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.