At PCXR, aspiration occurs in a dependent distribution that varies with patient position; in the supine patient this is most often perihilar and asymmetric in the superior segments of the lower lobes and posterior segments of the upper lobes.
Correlation with protected bronchial brushing cultures, PCXR has a positive predictive value of 0.35 and negative predictive value of 0.55.
(34) As much as 500 mL of pleural fluid may be necessary for radiographic detection, (35) and accurate estimation of pleural effusion size with supine PCXR is difficult.
As with pleural fluid, pleural air can be difficult or impossible to detect by PCXR. Occult pneumothorax (OPTX) has been described in 29-72% of trauma patients correlating PCXR with contemporaneous CT.