Sensitivity and Specificity of Histamine PC20 Determination in a Random Selection of Young College Students

Histamine provocative concentration causing a 20% drop in FEV1 (PC20) was measured in 500, randomly selected, young (20 to 29 years) university students. Participation was 500/619, or 81%. In this population, by a research assistant-administered questionnaire, we identified 17 subjects with current asthma, 16 with asthma only on allergen exposure, 19 with past (more than 1 year ago) asthma, 158 with rhinitis (77 atopic and 81 nonatopic subjects), and 290 subjects with neither asthma nor rhinitis. Histamine airway hyperresponsiveness (PC20 less than or equal to 8 mg/ml) was observed in 58 subjects and included the 17 subjects with current asthma, 6/16 with asthma only on allergen exposure, 2/19 subjects with previous asthma, 20/158 with rhinitis, and 13/290 subjects with neither asthma nor rhinitis. With "current symptomatic asthma" as the diagnosis and PC20 less than or equal to 8 mg/ml as the positive test, the sensitivity was 100%, the specificity was 93%, and the negative predictive value was 100%; the positive predictive value (for current symptoms of asthma) was only 29%. The strength of this test with a cutoff of 8 mg/ml is the high sensitivity, indicating that a PC20 greater than 8 mg/ml is likely to indicate that current asthma is not present. The weakness is the failure to predict current symptoms of asthma. As the cutoff is lowered, for example, to PC20, 1 mg/ml, the sensitivity falls to less than 50% and the positive predictive value approaches 100%. These data indicate that a PC20 greater than 8 (or 16) mg/ml rules out current asthma in most instances, whereas a PC20 less than 1 mg/ml is almost diagnostic of current asthma. Values between 1 and 8 mg/ml are intermediate in these regards.