Excerpt from pdf of the CDC 2019 Coronavirus Manual page 39 towards bottom:
Limit of Detection (LoD): LoD studies determine the lowest detectable concentration of 2019-nCoV at which approximately 95% of all (true positive) replicates test positive. The LoD was determined by limiting dilution studies using characterized samples. The analytical sensitivity of the rRT-PCR assays contained in the CDC 2019 Novel Coronavirus (2019- nCoV) Real-Time RT-PCR Diagnostic Panel were determined in Limit of Detection studies. Since
no quantified virus isolates of the 2019-nCoV are currently available,
assays designed for detection of the 2019-nCoV RNA were tested with characterized stocks of in vitro transcribed full length RNA (N gene; GenBank accession: MN908947.2) of known titer (RNA copies/µL) spiked into a diluent consisting of a suspension of human A549 cells and viral transport medium (VTM) to mimic clinical specimen. Samples were extracted using the QIAGEN EZ1 Advanced XL instrument and EZ1 DSP Virus Kit (Cat# 62724) and manually with the QIAGEN DSP Viral RNA Mini Kit (Cat# 61904). Real-Time RT-PCR assays were performed using the ThemoFisher Scientific TaqPath™ 1-Step RT-qPCR Master Mix, CG (Cat# A15299) on the Applied Biosystems™ 7500 Fast Dx Real-Time PCR Instrument according to the CDC 2019-nCoV RealTime RT-PCR Diagnostic Panel instructions for use.