Aim: To test the value of outflow fraction using air plethysmography (
APG) in the detection of suspected acute deep vein thrombosis (DVT) ex
tending proximal to the calf. Design: Prospective study in patients an
d normal volunteers. Setting: Academic Surgical Unit and Vascular Unit
, St Mary's Hospital Medical School, London, UK. Material and methods:
We prospectively examined 152 lower extremities of 150 patients with
clinical suspicion of DVT. Patients were examined by APG (outflow frac
tion) and either ascending venography or colour flow duplex imaging (C
FDI) 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. Res
ults: On venography or CFDI, DVT was detected in 96 limbs. Proximal DV
T was seen in 74; it was confined to the calf in 22. APG was 95% sensi
tive, 95% specific and 95% accurate in detecting proximal DVT whereas
it was 45% sensitive, 95% specific and 81% accurate for calf DVT. Conc
lusions: Using air plethysmography, the outflow fraction is a highly r
eproducible and accurate measurement in detecting DVT proximal to the
calf. The high positive predictive value means that if the test is pos
itive, treatment can start immediately; if it is negative, duplex scan
ning is necessary to exclude proximal partially occluded thrombi or ca
lf DVT.