ABIDING demonstrated that use of oscillometric devices by general practice nurses to determine ABI and therefore the presence of PAD had high specificity (92%) and negative predictive value (90%), good accuracy (84%) but modest sensitivity (62%) and positive predictive value (67%). The modest sensitivity and the LR+ 7.3 indicate that this test has little value for confirming the presence of PAD. On the contrary high specificity and negative predictive value suggests that the test has some value in ruling out the disease (ie, when the test is negative). Looking at the symptomatic individuals as determined by ECQ showed that, though the numbers were small, the pragmatic measure had a poor performance as a diagnostic test in this high-prevalence (61%) subgroup. Changing the cut point to improve sensitivity or specificity simply compromised the other measure and therefore did not improve test performance.
These findings were in contrast to the experience in a specialist centre where their test performance (both limbs in comparison to ABIDING lower of the two measures) was sensitivity left/right leg 88/73% (62%), specificity 85/95% (92%), positive predictive value 65/88% (69%), negative predictive value 96/88% (90%), LR+ left/right leg 5.9/14.6 (7.9) and LR− 0.14/0.28 (0.4).4
A good diagnostic test has a LR+ >10 and LR− <0.1.17
This difference in performance to some extent may be accounted for by patient selection but is more likely due to operator expertise. In the specialist centre, the mean age was 10 years younger and 53% were women compared to only 22% in ABIDING. The respective prevalence of PAD was 32% and 22%. In other studies reporting being conducted in primary care Mehlsen et al18
enrolled 1258 consecutive general practice patients for an oscillometric determination of ABI, with those with an ABI <0.9 referred for a Doppler measure in a vascular unit. Hence all ‘negatives’ including false negatives did not have a gold standard measure and therefore this was not a true measure of test performance in primary care. Nicholai et al19
had similar limitations. Verberk et al21
conducted a systematic review of automated oscillometric devices including a subgroup analysis on devices developed for arm blood pressure (BP) measurement. Only 1 of the 18 studies identified was conducted in primary care and that with an ABIgram and not a simple BP arm device.22
Although the investigators demonstrated its reliability, the use of this special piece of equipment would seem to effect is acceptability as is the current situation. ABI is a valid and reliable clinical measure although an indirect one. The true gold standard would be an intravascular perfusion study. Both methods have been compared to the true gold standard in 85 patients with claudication undergoing angiography.23
The oscillometric method showed 97% sensitivity, 89% specificity, 98% positive predictive value and 86% negative predictive value. The Doppler method showed 95% sensitivity, 56% specificity, 91% positive predictive value and 68% negative predictive value. This study suggests that the oscillometric method had greater diagnostic accuracy but the test was performed by physicians not specifically trained to use the Doppler probe. This said ABI is a practical tool and is superior to clinical examination for identifying PAD.20
However, screening whole populations is not always practical. ABI ascertainment of PAD is most effective by identifying high-risk patients as we have done in ABIDING. By including high-risk and overt CVD patients we were confident that we should get a distribution of ABI scores that included PAD diagnostic scores and the outcome of the study supports this (22% had PAD by the conventional method).
If our method had been reliable it would have been readily implementable as Australian GPs have ready access to oscillometric sphygmomanometers. More than 19 500 devices were distributed on behalf of the High Blood Pressure Research Council of Australia, mostly to GPs, over the years 2007–2009. Practice nurses were chosen rather than GPs as this approach is also more likely to be implementable. A survey by Mohler et al24
of primary care clinicians showed that most (88%) thought ABI to be feasible in that setting.
The intervention was kept as simple as possible by using practice nurses to do single measures on a device they were familiar with but did not receive extensive further training on. While this means that this is simple to introduce into clinical practice the practice nurse performance may have been improved by more intense training and repeated limb measurements.