Quantification of cytokines and inflammatory mediators in samples of nasopharyngeal secretions with unknown dilution

In the study of inflammatory mechanisms in the upper respiratory tract, the unknown dilution of collected samples of nasal secretions poses a serious problem for interpretation of the measured concentrations of various substa nces in the specimens. We investigated the magnitude of the dilution proble m in a true clinical research situation and determined the validity of usin g the levels of total protein, albumin, and secretory IgA in nasal secretio ns to correct for the unknown dilution. The study samples consisted of simu ltaneously obtained nasopharyngeal aspirates and nasal lavage specimens fro m 52 children with upper respiratory tract infection. The dilution factors of the nasal lavage specimens varied widely between 1.8 and 432 (median, 11 .2). Of the three proteins studied, total protein had the narrowest intersu bject variation in the nasal secretions of the children and thus seemed to provide the best standardization method for comparing levels of substances between individuals. Concentrations of IL-6 standardized with total protein correlated significantly better with the true IL-6 concentrations in the n asal secretions than did IL-6 levels measured in the nasal lavage specimens without standardization (p = 0.049). These findings suggest that the most common current practice of measuring substances in nasopharyngeal specimens , i.e. measuring without correction for the dilution, may produce "false-ne gative" results. Potentially important information on inflammatory mechanis ms may be undetected if false-negative results mask real differences betwee n groups. The use of exogenous markers of dilution might improve the accura cy of quantifying substances in nasal secretions.