C3 and infection: In our patient, the constellation of dyspnoea, pleuritic pain, hypoxia, and bilateral ground-glass opacities on CTPA, together with low procalcitonin (0.07 μg/L), normal complement levels (C3: 1.29 g/L, C4: 0.47 g/L), and mildly raised CRP (23 mg/L), favored ALP over infection [12-14].