Editors’ Note: This article was accepted on April 28, 2020, and was accurate at that time. Because information about SARS-CoV-2 and COVID-19 is evolving rapidly, please verify these recommendations and information.
Explore This IssueACEP Now: Vol 39 – No 06 – June 2020
When SARS-CoV-2 appeared, we had no tests. The disease spread unchecked and unmeasured.
Today, we have several tests, each with its own unique roles and limitations. How should these be used, and what strengths and weaknesses affect their use?
Nucleic Acid-Based Tests
These tests have been the work horses in diagnosing acute cases of COVID-19. The two primary analytic methods are reverse transcriptase polymerase chain reaction (RT-PCR) and loop-mediated isothermal amplification (LAMP). Regardless of the technique, the basic principle is the same: RNA from viral particles are bound to complementary DNA sequences, which are then copied. Repeated cycles of copying produce exponential amplification that, in sufficient quantity, reaches a defined threshold for a positive test. Absent specific SARS-CoV-2 RNA, minimal amplification occurs, and the test never reaches the threshold for positivity.
These tests are designed for those actively infected and shedding virus. Their analytic sensitivity and specificity are considered excellent, with limitations related to mismatches between the DNA primers and small alterations in the SARS-CoV-2 RNA genome. The DNA primers used may match fragments of other RNA found in samples but typically not to the extent where cross-reactivity impairs amplification of the target.
The sensitivities for these tests are limited by two main factors. The first issue is the process through which the specimen is obtained. The most widely recognized version of this test is the nasopharyngeal swab. An inadequate sampling technique will diminish the quality of the specimen, lowering sensitivity.
Second, viral load and shedding also decrease with time, contributing to diminishing sensitivity.1 Overall clinical sensitivity in practice appears similar to our expectations for common influenza tests, in a range approximating 70 to 80 percent.1 As many of us have already experienced in our clinical workflow, a single negative nasopharyngeal swab does not adequately exclude COVID-19 infection when the remaining clinical picture is supportive. A positive test is, however, virtually unimpeachable.
Serological tests are designed to detect the presence of antibodies to SARS-CoV-2. The antibodies of interest include the acute-phase immunoglobulin (IgM), the late-phase (IgG), and occasionally IgA.1 These tests have been promoted widely as a critical part of plans to reopen America and are in frequent use in population prevalence studies.
Antibody tests are more complicated than DNA-based tests, however. Two main types, enzyme-linked immunosorbent assay (ELISA) and lateral-flow immunoassay (LFIA), are in use. These tests are more difficult to develop because the developers of these assays must synthesize their own novel viral fragments. This involves an analysis of the actual protein coat of the virus, typically focusing on the unique features of the spike protein and cell-entry apparatus. When serum or plasma containing antibodies to SARS-CoV-2 are mixed with the assay antigens, the test reporter systems provide a positive result.