The Centers for Disease Control and Prevention advises that immune-competent adults are not infectious more than 10 days after symptom onset.7 RT-PCR testing detects noninfectious viral RNA up to 12 weeks after infection.7 To distinguish infectious from noninfectious viruses, cycle thresholds may be used. The cycle threshold is the number of cycles a sample must be amplified in the laboratory before the virus can be detected. A low cycle threshold value correlates with higher viral load and contagiousness because fewer cycles are required to detect the virus. Cycle threshold values are also affected by collection technique and vary by assay, reducing their reliability.8
FACTORS AFFECTING CLINICAL SENSITIVITY AND SPECIFICITY OF PCR/RAT
Days from symptom onset - Sn greatest at days 1 - 5 after symptom onset (> 90%), lower Sn earlier before symptoms or more than 5 days after symptoms
Sampling site - Lower Sn for sampling sites other than nasopharynx
RAT compared with rt-PCR testing - Sn - overall 62%-64% lower if asymptomatic, greater if pesymptomatic, to detect like contagious patients 87%-90% Sn lower if asymptomatic, specificity > 99%