Peripheral arterial disease (PAD) is common, with a prevalence of both symptomatic and asymptomatic disease estimated at 13% in the over 50 age group (Hirsch 2001). Symptomatic PAD affects about 5% of Western populations between the age of 55 and 74 years (Khan 2007). The deposition of atheromatous plaques (fatty deposits) and superadded thrombus (additional blood clot) results in arterial stenosis (narrowing) and occlusion. In the lower limb this leads to inadequate blood flow to the muscles during exercise causing muscle pain that is relieved by rest. This is known as intermittent claudication (IC). Intermittent claudication is the most common symptom of PAD, though in severe cases critical limb ischaemia (with symptoms of rest pain, ulceration, and gangrene) may develop, which, if untreated, can lead to lower limb amputation (Hooi 2007; Twine 2009). PAD is also associated with an increased risk of cardiovascular disease and stroke (Heald 2006).
The ankle brachial index (ABI) - or ankle brachial pressure index (ABPI) as it is also known - is used to diagnose PAD. A sphygmomanometer (device for measuring blood pressure) with an inflatable cuff is used to measure the systolic blood pressure at the upper arm or the ankle. This equipment can be either manual or digital to achieve readings. Manual equipment can use mercury or be aneroid (mechanical) in nature and require a stethoscope or a doppler flow auscultation. Digital equipment use oscillometric measurements and electronic calculation.
The ABI is widely used to assess PAD by a wide variety of healthcare professionals including specialist nurses, physicians, surgeons, and podiatrists working in secondary care settings. Division of the recorded blood pressure at the ankle with that recorded at the arm produces a ratio. Ratios of 0.90 to 1.30 are considered to be normal for adults, while ratios of less than 0.8 are indicative of arterial stenosis, and ratios between 0.75 to 0.5 are associated with severe ischaemia, poor healing and, in some cases gangrene (Bhasin 2007; MacLeod-Roberts 1995).
In secondary care a variety of imaging tests may be used to diagnose PAD; duplex ultrasound (DUS) allows the assessment of blood flow in the arteries, it is non-invasive and highly dependent on the experience of the radiologist to achieve useful images. Different types of angiography can also be used to image the blood vessels, namely; computerised tomography angiogram (CTA), magnetic resonance angiogram (MRA) and catheter angiogram.
The clinical severity of PAD is usually described using the Rutherford Classification Index (Rutherford 1997), or the Fontaine classification system (Fontaine 1954). In the former, patients are classified into one of four to six categories according to the degree of sensory loss, muscle weakness and arterial and venous measurements in acute and chronic PAD. These classified states which range from viable to threatened marginal, threatened immediate, and irreversible for acute PAD, and from asymptomatic, to mild, moderate or severe claudication, ischaemic rest pain and minor or major tissue loss for chronic disease, have different prognoses or outcomes (Rutherford 1997). The Fontaine classification index describes four disease states from no symptoms (stage I) to tissue necrosis, death and gangrene (stage 4) (Fontaine 1954).
People with IC may be prescribed naftidrofuryl oxalate and offered supervised exercise therapy, but those with incapacitating disease and significantly impaired quality of life may require some form of surgical arterial revascularisation (Bendermacher 2006; Cassar 2003; Chang 2011; de Backer 2012; NICE 2012; Rutherford 1997; Watson 2008).
As well as identifying people with PAD, the ABI can be used to predict the risk of cardiovascular events (Ankle Brachial Index Collaboration 2008; SIGN 2007). People with symptomatic PAD are known to have an increased risk of cardiovascular mortality, and subsequently receive treatment to manage their cardiac risk (Bhasin 2007).
As the ABI test is non-invasive, inexpensive and in widespread clinical use, a systematic review of its diagnostic accuracy is highly relevant to routine clinical practice.
Target condition being diagnosed
Presence or absence of peripheral arterial disease.
Ankle brachial index (ABI) is used to diagnose peripheral arterial disease (PAD). It is calculated by dividing the systolic pressure measured in the arteries at the ankle (dorsalis pedis and posterior tibial arteries) by the systolic blood pressure at the arm (brachial artery).
There are several ways to calculate an ankle brachial ratio. UK clinical guidelines recommend that the patient is rested in a supine position. The blood pressure is taken using a sphygmomanometer with an appropriately-sized cuff at the brachial artery and the posterior tibial, dorsalis pedis, and, where possible, peroneal arteries. The audible systolic pressure is detected with a doppler scanning probe (McDermott 2000; NICE 2012).
For each leg the ABI is calculated by dividing the highest ankle pressure by the highest pressure reading taken from the arm (McDermott 2000). The ABI values have been classed as follows (MacLeod-Roberts 1995):
Normal values range: 0.90 to 1.30;
Mild disease range: 0.7 to 0.9;
Moderate disease range: 0.41 to 0.69;
Severe disease (critical limb ischaemia): less than, or equal to (≤) 0.4.
In this review, we shall use the threshold of less than 0.90 to distinguish between test positive (< 0.90) and test negative (≥ 0.90) results. This threshold is commonly used in clinical practice and is cited in current guidelines (NICE 2012).
The position of the patient at the time the blood pressure is taken is important: for each inch that the ankle is positioned below the heart there is a 1 mmHg increase in systolic ankle blood pressure (MacLeod-Roberts 1995).
False negatives are known to occur in people who have calcification of the ankle artery wall, which creates incompressibility and an artificially high reading. This may occur in some patients with diabetes (Bhasin 2007; MacLeod-Roberts 1995).
There are a number of automated blood pressure machines all of which will be eligible for inclusion in the review.
There are several clinical pathways for people to be diagnosed with mild to moderate PAD using the ABI: during routine diabetic foot checks in primary care, community health settings or hospital settings. Similarly, members of the general population who report symptoms such as pain on walking may be diagnosed by means of the ABI. People may also be identified in accident and emergency departments, having presented with an acute episode of intermittent claudication and referred to either primary care or other secondary, hospital-based departments such as vascular surgery and vascular laboratories for treatment or further assessment.
Role of index test(s)
As outlined above, this review will include studies that evaluate the diagnostic test accuracy of the ABI in a variety of healthcare settings, with different patient groups by a range of healthcare professionals.
The range of uses of the ABI in clinical practice is diverse and there are no standard alternative tests to consider.
The success of management strategies for PAD depend upon the quality of the diagnostic process, which involves a careful assessment of the underlying pathology using diagnostic tests that possess a high level of accuracy, to allow the detection and measurement of an arterial stenosis and its distribution in the blood vessels.