Air Plethysmography in the Detection of Suspected Acute Deep Vein Thrombosis

To test the value of outflow fraction using air plethysmography (APG) in the detection of suspected acute deep vein thrombosis (DVT) extending proximal to the calf. Prospective study in patients and normal volunteers. Academic Surgical Unit and Vascular Unit, St Mary's Hospital Medical School, London, UK. We prospectively examined 152 lower extremities of 150 patients with clinical suspicion of DVT. Patients were examined by APG (outflow fraction) and either ascending venography or colour flow duplex imaging (CFDI) as gold standards to establish the diagnosis. The reproducibility of outflow fraction was tested in two normal volunteers and was shown to have coefficients of variation of 6.1% and 8.2%, respectively. On venography or CFDI, DVT was detected in 96 limbs. Proximal DVT was seen in 74; it was confined to the calf in 22. APG was 95% sensitive, 95% specific and 95% accurate in detecting proximal DVT whereas it was 45% sensitive, 95% specific and 81% accurate for calf DVT. Using air plethysmography, the outflow fraction is a highly reproducible and accurate measurement in detecting DVT proximal to the calf. The high positive predictive value means that if the test is positive, treatment can start immediately; if it is negative, duplex scanning is necessary to exclude proximal partially occluded thrombi or calf DVT.