The dynamic change of antibody index against Covid-19 is a powerful diagnostic tool for the early phase of the infection and salvage PCR assay errors.

Background: Currently, PCR assay is a golden standard for diagnosis of Covid-19. However, it needs nasopharyngeal swabs, expensive instruments and expertise. It even causes PCR errors.
Methods: We validated the antibody assay (Roche) in 36 followed patients and 1879 controls (medical staffs).
Results: Of 1879 medical staffs, only two (0.11%) were positive by Cut off Index (COI; 1.0) (mean ± SD, 0.094 ± 0.047). Thirty six patients were composed of three groups; Group A,4 from Diamond Princess cruise ship, Group B, 2 infected in Africa, and Group C, 30 infected in Japan. PCR assays were conducted at outside laboratories before and repeated in house after hospitalized. Of 36 at admission, positive antibody was seen in 4/4 from the ship, 0/2 from Africa, and 5/30 from Japan. Two from Africa showed the increase of COI and became positive on days 8 and 13. Thirty Japanese was divided in two groups, e.g., 23 showed dynamic increase of COI up to 84.4 within 3 days while active virus replication present (Group C). In remaining 7 (7/30, 23%) (Group C'), no rise of antibody nor positive in house PCR assays, indicative of false positive results of PCR at the beginning.
Conclusion: This antibody testing has a wide dynamic ranges of COI and, thus, could be utilized in the early infection phase. This may also compliment and even help to avoid possible PCR errors. Therefore, this can serve as a powerful diagnostic tool, needed in the frontline of the clinic and hospitals.
Competing Interests: Declaration of competing interest None.
(Copyright © 2021. Published by Elsevier B.V.)