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How is the threshold for negative and positive results determined?

Ask the Lab Scientist Question:

I have a few questions on the LabCorp antibody test offered under the FDA EUA (164090:  SARS-CoV-2 Semi Quantitative Total Antibody Spike).

A. Results measuring IgG, IgM and IgA are provided in Units/mL. The published threshold for negative and positive results are: Negative <0.79 U/mL; Positive >0.79 U/mL.

  • What is a unit?
  • How is the threshold for negative and positive results determined?

B. LabCorp’s antibody test uses Roche’s ECLIA system. I think a unit is related to the quantity of light (collection of photons) determined by the ECLIA system’s computer program, but how is the quantity of light specifically measured?

Answer: This question is difficult to answer. The research laboratories who developed the test first confirmed that there were few to no false results and then ran the tests on multiple specimens from agreeable research volunteers to determine the reference range. There is no public explanation of “unit.” For example, a test using diffusion would have a different set of units from a test using agglutination.