Not as good as one would hope. In an often quoted study involving 768 patients with suspected PE who underwent angiography, a combination of normal A-a gradient (<20 mm Hg ), normal PaO2 (>80 mm Hg), and normal PaCO2 (>35 mm Hg) was examined to help exclude PE (1). Among patients with no known cardiopulmonary disease and normal values in all 3 parameters, over 30% still had PE, while among those with cardiopulmonary disease and normal parameters 17% had PE. In short, normal arterial blood gases may make PE less likely, they do not by any means exclude the possibility of PE.
1.Stein PD, Goldhaber SZ, Henry JW, et al. Arterial blood gas analysis in the assessment of suspected acute pulmonary embolism. CHEST 1996; 109:78-81.