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Laboratory Tests and Ancillaries
Diagnostic
testing involves detecting the virus itself (viral RNA or antigen) or detection
of antibodies or other biomarkers. SARS-CoV-2 virus is detectable in the upper
respiratory tract 1-3 days before the onset of symptoms. The concentration of
the virus in the upper respiratory tract is highest around the time of onset of
symptoms and then gradually declines.
The
presence of viral RNA in the lower respiratory tract, and for a subset of
individuals in the feces, increases during the second week of illness. SARS-CoV-2
can be detected in a wide range of other body fluids and compartments but is
most frequently detected in respiratory material and, therefore, respiratory
samples remain the specimen of choice for diagnostics.
Nucleic Acid Amplification Test (NAAT) including
Real-time Reverse Transcription-Polymerase Chain Reaction (RT-PCR) Assay
NAAT
including real-time RT-PCR assay is the recommended or gold standard
confirmatory test of COVID-19 that detects viral RNA. A nasopharyngeal specimen
is preferred but alternatives can be oropharyngeal or saliva specimens. It is used
for the diagnosis of current infection but is not used to monitor response to
treatment. In ideal settings, it has high analytic sensitivity and specificity.
Rapid Tests Based on Antigen Production
Rapid
tests based on antigen production involve a lateral flow immunofluorescent
sandwich assay intended for the qualitative detection of the nucleocapsid
protein antigen from SARS-CoV-2 in nasopharyngeal and nasal swab specimens
directly or after the swabs have been added to either the universal transport
media or viral transport media.
They
detect the presence of antigens expressed by the COVID-19 virus. If the target
antigen is present in sufficient concentrations in the sample, it will bind to
specific antibodies fixed to a paper strip enclosed in a plastic casing and
generate a visually detectable signal, typically within 30 minutes.
Antigen(s)
detected are expressed only when the virus is actively replicating; therefore,
such tests are best used to identify acute or early infection. They can be used
as an alternative to RT-PCR assay for the diagnosis of COVID-19 among
symptomatic patients during the first week of illness. Negative results should
be confirmed with an RT-PCR test before making treatment decisions to prevent
undue transmission.
Rapid Antibody Tests
Rapid
antibody tests are point-of-care and laboratory-based antibody immunoassays used
for the diagnosis of prior infection or infection of at least 3 to 4 weeks’
duration. They are not recommended as a sole test for the diagnosis of COVID-19
and for mass testing and clearance for work of asymptomatic individuals because
of its low sensitivity and high false-negative rates.
Other Diagnostic Tests
Other
diagnostic tests that may be requested for suspected COVID-19 patients,
depending on the severity of the disease, are complete blood count (CBC); metabolic
panel including creatinine, liver function tests (LFTs), sodium, potassium,
magnesium, calcium, albumin; inflammatory markers including lactate
dehydrogenase (LDH), ferritin, C-reactive protein (CRP), and procalcitonin; prothrombin
and D-dimer; arterial blood gas (ABG) measurement; and electrocardiogram (ECG).
Blood cultures may be requested if concomitant
bacterial infection is suspected and respiratory tract specimens for influenza
testing if prevalent. Sputum, endotracheal aspirate (ETA), or bronchoalveolar
lavage fluid culture and sensitivity may also be necessary in some cases.
Imaging
Imaging studies that may be requested include a chest X-ray, high-resolution plain chest computed tomography (CT) scan, or CT pulmonary angiography (CTPA).