COVID-19 Cycle Threshold/Cycle Number Testing at a Community Living Center

Ten of the 25 patients testing positive were admitted to the hospital, including 1 admitted 15 days before diagnosis (patient 20) and 1 admitted 80 days after diagnosis (patient 7). Among these 10 patients, 6 were admitted to the intensive care unit, including patient 7. None of the patients were intubated. Three of the 10 admitted patients died (patients 7, 20, and 24). Patient 7 was a 79-year-old male with a history of dementia, cerebrovascular accident, hypertension, hyperlipidemia, and chronic kidney disease with symptoms of lethargy and refusal of oral intake when he was diagnosed with COVID-19. He was admitted 80 days after diagnosis for hyponatremia and acute renal failure, with death on day 87 recorded as complications from the earlier COVID-19 infection. Patient 20, an 89-year-old male with a history of dementia, chronic kidney disease, and hyperlipidemia, had been admitted with fever, cough, and leukocytosis 17 days before COVID-19 diagnosis. He continued to be symptomatic after diagnosis with development of hypotension, dehydration, and refusal of oral intake while on comfort measures/endof- life care and died 15 days after COVID- 19 infection diagnosis. Patient 24 was a 96-year-old male with history of heart failure, hypertension, coronary artery disease, prostate carcinoma, and dementia who developed a cough at the time of diagnosis; because of his underlying condition, he remained in the CLC on comfort care. His symptoms, including hypoxia, worsened until he died 7 days after diagnosis.

Among the 25 patients, 17 were symptomatic at the time of diagnosis; the 14 initially symptomatic patients who survived improved clinically and returned to baseline. Eight of the 25 patients were asymptomatic initially and 3 developed symptoms 2 to 5 days after diagnosis. Only 1 patient who remained asymptomatic was admitted for inability to adhere to quarantine at the CLC. Review of the health records of all surviving symptomatic patients showed symptom resolution with return to baseline that corresponds to an increasing CT/CN value. A 1-tailed t test comparing the initial CT/ CN at the time of diagnosis to the last CT/CN value for symptomatic patients who recovered revealed a statistically significant increase (P < .05). For the symptomatic, symptom resolution and hospital discharge took (if required) a mean 20 days (range, 7-46). Among those who were not hospitalized, symptoms resolved in 7 to 36 days (18 days). Among those requiring hospitalization at any time (excluding patients who died or were asymptomatic), symptom and hospitalization resolution took a mean 22 days (range, 10-46). Asymptomatic patients (patients 8, 10, 15, 16, and 25) also showed increasing CT/CN value during the infection course, although there was no correlation with the continued lack of symptoms.

During the initial validation of the Abbott m2000 instrument, an LOD study included concentrations of 1000, 500, 250, 100, 70, 60, and 50 virus copies/mL (eAppendix 2, available online at doi:10.12788/fp.0276).21 The average CN at 100 virus copies/mL—the manufacturer provided LOD in the instructions for use—was 25.74.20 At a concentration of one-half that (50 virus copies/mL), the average CN was 28.39.

Discussion

This is the first study in the English literature to track CT/CN values as part of serial testing of a veteran CLC. Widescale testing and repeat screening in the absence of symptoms of nursing home residents would identify those who are infected and allow providers to track viral load clearance.^9-14 CT/CN values, when serially tracked during the infection course, appear to increase with illness resolution, consistent with earlier reports that CT/CN correlates with viral load.^8-14 Serial CT/CN values that are high (> 25) and continue to increase with each test suggest progression toward disease resolution or viral RNA clearance.^8-14 After symptom resolution, patients can have a persistent low level of viral shedding (corresponding to a high CT/CN value).^10-14,25 Near the end of disease resolution, a negative serial RT-PCR sample test before a subsequent positive might be a promising clinical sign of near disease recovery. Once the viral load is low with a CT/CN significantly higher than 25, some specimens might result as negative but turn up positive on subsequent sampling with a high CT/CN value. This pattern, with attendant high CT/CN values for the positive results, are consistent with the known effect of viral load (ie, a low viral load correlates to a high CT/CN) and adequacy of specimen collection on CT/CN values^.25 If the patient’s viral load is low, the sample collected might have a viral load at or near the testing platform’s LOD.

For Abbott m2000, the manufacturer provided LOD is 100 virus copies/mL, although the instrument was able to detect virus concentrations below that level during the initial validation.²⁰ The actual LOD of the instrument at our institution is < 100 virus copies/mL. For the Cepheid Xpert Xpress SARS-CoV-2 assay, the manufacturer-provided LOD is 250 virus copies/mL.19 An LOD study including samples below the manufacturer-provided LOD was not part of the initial validation study for the Xpert Xpress assay. Nonetheless, the virus concentration of samples with very high CT values at or near the maximum CT value of 45 is expected to be at or near the platform’s actual LOD.

If the samples collected near the end of the patient’s disease course have viral loads near these low concentrations, the encouraging positive-negative-positive pattern with high CT/CN values might be a promising sign for viral clearance. On the other hand, a positive-negative-positive pattern in the setting of low CT/CN values before and after the negative test might indicate poor sampling for the negative specimen. The back-and-forth or positive-negative-positive pattern generally appears to indicate near resolution of the infection course, although clinical correlation is necessary to rule out inadequate sampling earlier in the disease course or prolonged viral RNA shedding.^9-14 In all of the surviving symptomatic patients who showed the positive-negative-positive pattern, this sign occurred around or after symptom resolution. It also is important to consider that in some patients, SARS-CoV-2 RNA might remain detectable with increasing CT/CN after symptom resolution, and samples from these patients might not result positive. Therefore, CT/CN values cannot be interpreted without considering the clinical picture.²⁵

Studies on infectiousness and virus culture from COVID-19 samples with CT/ CN correlation have shown that patients with high CT/CN at the end of their disease course might not be as infectious.^9-14,25 Because 1 patient had a presumptive positive result after the negative result, this study shows that this positive-negative-positive pattern could include presumptive positive results. Also, in the setting of a recent positive result on the same testing platform, a patient with this pattern is presumed to be positive for COVID-19 RNA because of scant viral material.

Taiwan’s public health response to the outbreak illustrates the ability to mitigate an outbreak throughout a society.²⁶ These actions could help blunt an outbreak within a civilian nursing home population.⁵ Mitigation within a veteran CLC population has been documented, but the study, which focused on mitigation, did not consider CT/CN values, demographic distribution, testing access of the studied population, or laboratory findings related to disease pathophysiology.¹⁵ A key ingredient in widescale, serial testing is the availability of a rapid turnaround from testing in-house that allowed identification within 24 hours instead of several days at a reference laboratory. ¹⁵ Rapid widescale testing would allow clinical teams to optimize the Triangle of Benefit of Widescale Timely Tests for CLC (Figure 3).¹⁵ Timely laboratory testing remains pivotal for CLC veteran residents to aid successful clinical triage and management. Reporting serial CT/CN values can provide additional information to clinicians about the disease course because CT/ CN correlates with viral load, which varies based on where the patient is in the disease course.^9-14 CT/CN values carry significant prognostic value, particularly with respect to intubation and mortality.⁸