The US Centers for Disease Control and Prevention (CDC) published in their December 12, 2003 Morbidity and Mortality Weekly Report (MMWR) a “Revised US Surveillance Case Definition for Severe Acute Respiratory Syndrome (SARS) and Update on SARS Cases---United States and Worldwide, December 2003”. This CDC report summarized the new US surveillance case definition for SARS issued November 3, 2003 as a new interim position statement by the US Council of State and Territorial Epidemiologists (CSTE). This revised SARS case definition includes four criteria: clinical, epidemiologic, laboratory, and exclusion criteria.
The clinical criteria includes 3 possibilities:
1. “Early” disease (a new term that replaces the former terms “asymptomatic” or “mild”),
2. Mild-to-moderate respiratory illness
3. Severe respiratory illness
The epidemiologic criteria includes 2 new terms (“possible” and “likely”) as possibilities:
1. “Possible” exposure to the SARS-CoV within 10 days of symptom onset
2. “Likely” exposure to the SARS-CoV within 10 days of symptom onset
Laboratory criteria include 3 possibilities for confirmation of SARS-CoV:
1. Detection of serum antibody to SARS-CoV by a test validated by CDC
2. Isolation in cell culture of SARS-CoV
3. SARS-CoV RNA detection by a PCR test validated by CDC, AND with subsequent validation in a reference lab such as at the CDC.
Exclusion criteria includes 3 possibilities:
1. An alternative diagnosis can explain the illness fully, or
2. Antibody to the virus is not present in a serum obtained >28 days after the onset of illness.
3. The case was reported on the basis of contact with a person who was excluded subsequently as a cause of SARS-CoV disease; then the reported case is also excluded as long as there are no other epidemiologic or lab criteria for SARS present.
Importantly, a new term “SARS RUI” (SARS report under investigation” is added. SARS-RUI includes 4 possibilities:
1. SARS RUI-1: Cases compatible with SARS in groups likely to be first affected by SARS-CoV, if SARS-CoV is introduced from someone who does not have clear epidemiologic links to known cases of SARS disease or places with known ongoing transmission of SARS-CoV. These will be reports of patients from areas where SARS is not known to be occurring. Specific consideration as such initial SARS cases should be given to:
a. Persons who have traveled within 10 days of onset of symptoms to mainland China, Hong Kong, or Taiwan, OR close contact with an ill person who has recently traveled to one of these areas.
b. Health care worker or lab worker with potential exposure to SARS-CoV
c. Part of a cluster of cases of unexplained pneumonia.
In contrast, SARS RUI 2-4 will be from areas where SARS is occurring:
2. SARS RUI-2: Cases of “mild-to-moderate illness” and epidemiologic criteria for ”possible” exposure.
3. SARS RUI-3: Cases of “severe illness” and epidemiologic criteria for “possible” exposure.
4. SARS RUI-4: Cases of “early” or “mild-to-moderate illness” and epidemiologic criteria for “likely” exposure.
SARS-CoV disease includes two possibilities:
1. “Probable case:” “severe” respiratory illness and epidemiologic criteria for “likely” exposure.
2. “Confirmed case:” Any clinically compatible illness whether “early”, mild-to-moderate” or “severe” that is laboratory confirmed.
This December 12, 2003 CDC document also provides two tables further demonstrating these case classification terms depending on whether laboratory testing has, or has not, been performed. Additional discussion nationally can be expected of this revised US surveillance case definition for SARS. In my opinion, while this case definition initially appears more complex than its predecessor, use of the term “SARS report under investigation (SARS-RUI)” will be useful if SARS returns. While working in a SARS-designated Toronto hospital during the outbreak I found that use of the official term in their SARS classification system of “person under investigation (PUI)” to categorize certain patients was clinically and epidemiologically useful.
Daniel R. Lucey, MD, MPH
Director, Center for Biologic Counterterrorism & Emerging Diseases
Washington Hospital Center
Washington, DC