The ankle brachial pressure index (ABPI) is the ratio of systolic leg blood pressure to systolic arm blood pressure. The blood pressure readings in the legs are taken at the ankle, while the blood pressure readings in the arms are taken just above the elbow at the brachial artery, hence the name of the test. Doctors measure ABPI for each leg to check if the arteries become narrower as they reach the feet. When a patient's ABPI is less than 0.9--that is, the blood pressure in one leg is 10 per cent lower than in the arms--it may signal peripheral artery disease.
You must visit your doctor or cardiologist to have your blood pressure taken to calculate ABPI because you need to apply a particular technique to obtain accurate readings. At your doctor's office, your doctor will have you lie on your back while she uses a Doppler instrument and standard inflatable cuffs to measure the blood pressure at the ankles and brachial artery.
Find the systolic blood pressure of each ankle on your blood pressure report. Systolic blood pressure is the larger of the two blood pressure numbers given; diastolic blood pressure is the smaller number. Call the systolic blood pressure in the left ankle P-left, and that in the right ankle P-right. For example, if your blood pressure is 125/85 at the left ankle and 120/85 at the right ankle, then P-left = 125 and P-right = 120.
Find the systolic blood pressure of each arm on your blood pressure report. Call the higher of the two systolic arm pressures P-arm. For example, if your blood pressure is 130/90 in the left arm and 128/88 in the right arm, then P-arm = 130.
Divide P-left by P-arm to calculate the ABPI for the left leg. If P-left = 125 and P-arm = 130, your left ABPI = 0.96
Divide P-right by P-arm to calculate the ABPI for the right leg. If P-right = 120 and P-arm = 130, then your right ABPI = 0.92
Schedule regular checkups if your ABPI is lower than 0.9. ABPI values in the range of 1 to 1.3 are considered healthy, while values between 0.9 and 1 may signal risk of future vascular disease. Your doctor will perform more tests to check for artery blockages if your ABPI is less than 0.9, especially if it is below 0.8.