Measuring inter-leg difference to diagnose peripheral artery disease?

We read with a great interest the paper of Herraiz-Adillo et al. [1] concluding that inter-leg systolic pressure difference (ILSPD) shows an acceptable diagnostic accuracy for the detection of peripheral artery disease (PAD). The paper is interesting nevertheless a few points warrant discussion.

First, although the authors have already used this approach in their previous work [2], we believe that encoding absence of results on the automatic sphygmomanometer as a zero mmHg pressure induces a bias in the results. Indeed an absent value does not necessarily rely on low or non-measurable arterial pressure. Cardiac arrhythmia, ankle artery non compressibility, movement artifacts may result in the system being unable to define arterial pressures adequately. Further this encoding contributes to increase the mean value of the between-leg difference. As underlined in the legend of table 4, for all the 10 PAD patients with inter-leg differences in excess of 100 mmHg, the difference resulted from a missing value. Considering that authors studied 23 patients, our interpretation of these 43.5% absent results is that oscillometry is of poor applicability to measure ABI in PAD patients. Authors underline that two consecutive measurements were performed. It would be interesting to know how many missing results were observed on one of the two recording. Similarly it is not clear from the method section whether or not the final value was the mean of the two tests.

Second, the authors have only 22 (15%) of the 146 non-PAD patients with an ILSPD >15 mmHg. This number is in the range of previous studies. For example, in study of 1059 patients only 26.2% of the 145 patients with an ILSPD >15 mmHg were PAD patients. [3] Nevertheless, the suggestion that inter-leg pressure difference is of acceptable diagnostic accuracy is, at least, debatable. Indeed with a prevalence of 13.5% of PAD (23/171) in the studied population, the positive predictive value (PPV) of the test is 42.1%. If the test has to be used for diagnostic purpose this PPV is clearly not sufficient. Specifically, PPV for a defined sensitivity and specificity is highly dependent on prevalence. The prevalence of PAD in the study is 2 to 3 times higher than other Spanish studies analyzing adult patients [4,5] and applying the technique to these populations would probably results in extremely low PPV.

Third, the reference used by the authors to support the claiming that atherosclerosis is habitually not symmetrically distributed is incomplete and the article was published in a German journal. [6] We could not access the original paper through the journal internet website but only the abstract on the Researchgate network. The abstract suggests that bilateral symptoms at symptom occurrence is rare (6% of patients) without analyzing the presence of symptoms when exercise is performed up to maximal walking distance, and that bilateral artery occlusion is rare (19%) without analyzing the presence of stenosis contralateral to occlusion. These observations may question the perfect symmetry of PAD but should certainly not question PAD bilaterality. We believe that it must be recalled that PAD is a bilateral and systemic disease.

Disclosure statement

The authors report no conflicts of interest.