Severe Acute Respiratory Syndrome - Diagnosis

Diagnosis

SARS may be suspected in a patient who has:

1. Any of the symptoms, including a fever of 38 °C (100.4 °F) or higher, and
2. Either a history of:
   1. Contact (sexual or casual, including tattoos) with someone with a diagnosis of SARS within the last 10 days OR
   2. Travel to any of the regions identified by the WHO as areas with recent local transmission of SARS (affected regions as of 10 May 2003 were parts of China, Hong Kong, Singapore and the province of Ontario, Canada).

A probable case of SARS has the above findings plus positive chest X-ray findings of atypical pneumonia or respiratory distress syndrome.

With the advent of diagnostic tests for the coronavirus probably responsible for SARS, the WHO has added the category of "laboratory confirmed SARS" for patients who would otherwise fit the above "probable" category who do not (yet) have the chest X-ray changes, but do have positive laboratory diagnosis of SARS based on one of the approved tests (ELISA, immunofluorescence or PCR).

The chest X-ray (CXR) appearance of SARS is variable. There is no pathognomonic appearance of SARS, but is commonly felt to be abnormal with patchy infiltrates in any part of the lungs. The initial CXR may be clear.

White blood cell and platelet counts are often low. Early reports indicated a tendency to relative neutrophilia and a relative lymphopenia — relative because the total number of white blood cells tends to be low. Other laboratory tests suggest raised lactate dehydrogenase and slightly raised creatine kinase and C-reactive protein levels.

With the identification and sequencing of the RNA of the coronavirus responsible for SARS on 12 April 2003, several diagnostic test kits have been produced and are now being tested for their suitability for use.

Three possible diagnostic tests have emerged, each with drawbacks. The first, an enzyme-linked immunosorbent assay (ELISA) test detects antibodies to SARS reliably, but only 21 days after the onset of symptoms. The second, an immunofluorescence assay, can detect antibodies 10 days after the onset of the disease, but is a labour- and time-intensive test, requiring an immunofluorescence microscope and an experienced operator.

The last test is a polymerase chain reaction (PCR) test that can detect genetic material of the SARS virus in specimens from blood, sputum, tissue samples and stools. The PCR tests so far have proven to be very specific, but not very sensitive. This means while a positive PCR test result is strongly indicative the patient is infected with SARS, a negative test result does not mean the patient does not have SARS.

The WHO has issued guidelines for using these diagnostic tests. There is currently no rapid screening test for SARS and research is ongoing.