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from ProMed, regarding the testing for SARS:

"Unlike [influenza] virus, which has been studied for decades, SARS is
so new that little is known about its infectious course. So, as the
first SARS diagnostic tests are used, we will also be learning how to
use them wisely. For example, while a positive SARS-PCR result from
the sputum of a sick patient very likely means he has SARS, a
negative result may simply mean that we missed a window of viral
shedding. Therefore, a single negative test does not mean that the
person is disease-free. Does a positive test mean that someone is
infectious? We simply do not know. Our PCR tests detect viral nucleic
acids and cannot tell us whether the infectious particle is still
intact. So if a recovered SARS patient still excretes viral nucleic
acids in his stool, does this mean he is still infectious? And if so,
how infectious? We will need to continue the study of these
individuals as we treat them."

Further reading here tells me that the tests are not very reliable.