COVID-19 Cycle Threshold/Cycle Number Testing at a Community Living Center
Background: COVID-19 imposes a special risk to the nursing home population, including community living centers (CLCs) for veterans. Cycle threshold/cycle number (CT/CN) values obtained by serial reverse transcriptase polymerase chain reaction (RT-PCR) testing could yield valuable information about viral load and potential infectiousness. Serial testing for COVID- 19 with CT/CN correlates in a nursing home population during an outbreak has not yet been reported in the literature.
Methods: A retrospective review of serial RT-PCR testing for COVID-19 during an outbreak at a CLC was performed from March 28 to April 4, 2020, with follow-up of identified patients until November 10, 2020. Testing was performed on the Abbot m2000 or Cepheid platform.
Results: Of 80 patients tested, 25 (31%) were positive for COVID-19. CT/CN values corresponded to the infection course as expected.
Conclusions: Repeat testing for COVID-19 accompanied by CT/ CN values could provide clinical and epidemiologic information about the likely stage of the patients’ disease course, which may aid public health measures and clinical management.
Limitations
Important limitations to our study include the use of 2 separate RT-PCR platforms. Using different RT-PCR platforms is common in clinical laboratories trying to take advantage of the unique characteristics of different platforms—for example, turnaround time vs high throughput— to manage COVID-19 testing workflow.25 However, the exact CT/CN values obtained from each platform might not translate to the other, and the general trend (CT/CN values are rising or falling across serial tests) rather than a single value could be useful for clinical correlation. Even when the same platform is used for the serial testing, CT/CN values can be affected by adequacy of specimen collection; therefore, clinical correlation and considering the trend in CT/CN values is necessary for interpretation.10-14,25 Because of the known trend in viral dynamics, a positive specimen collected with a high CT/CN followed by a subsequent (within 2 days) positive specimen collected with a low CT/CN might be compatible with early detection of COVID- 19 infection in the appropriate clinical context. 10-14 However, detection late in the infection course or even after the symptomatic disease resolved with prolonged viral shedding might show serial positive samples with increasing CT/CN values.10-14
Patients with prolonged viral shedding might not be infectious.27 Because of the clinical correlation required for interpretation and the other factors that might affect CT/CN values, recommendations advise against using CT/CN values in clinical practice at this time, although these recommendations could change with future research.25 Serial CT/CN values have the potential, if appropriately correlated with the clinical picture, to provide useful information, such as whether the viral load of the sample is relatively high or low and increasing or decreasing.
Veterans, as a population, are more susceptible to poor health outcomes and morbidity compared with similar civilian counterparts.2,14-16 Veteran CLC patients likely would experience worse outcomes with COVID-19, including more infections, expiration, and morbidity compared with similar general population nursing homes. Similar to what had been reported for the civilian population, a trend (high CT/CN values early in the disease course with repeat testing needed to detect all positives followed by lower CT/CN value to correlate with increased viral load and then increased CT/CN value as the infection resolved) also was observed in this veteran population.
It has been extensively documented that minority groups experience decreased health care access and worse health outcomes. 28-30 Considering the critical medical supply shortages, including personal protective equipment, ventilators, and even testing supplies, there is the potential for a resource access disparity by ethnicity.28-31 Because the VA does not depend on measures of wealth and privilege such as health insurance, there was no disparity noted in access to testing by race or ethnicity at the VAMC CLC. When considering the health outcome of viral load from the measured CT/CN value, the viral loads of African American patients and those of other ethnicities was not significantly different in this study.
Conclusions
This is the first study to bring up critical points including serial CT/CN value correlation in RT-PCR tests, demographic distributions demonstrating easy and equal access in a veteran nursing home to COVID-19 testing, and clinical laboratory signs related to disease pathophysiology. Unlike other populations who have undergone serial CT/CN monitoring, nursing homes represent a particularly vulnerable population who require measures to prevent the spread and mitigate outbreaks of COVID-19.2,4,5 Test measurements obtained such as the CT/CN value during routine clinical care can provide useful information for public health, epidemiologic, or clinical purposes with appropriate correlation to clinical and other laboratory parameters. This study demonstrates early intervention of serial testing of an outbreak in a veterans nursing home with CT/CN value correlation.
