REPLY
Duplex Scanning of Renal Arteries for Stenosis
Jeffrey W. Olin, DO;
Marion Piedmonte, MA; and
Jess R. Young, MD
1 February 1996 | Volume 124 Issue 3 | Page 371
IN RESPONSE:
Dr. Chertow asserts that we erred in our calculations of positive and negative predictive values. Our definitions and our results, however, are correct. In fact, the formula provided by Dr. Chertow defines positive predictive value and negative predictive value exactly as we did.
Positive and negative predictive values depend heavily on disease prevalence; however, when sensitivity and specificity are high, as in our study (both more than 98%) [1], positive and negative predictive values will be very high, given prevalence rates between 20% and 80%. The positive predictive value will be poor only if renovascular disease is very rare. Similarly, the negative predictive value will be poor only if the prevalence of renovascular disease is extremely high.
As stated in our manuscript, our study cohort had a high pretest likelihood of disease [1]. Most patients who were studied had either hypertension that was difficult to control, unexplained azotemia, or both. In a previous study, we showed a disease prevalence of 70% among patients in whom renal artery stenosis was initially suspected. Fewer patients were evaluated because of associated peripheral vascular disease or abdominal aortic aneurysm. Similarly, we have shown that the prevalence of renal artery stenosis in this subgroup was approximately 40% [2]. Therefore, calculations of positive predictive value based on prevalence rates of 1%, 5%, and 10% seriously underestimate the prevalence of renovascular disease in our patient population.
We agree with Appel and colleagues that a normal (or even low) end diastolic velocity does not exclude critical renal artery stenosis. Appel and others have shown that in patients with parenchymal renal disease, the end diastolic velocity may not be elevated in the presence of severe renal artery stenosis. We believe that the increased resistance (resistive index) within the renal circulation may have prevented an increase in the end diastolic velocity. If the end diastolic velocity was 150 cm/sec or greater, 81% of the arteries had renal artery stenosis of 80% or greater [1]; however, an end diastolic velocity of less than 150 cm/sec does not exclude high-grade renal artery stenosis.
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Author and Article Information
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Cleveland Clinic Foundation; Cleveland, OH 44195
1. Olin JW, Piedmonte MR, Young JR, De Anna S, Grubb M, Childs MB. Utility of duplex ultrasound scanning of the renal arteries for diagnosing significant renal artery stenosis. Ann Intern Med. 1995; 122:833-8.
2. Olin JW, Melia M, Young JR, Graor RA, Risius B. Prevalence of atherosclerotic renal artery stenosis in patients with atherosclerosis elsewhere. Am J Med. 1990; 88:46N-51N.
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