A cross-sectional study of quantitative CT measurements associated with the diffusion capacity of the lung in recovered COVID-19 patients with normalised chest CT

Impairment of the diffusion capacity of the lung for carbon monoxide (DLco) is commonly reported in convalescent and recovered COVID-19 patients, although the cause is not fully understood especially in patients with no radiological sequelae. In a group of 47 patients at 7 – 51 weeks post infection with either none or minimal scarring or atelectasis on chest CT scans (total < 0.1% of lung volume), dispersions in DLco-adj % and total lung capacity (TLC) % of predicted were observed, with median(quartiles) of 87(78, 99)% and 84(78, 92)%, respectively. Thirteen(27.1%) patients had DLco-adj% < 80%. Although the DLco-adj% did not significantly correlate with the severity of the illness in the acute phase, time since the onset of symptoms, the volume of residual lesions on CT, age or sex, DLco-adj/alveolar volume (Kco-adj) % predicted was correlated with the measurements of small blood vessel volume fraction (diameter <= 5mm) and parenchyma density on CT. Multivariate analysis revealed that these two CT metrics significantly contributed to the variance in DLco-adj% independent of TLC%. Comparing to between-subject variability of DLco-adj in healthy individuals, patients in this cohort with DLco-adj% < 80% were likely abnormal with a degree of disease not visually detectable on CT. However, it is not clear whether the associated variance of parenchyma density and small vessel volume fraction were a consequence of the COVID-19 disease or a pre-existing background variance.