Biomarkers, or “markers,” are used to measure an aspect of the biology or physiology of health and clinical disease. In the case of asthma, the most desirable biomarkers measure airflow obstruction caused by inflammation. Although many biomarkers have been proposed to assist in the diagnosis of asthma and to indicate the degree of disease severity or responsiveness to treatment, few are noninvasive, inexpensive, or measure clinically important aspects of disease activity. Currently, spirometry is the only established biomarker used to confirm the diagnosis and determine lung health. However, the National Heart Lung and Blood Institute’s Expert Panel characterizes other biomarkers of particular interest in asthma. This review will compare and contrast spirometry (something old) with one of the newly proposed biomarkers–fractional exhaled nitric acid (something new)–with respect to what aspects of asthma they propose to measure, what they do measure, and what sources of systematic and random error arise in the measurement of these variables.
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