Allergic rhinitis is an inflammatory nasal disorder in which a range of different cells participates. A variety of approaches has been used to monitor nasal inflammation objectively to investigate disease processes and to evaluate the effect of therapeutic intervention. These approaches include nasal lavage, nasal cytology, and nasal biopsy, together with the more recently established measurement of nasal nitric oxide (NO) concentration. Although all provide information about nasal mucosal inflammation, the extent of information that can be obtained by each approach, the ease of sampling, and the complexity of sample handling differ. Such considerations influence the choice of approach when measurement of nasal inflammation is to be an objective outcome parameter in a clinical trial. In addition, the choice of approach is also determined by the questions or hypotheses that are to be addressed.

Nasal lavage is simple and rapid to perform, is well tolerated, and provides a sample that can provide information about luminal cell recruitment, cell activation, and plasma protein extravasation. Nasal cytology involves sampling and recovering mucosal surface cells. It is also easy to perform and is well tolerated in general, although some find that the procedure causes a transient unpleasant sensation. A differential cell count from the sample provides information about relative cell populations. Both nasal lavage and nasal cytology are readily applicable to clinical trials. Nasal cytology sample handling is easier, but nasal lavage offers the advantage of providing considerably greater information from the sample.

Nasal biopsy is a considerably more invasive procedure and requires expertise not only in tissue sampling but also in biopsy processing. Therefore, it is applicable only in specialist centers. However, nasal biopsy is the only sampling technique that directly informs about tissue cellular events, although these may be implied, in part from the other sampling approaches. Tissue specimens can be used to evaluate both protein and gene expression.

Measurement of nasal NO involves expensive equipment but provides an instantaneous result, unlike the other approaches, all of which require sample processing and analysis. Recommendations for standardization of measurement have been made, and measures are considered in part to reflect allergic inflammation within the nasal mucosa. The limitations of nasal NO are that it reflects only a certain aspect of allergic mucosal inflammation, and that because a proportion of nasally measured NO is derived from the sinuses under normal circumstances, nasal NO is not specific for nasal disease. The high contribution from the sinus mucosa limits the discriminatory ability of nasal NO to reflect nasal tissue–specific alterations.

The incorporation of measures of nasal inflammation in clinical trials has distinguished anti-inflammatory therapy from symptomatic therapy and has the potential to provide information about the efficacy of novel therapies for allergic rhinitis.