Limits of detection for FDA-authorized COVID-19 diagnostics

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As more tests that diagnose active infection with SARS-CoV-2 come online, their limits of detection offer a window into their sensitivity and potential to produce false negative results.

More than 20 diagnostic tests have received Emergency Use Authorization (EUA) by FDA for COVID-19 testing in the last two months. The limits of detection (LoD) for the virus range from 40 copies/mL to 100,000 copies/mL; four tests use the measure copies per reaction, and range from three copies to 200 copies per reaction.

Two use TCID50 (median tissue culture infectious dose) per mL, and have very close values at 0.009 and 0.01 TCID50/mL. TCID50 represents the viral load at which 50% of cells are infected when a solution containing the virus is added to cell culture.

Table: COVID-19 diagnostic tests with EUA

An assay's LoD is the lowest amount of the target that the test can detect at least 95% of the time. A lower LoD indicates a more sensitive test.

LoDs reported as viral genomic sequence copies per volume are primarily reported in copies per mL; asterisks denote LoDs originally reported as copies per μL.

The CDC reported separate LoDs for two versions of its test involving different PCR primers.

Most of the diagnostics that have received EUA from FDA are reverse transcription polymerase chain reaction (RT-PCR) tests; the ID NOW COVID-19 test from Abbott Laboratories (NYSE:ABT) is based on non-thermocycling probe technology (see "COVID-19 Diagnostic Tech Tableau").

FDA has also granted EUA to a serological test from Cellex Inc. that detects patient antibodies against COVID-19; because these tests target variable human proteins, they are evaluated by their clinical sensitivity, rather than an analytical LoD.

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