Public Health Guidance for Community-Level Preparedness and Response to
Severe Acute Respiratory Syndrome (SARS) Version 2

Supplement F: Laboratory Guidance

Appendix F7

Fact Sheet for Clinicians: Interpreting SARS-CoV Test Results from CDC
and Other Public Health Laboratories

**Definitions**

+----------------+-----------------------------------------------------+
| SARS           | Severe acute respiratory syndrome                   |
+----------------+-----------------------------------------------------+
| SARS-CoV       | SARS-associated coronavirus; a newly described      |
|                | coronavirus that is genetically and antigenically   |
|                | distinct from other human coronaviruses             |
+----------------+-----------------------------------------------------+
| Labora         | -   Detection of any of the following by a          |
| tory-confirmed |     validated test, with confirmation in a          |
| SARS-CoV       |     reference laboratory:                           |
| infection      |                                                     |
|                |     -   Serum antibodies to SARS-CoV in a single    |
|                |         serum specimen, *or*                        |
|                |                                                     |
|                |     -   A four-fold or greater increase in SARS-CoV |
|                |         antibody titer between acute- and           |
|                |         convalescent-phase serum specimens tested   |
|                |         in parallel, *or*                           |
|                |                                                     |
|                |     -   Negative SARS-CoV antibody test result on   |
|                |         acute-phase serum and positive SARS-CoV     |
|                |         antibody test result on convalescent-phase  |
|                |         serum tested in parallel; **or**            |
|                |                                                     |
|                | ```{=html}                                          |
|                | <!-- -->                                            |
|                | ```                                                 |
|                | -   Isolation in cell culture of SARS-CoV from a    |
|                |     clinical specimen, with confirmation using a    |
|                |     test validated by CDC; **or**                   |
|                |                                                     |
|                | ```{=html}                                          |
|                | <!-- -->                                            |
|                | ```                                                 |
|                | -   Detection of SARS-CoV RNA by RT-PCR validated   |
|                |     by CDC, with confirmation in a reference        |
|                |     laboratory, from:                               |
|                |                                                     |
|                |     -   Two clinical specimens from different       |
|                |         sources, *or*                               |
|                |                                                     |
|                |     -   Two clinical specimens collected from the   |
|                |         same source on two different days           |
+----------------+-----------------------------------------------------+
| Confirmed case | A person with clinically compatible illness and     |
| of SARS-CoV    | laboratory-confirmed SARS-CoV infection             |
| disease        |                                                     |
+----------------+-----------------------------------------------------+

The Centers for Disease Control and Prevention (CDC) and other
institutions have been working to develop strategies to detect and
control the spread of severe acute respiratory syndrome (SARS). The
cause of SARS has been determined to be infection with a previously
unrecognized human coronavirus, SARS-associated coronavirus (SARS-CoV).
Information on SARS and SARS-CoV is provided on CDC's SARS website:
[www.cdc.gov/sars/](http://www.cdc.gov/sars/). All information and
guidelines, including those on SARS-CoV laboratory testing, may change
as we continue to learn more about this disease. Please check CDC's SARS
website regularly for the most current information.

Previous experience with SARS-CoV disease demonstrates that the best
guide to diagnosis is exposure to a person with SARS-CoV disease, a
setting where SARS-CoV transmission is occurring, or persons who are
part of a cluster of pneumonia without a known cause. Information in
diagnosing SARS-CoV disease is provided in *Clinical Guidance on the
Identification and Evaluation of Possible SARS-CoV Disease among Persons
Presenting with Community-Acquired Illness*
([www.cdc.gov/ncidod/sars/clinicalguidance.htm](http://www.cdc.gov/ncidod/sars/clinicalguidance.htm)).
Persons without a potential risk of exposure should usually not be
tested for SARS-CoV. Clinicians should seek guidance from the state or
local health department regarding current guidelines for SARS-CoV
testing.

Clinicians providing care for patients with possible SARS-CoV disease
may find the following information useful when interpreting SARS-CoV
test results.

**What tests for SARS-CoV are available?**\
CDC has developed and validated an enzyme immunoassay (EIA) for
detection of serum antibody
([www.cdc.gov/ncidod/sars/lab/eia/](http://www.cdc.gov/ncidod/sars/lab/eia/))
to SARS-CoV and a reverse transcription-polymerase chain reaction
(RT-PCR) assay
([www.cdc.gov/ncidod/sars/lab/rtpcr/](http://www.cdc.gov/ncidod/sars/lab/rtpcr/))
for detection of SARS-CoV RNA. The EIA has been distributed to most
state public health laboratories, and the RT-PCR has been distributed to
most laboratories in the Laboratory Response Network (LRN). Both the EIA
and the RT-PCR tests are sensitive and highly specific for SARS-CoV. The
ability to diagnose SARS-CoV infection in a patient is often limited,
however, by either the low concentration of virus in most clinical
specimens (RT-PCR assays) or the time it takes a person to mount a
measurable antibody response to SARS-CoV (serologic assays). The
likelihood of detecting infection is increased if multiple specimens
(e.g., stool, serum, respiratory tract specimens) are collected at
several times during the course of illness.

