# 13-May-2020 COVID-19: Calculating the detection limit for a SARS-CoV-2 RT-PCR assay using Analyse-it

Recent improvements to the , in version 5.50 and later, include the addition of probit regression. Probit regression is useful when establishing the detection limit (LoD) for an RT-qPCR assay.

The protocol provides guidance for estimating LoD and is recognized by the FDA. In this blog post, we will look at how to perform the relevant part of the CLSI EP17-A2 protocol using Analyse-it.

For details on experimental design, see section 5.5 in the CLSI EP17-A2 guideline. In Analyse-it, you should arrange the data in 2 columns: the first should be the concentration, and the second should be the result, positive or negative. You should have a minimum of 20 replicates at each concentration. We have put together a hypothetical example in the workbook which you can use the follow the steps below:

To Analyse-it:

The analysis task pane opens.

NOTE: If using Analyse-it pre-version 5.65, on the **Fit** panel, in the **Predict X given Probability** edit box, type 0.95.