at around 15 days after the onset of symptoms (Peiris).

In the same study, SARS-associated coronavirus RNA was detected in nasopharyngeal aspirates by RT-PCR in 20 patients (32%) at initial presentation (mean 3.2 days after the onset of illness) and in 68% at day 14 (Peiris). Quantification revealed that the viral load peaked on day 10 with a mean geometric value of 1.9*107 copies per ml, compared to values of 2.3*105 copies per ml and 9.8*104 copies per ml on days 5 and 15, respectively (Peiris).

Furthermore, viral RNA was detected in 97% of stool samples collected later in the illness (a mean of 14.2 days after onset). Similarly, viral RNA was detected in 42% of urine samples collected at a mean of 15.2 days after the onset of symptoms (Peiris).

The authors therefore conclude that although viral RNA detection in the nasopharyngeal aspirate has a sensitivity of only 32% at presentation, testing of multiple nasopharyngeal and fecal samples is able to increase the predictive value of the RT-PCR assay (Peiris).

Laboratory tests

Due to the efforts of the WHO-led international multi-center collaborative network of laboratories testing for SARS, tests for the novel coronavirus have been developed with unprecedented speed (SARS: Laboratory diagnostic tests – 29 April 2003; http://www.who.int/csr/sars/diagnostictests/en/). Samples from suspected and probable SARS cases have been tested for SARS-CoV for some time in several countries, including Canada, France, Germany, Hong Kong SAR, Italy, Japan, the Netherlands, Singapore, the United Kingdom and the United States of America.

Nevertheless, until standardized reagents for virus and antibody detection become available and methods have been adequately field tested, the diagnosis of SARS remains based on clinical and epidemiological findings. The revised case definition from May 1, 2003, (see: http://www.who.int/csr/sars/casedefinition/en/) includes laboratory results for the first time: a suspected case of SARS, that is positive for SARS-CoV in one or more assays, should be reclassified as a probable

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case. At present there are no defined criteria for SARS-CoV test results to confirm or reject the diagnosis of SARS.

Positive laboratory test results for other known agents that are able to cause atypical pneumonia such as Legionella pneumophila, influenza and parainfluenza viruses, Mycoplasma pneumoniae etc. may serve as exclusion criteria: according to the case definition, a case should be excluded if an alternative diagnosis can fully explain the illness. However, the possibility of dual infection must not be ruled out completely.

Molecular tests

SARS-CoV-specific RNA can be detected in various clinical specimens such as blood, stool, respiratory secretions or body tissues by the polymerase chain reaction (PCR). A number of PCR protocols developed by members of the WHO laboratory network are available on the WHO website (http://www.who.int/csr/sars/primers/en/). Furthermore, a 5´-nuclease RT-PCR test kit containing primers and positive and negative controls, developed by the Bernhard Nocht Institute (http://www.bni-hamburg.de/; Drosten et al.), is available commercially (http://www.artus-biotech.de). An inactivated standard preparation is also available for diagnostic purposes through the European Network for Imported Viral Infections (ENIVD; http://www.enivd.de). ENIVD is also preparing an international external quality assessment scheme for SARS-CoV assays.

Despite their sometimes high sensitivity, the existing PCR tests cannot rule out, with certainty, the presence of the SARS virus in patients (Peiris, McIntosh, Poon). On the other hand, contamination of samples in laboratories might lead to false positive results. Stringent guidelines on laboratory quality control and confirmatory testing have therefore been issued by the WHO (http://www.who.int/csr/sars/labmethods/en/).

A valid positive PCR result indicates that there is genetic material (RNA) from the SARS-CoV in the sample. It does not mean, however, that the virus present is infectious, or that it is present in a large enough quantity to infect another person.

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