CDC considers detection of SARS-CoV antibody to be the most reliable
indicator of infection. Since previous infection is still rare in most
populations, seroconversion is not needed to diagnose infection.
Therefore, the presence of SARS-CoV antibody in someone without a
previous history of SARS is indicative of recent infection. A negative
serologic test can rule out SARS-CoV infection if the serum specimen is
collected \>28 days after onset of illness. Some persons do not mount an
antibody response (test positive) until more than 28 days after onset of
illness. Patients with a negative antibody test result whose specimens
were obtained 28 days before illness onset or before should have another
serum specimen collected \>28 days after onset of symptoms.

RT-PCR for SARS-CoV RNA is a very sensitive and specific assay when
performed appropriately. This test can detect SARS-CoV RNA in serum,
stool, upper and lower respiratory specimens, various tissues, and
occasionally urine specimens. Testing of multiple specimen types at
several times during the course of illness should increase the
likelihood of detecting infection.

Other tests for detection of SARS-CoV include immunofluorescence assay
(IFA) for SARS-CoV antibody, SARS-CoV isolation studies, electron
microscopic studies, and immunohistologic or in situ hybridization
studies on tissue specimens. The IFA for SARS-CoV antibody gives results
essentially identical to those for the EIA for SARS antibody. Cell
culture, electron microscopy, and histologic studies are less frequently
used and less sensitive than RT-PCR. Cell culture for SARS-CoV should be
done only in a BSL-3 laboratory using BSL-3 procedures (see Appendix
F5).

**What does it mean if a specimen tests positive for SARS-CoV?**\
Laboratory test results should always be considered with clinical
observations and epidemiologic data in making a final diagnosis. A
positive RT-PCR result should be confirmed by testing a second specimen
and confirming the result at a qualified second laboratory to ensure
that the result is not an artifact of laboratory contamination. A
positive serologic result is less likely to result from a laboratory
artifact but should also be subjected to confirmatory testing. If the
results are confirmed, then a positive RT-PCR or serologic test result
indicates that the patient has been recently infected with SARS-CoV
(unless the patient has a previous history of SARS-CoV disease).
Guidelines for managing patients with SARS-CoV disease are provided in
Supplement C and Supplement I.

**How is a SARS-CoV test confirmed?**

Positive antibody and RT-PCR test results should be confirmed by repeat
testing of the original specimen AND by testing of the same specimen in
an independent laboratory using a validated assay.

**What is difference between a laboratory-confirmed clinical specimen
and laboratory-confirmed SARS-CoV disease?**

This distinction is made for PCR test results because of concerns about
false-positive results. For serology, virus isolation, and
histopathologic studies, if a specimen is confirmed positive, the
patient is also considered to be confirmed positive. For PCR, a second
specimen is required to be confirmed positive to decrease the chance of
misclassifying a patient due to a false-positive result. In all
instances, laboratory results must be considered in the context of
clinical and epidemiologic information on the patient.

**What does it mean if a patient with an illness suggestive of SARS has
a negative SARS-CoV test result?**\
A negative antibody result on a serum specimen collected \>28 days after
onset of illness is sufficient to eliminate SARS-CoV as the cause of
illness. A negative antibody result on serum specimens collected ≤28
days after onset of illness or a negative RT-PCR test does not rule out
SARS-CoV infection. Clinical specimens do not always have sufficient
virus to be detected by RT-PCR. An antibody response may not be detected
in some patients until \>28 days after onset of illness.

**What does it mean if test results are positive for other respiratory
diseases?**\
A positive test result for another respiratory pathogen does not rule
out SARS-CoV disease. SARS patients can be co-infected with SARS-CoV and
other respiratory pathogens. Thus, detection of another respiratory
pathogen does not eliminate the possibility of SARS-CoV disease. In some
circumstances (e.g., another pathogen is detected in multiple patients
in a cluster of cases and can fully explain the severity of illness),
detection of another respiratory pathogen may make SARS-CoV disease less
likely. Factors that may be considered in assigning alternate diagnoses
include the strength of the epidemiologic exposure criteria for SARS-CoV
disease, the specificity of the diagnostic test, and the compatibility
of the clinical presentation and course of illness with the alternative
diagnosis.

**Does a negative SARS-CoV test result affect patient management?**\
As noted above, the interpretation of negative SARS-CoV test results
varies depending on the type of specimen, the timing of specimen
collection, and the test that was performed. With the exception of a
\>28-day negative serologic test result, a negative SARS-CoV test result
should not affect patient isolation or management decisions. The
clinical features of the illness and the type and risk of exposure are
the keys to making decisions on patient management and isolation.

**Has the identification of SARS-CoV as the etiologic agent changed the
recommendations for medical treatment of patients with SARS?**\
No. The discovery that SARS-CoV is the cause of SARS has not changed
treatment recommendations. Research on antiviral treatment for SARS-CoV
disease is currently under way.

**Should a person who may have been exposed to a location with
transmission of SARS-CoV or who had contact with a SARS patient be
tested even if not ill?**\
Persons who have potentially been exposed to SARS patients and are well
should be tested only as part of research studies. The exposed person
may contact their state health department or CDC about participating in
studies of persons exposed to SARS-CoV.

For more information, visit
[www.cdc.gov/ncidod/sars](http://www.cdc.gov/ncidod/sars/) or call the
CDC public response hotline\
at (888) 246-2675 (English), (888) 246-2857 (Español), or (866) 874-2646
(TTY)