Pulmonary embolism

The A-a gradient is the difference between the calculated alveolar oxygen tension and the measured arterial oxygen tension. A gradient that is normal for age suggests hypoventilation or a low inspired oxygen tension (such as altitude), whereas a widened gradient points to V/Q mismatch, right-to-left shunt, or impaired diffusion. The expected upper limit rises with age, roughly (age / 4) + 4 mmHg on room air at sea level.

A weighted risk assessment model that totals individual venous thromboembolism risk factors (worth 1, 2, 3, or 5 points each) to place a surgical patient into a risk category from very low to high. The category guides whether mechanical prophylaxis, pharmacological prophylaxis, or both are appropriate, always balanced against bleeding risk.

A weighted score that stratifies hospitalised medical (non-surgical) patients by venous thromboembolism risk. A total of 4 or more identifies high risk: in the derivation cohort these patients had a markedly higher VTE rate, and pharmacological thromboprophylaxis is recommended unless contraindicated. A score below 4 is low risk.

Eight clinical criteria for patients in whom PE is being considered but gestalt pretest probability is already low. If none of the eight are present, PE can be excluded without D-dimer or imaging. Any single positive criterion means PERC cannot exclude PE and further work-up is required.

A clinical prediction rule that places a patient with suspected deep vein thrombosis into a pretest-probability category. The modern two-level version (DVT likely vs unlikely) is paired with D-dimer: an unlikely score plus a negative D-dimer can safely defer or avoid ultrasound. It is a triage tool, not a stand-alone rule-out.

Seven weighted clinical criteria estimate the pretest probability of pulmonary embolism. The result can be read three-tier (low, moderate, high) or two-tier (PE unlikely at 4 or less, PE likely above 4), which guides whether D-dimer or direct imaging is the next step.