Background. Nasal lavage is a common method for obtaining samples of nasopharyngeal (NP) secretions. The major disadvantage of this method is that the degree of dilution in the collected specimens is unknown and varies between subjects, which may severely distort the results. Several proteins have been used to correct for this varying dilution, but there are no reports comparing the suitability of different proteins for this purpose. An optimal standardization protein should show a minimal variation between different individuals and remain stable in the course of time. We compared the suitability of total protein, albumin, and secretory IgA (sIgA) for standardization by determining the variation in the levels of these proteins in NP secretions from children with upper respiratory infection (URI).

Methods. NP aspirate specimens from 52 children with URI and acute otitis media were studied. Specimens were obtained at the time of diagnosis of otitis and 2-3 days later. Total protein content in the samples was measured by the bicinchoninic acid protein assay, and albumin and sIgA levels were determined by enzyme immunoassay. To avoid the impact of extreme values, comparisons were based on the 10th and 90th percentile values for each protein.

Results. The concentration of total protein in the acute secretions ranged from 3.7 to 17.7 mg/ml, albumin from 0.8 to 12.8 mg/ml, and sIgA from 0.09 to 0.9 mg/ml. Total protein had the narrowest variation between the children: the ratio of the 90% and 10% values for total protein was 4.8, compared with 15.4 for albumin and 10.2 for sIgA. Between the first and second samples, the levels of total protein and albumin decreased significantly. The mean sIgA levels did not change significantly, but the sample size was too small to rule out a moderate change. When individual levels were analyzed, 72% of the children had ≥ 2-fold change in the concentration of sIgA between the first and second specimens.

Conclusions. Compared with albumin and sIgA, total protein showed the smallest intersubject variation in the NP secretions of the children. Therefore, the use of total protein appeared to provide the most reliable method of standardizing the concentrations of other substances in nasal lavage specimens from children. On the other hand, none of these proteins remained stable during the infection and thus did not seem to be suitable for use in standardization if changes in the levels of substances during URI are to be determined.