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To the Editor: Analyzing reverse causality in dialysis, Kalantar-Zadeh et al [1] address the blood pressure (BP) control issue and refer to Tassin unit experience in this issue of Kidney International.In the studies cited by the authors, BP figures used as risk predictor are baseline values, most patients are hypertensive, antihypertensives are used in 65%, followup time is <5 years, and extracellular volume (ECV) is not mentioned. Conversely, in our own studies [2, 3], integrated BP values are used, BP is strictly normal, antihypertensives are seldom used, follow-up time is >10 years, and ECV control is a key point. Among 1235 Tassin patients, the lowest initial predialysis BP decile (1st dialysis month mean arterial pressure <90 mm Hg) mortality is significantly high (2 years odds ratio = 1.96, P < 0.02). Therefore, in Tassin as elsewhere, initially low BP does predict early mortality.BP predictive value at dialysis initiation is poor: 90% of patients then are hypertensive, whatever their former hypertension exposure duration, and whether or not they become normotensive in dialysis. The effect of hypertension on target organs takes many years and very often we don't know how long a patient starting dialysis has been hypertensive. A recent report [4] clearly confirms the crucial predictive importance of hypertension duration before dialysis.Reverse epidemiology has misleading relevance on dialysis management. The high early mortality universally associated with low baseline BP figures does not contradict the need to achieve normal BP in dialysis patients to reduce long-term cardiovascular mortality. Besides, the eventual noxious/beneficial role of antihypertensive medications in dialysis patients needs to be investigated.

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