Viral disease testing

In March 2020, Abbott received emergency use authorization (EUA) for its isothermal nucleic acid test for COVID-19. It produces test results in 5 minutes using its ID NOW portable testing system.[1] It also received EUA for its m2000-based laboratory nucleic acid test for COVID-19.[2]

In April 2020, Abbott received EUA for its ARCHITECT IgG laboratory antibody test for COVID-19.[3] Also in April, Abbott's ID NOW test was reported to have sensitivity of 85.2%.[4] A later study found sensitivity of only 52%, inducing the FDA to issue an alert.[5] A different study found sensitivity of 91%.[6] The UK spent $20 million for antibody tests that proved flawed.[7]

In May 2020, a rapid antigen test from Quidel Corporation received EUA for detecting SARS-CoV-2.[8] Test results were said to be available at point of care within 15 minutes.[9] Sensitivity is 85%.[1] Also in May 2020, a CRISPR genetic test was approved for emergency use that claimed 100% selectivity and 97.5% sensitivity.[10] That month Abbott received EUA for its Alinity antibody test for COVID-19. The company claimed 100% sensitivity and 99.6% specificity for patients tested 14 days after symptoms began.[10] Another review found that the accuracy of PCR tests depended on the interval between the infection and the test. Immediately after infection, the sensitivity was 0, rising to 80% after three days and then declining thereafter.[11]

In May 2020 the FDA withdrew approval for 29 of 41 antibody tests for which it had given EUAs.[12]

Tests for viral DNA can look for either DNA or RNA. They typically use reverse transcriptase polymerase chain reactions to multiply the amount of genetic material in a small sample into an amount that is measurable. The genetic material is generally taken from the nose/sinus. One study found clinical sensitivity was found to range from 66 to 80%.[13] Another found accuracy of 70%.[14]

A newer form uses CRISPR to produce a test result without requiring lab equipment. The test reacts with genetic material to produce a visible signal without requiring amplification.[10]

Serological tests look for the presence of antibodies in a test sample. Antibodies are materials that the body produces to fight off an infection. The primary antibodies sought in viral testing are IgE and IgG.

A rapid antigen test quickly searches for antigens, protein fragments that are found on the surface of or within a virus. Antigen tests can be analyzed within a few minutes. Antigen tests are less accurate than PCR tests. It has a low false positive rate, but a higher false negative rate. A negative test result may require confirmation with a PCR test.[8] Advocates claim that antigen tests are less expensive and can be scaled up more rapidly than PCR tests.[8] Antigen tests are available for a variety of conditions, including streptococcus,[15] influenza, giardia,[16] Ebola and Helicobacter pylori.[17] Antigens can be detected via blood, urine or stool.

Imaging such as computerized tomography can be used to inform a diagnostic process. CT scans are considerably more expensive than nucleic acid tests and involve a small dose of radiation. For COVID-19, they are seen as the most accurate diagnostic tool, because the disease creates patchy "ground glass" areas in the lungs that are revealed by a scan. One study found 97% sensitivity.[14]

Sensitivity indicates whether the test accurately identifies whether the virus is or is present. A 90% sensitive test will correctly identify 90% of infections, leaving 10% with a false negative result.[13]

Specificity indicates how well-targeted the test is to the virus in question. Highly specific tests pick up only the virus in question. Non-selective tests pick up other viruses as well. A 90% specific test will correctly identify 90% of those who are uninfected, leaving 10% with a false positive result.[13]