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 and 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 Antigen Tests
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
offer timely results and are widely used for point-of-care testing but have
lower sensitivity than RT-PCR, especially in asymptomatic individuals or
early/late in the course of the infection. Negative
results should be confirmed with an RT-PCR test before making treatment
decisions to prevent undue transmission.
Coronavirus Disease 2019 (COVID-19)_DiagnosticsAncillary/Supportive 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 done.
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). Chest imaging may show bilateral infiltrates.