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Average daily change in SARS-CoV-2 reverse transcription polymerase chain reaction thresholds among elderly patients with COVID-19: A basis for the proposed determination of isolation periods

Hideo Nagai*

After a cluster of coronavirus disease 2019 (COVID-19) due to nosocomial infection occurred among elderly patients, I suspected that the isolation period for COVID-19 might be too short to protect patients and staff members. This study attempted to define an appropriate isolation period to minimize the risk of disease transmission. Nineteen patients contracted COVID-19 during an outbreak at the General Medicine Ward of Saitama Memorial Hospital, Japan, between July and August 2022. One patient died soon after the infection. For the remaining 18 patients, daily changes (Δ) in cycle threshold (CT) values were calculated using severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) reverse transcription polymerase chain reaction testing, which was performed at least twice. The isolation release date was based on data when the CT value reached 30, using Δ of the CT values. The isolation periods ranged from 8 to 32 days (median: 15.5 days). Isolation lasted for 25–32 days in three patients. In conclusion, the conventional isolation period of 10 days plus symptom-based extra days after onset appears to be too short to prevent nosocomial SARS-CoV-2 infection among the elderly. It would be necessary to define the isolation period based on the average Δ in CT values.


Covid-19; Sars-Cov-2 Rt-Pcr; Cycle Threshold Value; Isolation Period; Nosocomial Infection

Published Date: 2022-11-14; Received Date: 2022-10-16