3.4 Temporal changes of initial chest CT and RT-PCR
The cumulative percentage of cases identified by RT-PCR and initial chest CT is shown in Figure 1. In the first week after the onset of symptoms, the cumulative percentage of RT-PCR and CT both increased rapidly. On the 4th day after the onset of symptoms, 72 (63%) patients had a positive RT-PCR, but only 36 (42%) patients had a lung invasion in the chest CT at the same time (χ2=22·8,P <·05). On the seventh day after the onset of symptoms, RT-PCR confirmed 89 cases (78%) of COVID-19 pneumonia, while chest CT confirmed only 53 cases (46%) (χ2=24·2,P <·05). By the 14th day of symptom onset, 112 cases (98%) were confirmed by RT-PCR, but only 76 cases (67%) were positive for chest CT (χ2=39·3, P <·05). Eighteen days after the onset of symptoms, the last patient was confirmed by RT-PCR, comparing to 77 (68%) identified cases by CT (χ2=44·2,P <·05). As the chest CT of 32 children was normal, the cumulative percentage of cases identified by CT was only 72% (82), comparing to 114 (100%) cases identified by RT-PCR (χ2=37·2, P <·05). Through the whole duration of COVID-19 in children, the diagnostic positive rate of RT-PCR has been far higher than that of CT (All P<·05), as shown in E-table 2.
At the same time, it was found that after the first week of symptoms, the rise of cumulative percentage of cases identified by RT-PCR and chest CT became less, and reached a stable level after 11 days. Compared with the initial chest CT, the cumulative percentage of cases identified by RT-PCR is more significant.