Importance of using markers of dilution when measuring inflammatory mediators in nasal lavage fluid.

To the Editor-In their recent article on nasal cytokine production in children with upper respiratory infections (URIs), Noah et al. [1] state "accurate calculation of the volume ofepithelial lining fluid in respiratory lavage fluid is not possible with current techniques. " While this may be true in bronchoalveolar lavage (BAL), it is not the case with nasal lavage. I have used inulin as an exogenous marker of dilution when performing nasal lavages to study IgA and total protein and cytokine production in infants 1-31 months old with acute viral wheezing [2, 3]. By measuring the inulin concentration in the wash solution and in the lavage fluid obtained, the proportion of nasal lavage fluid consisting of nasal secretions can be calculated. This allows the substance being measured (e.g., tumor necrosis factor [TNF]-a) to be expressed per volume of nasal secretions by use of the simple formula: ([TNF-a]nasal wash/[TNF-a]nasal secretions) = {I ([inulin]nasallavage fluiJ[inulin]nasal wash)}. It is not possible to differentiate the nasal (epithelial) lining fluid recovered from the rest of the nasal secretions, but that is unimportant. Inulin, a chemically inert polysaccharide, is safe and easy to measure. Its use in nasal lavage was first described in 1970 [4], and it has recently been used in BAL [5]. There were early problems due to the difficult and inexact inulin assay, but by using an easier and more robust assay [6], I obtained a coefficient of variation of <2% for the method. I have also compared the use of exogenous inulin with that of endogenous urea, the method still in favor for calculating dilution factors in BAL, and found that the urea method underestimates the dilution factor in the majority of samples [7]. The concentration of a substance in nasal lavage fluid does not necessarily reflect its concentration in the nasal lining fluid. Simply