For pneumonia, is a posteroanterior (PA) view or anteroposterior (AP) view recommended for initial radiographic assessment?

PA View is Preferred for Initial Pneumonia Assessment

For initial radiographic assessment of pneumonia, posteroanterior (PA) and lateral chest radiographs are strongly recommended over anteroposterior (AP) views, as PA/lateral imaging demonstrates significantly superior sensitivity (83.9% vs 67.3%) for detecting clinically relevant parapneumonic effusions. 1, 2

Why PA View Outperforms AP View

The evidence clearly demonstrates PA radiography's superiority:

• PA and lateral radiographs detect parapneumonic effusions with 83.9% sensitivity compared to only 67.3% for AP radiographs when using CT as the reference standard 1, 2
• Among effusions initially missed by AP radiography, approximately 44% (7 of 16) either required drainage or developed empyema within 30 days, while no complicated effusions or empyemas were found among those missed by PA/lateral radiography 2
• The American College of Radiology explicitly endorses chest radiography as the initial imaging modality, with upright PA and lateral views considered the reference standard and preferred over AP portable radiography when feasible 1, 3

Clinical Decision Algorithm

Start with PA and lateral chest radiographs for all patients with suspected pneumonia, except for well-appearing children with uncomplicated community-acquired pneumonia who don't require hospitalization 3

For patients who cannot stand or are critically ill:

• AP portable radiography remains acceptable when PA views are not feasible 1
• Be aware that AP views miss more clinically significant effusions, particularly in patients with lower lobe consolidation 4, 2
• Consider obtaining additional imaging (ultrasound or CT) if clinical suspicion remains high despite negative AP radiograph 4

Important Caveats and Pitfalls

All chest radiograph views (PA, lateral, and AP) miss more than 10% of parapneumonic effusions, with most missed effusions occurring in patients with coexistent lower lobe consolidation 1, 4

The lateral view specifically helps identify left lower lobe pneumonia that may be obscured by the heart on PA view, particularly when the "spine sign" (interruption in progressive vertebral lucency) is present 5

Single-view radiographs of any type (PA alone, lateral alone, or AP alone) have statistically equivalent sensitivities for detecting parapneumonic effusions, which is why the combination of PA and lateral views is recommended 1

When to Consider Advanced Imaging

If chest radiograph is negative but clinical suspicion remains high, chest ultrasound demonstrates 93-96% sensitivity and 93-96% specificity compared to clinical criteria and chest radiographs, particularly for peripheral lesions 3

CT chest is not recommended for initial imaging but is indicated for:

• Treatment-refractory or recurrent infiltrates 3
• Suspected complications like empyema (use CT with IV contrast acquired 60 seconds post-bolus) 1, 3
• Immunocompromised patients or those with pre-existing structural lung disease 6