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Correlation Of Pcr Ct Values With Symptom Onset Across Diagnostic Platforms For Sars-Cov-2


 
 
 Following the definition of SARS-CoV2 outbreak as a pandemic by WHO, FDA gave EUA approval to the CDC real time polymerase chain reaction (PCR) assay and soon to other vendors to increase test availability. Most of testing platforms are PCR based, which test for multiple gene targets. We aimed to compare distribution of crossing threshold (cT) values of Diasorin (DIA), NeuMoDX (NDX) and Cepheid GenXpert (GX) for symptomatic and asymptomatic patients and assess performance of individual gene targets within the assays. We also correlated cT values with time from symptom onset.
 
 
 
 Retrospective review of medical and laboratory records of patients who tested positive for SARS-CoV2 between 08/01/2020 and 10/10/2020 on DIA, NDX, and GX platforms.
 
 
 
 We included 212 patients in our study. Days since symptom onset included 1 to 16 days. For DIA, mean Ct values for 46 symptomatic patients were 21.75 (S gene) and 22.74 (ORF1 gene); whereas 23.49 (S gene) and 25.49 (ORF1 gene) for 12 asymptomatic patients. Similarly, on NDX mean was 22.21 (N gene) and 23.13 (NSP2 gene) for 69 symptomatic, though 28.09 (N gene) and 28.61 (NSP2 gene) for 35 asymptomatic patients. GX manifested mean Ct value of 27.13 (E gene) and 31.22 (N2 gene) for 19 symptomatic; while 33.85 (E gene) and 36.42 (N2 gene) for 31 asymptomatic patients. Correlation coefficient for cT values versus days since symptom onset are DIA (r2 0.19), NDX (r2 0.22), and GX (r2 0.02).
 
 
 
 The difference in cT values was statistically significant for symptomatic versus asymptomatic patients. There was positive correlation between days since symptom onset and cT values for DIA and NDX but not for GX, which may be due to difference in population tested in these platforms. These observations may be used to predict viral load and thus infectivity of patients who test positive for SARS-CoV2.
