Criteria: Table 5-9 Impairment Classification for Asthma Severity (104), Table 5-10 Impairment Rating for Asthma (104), Table 5-4b Lower Limit FEV1 for Men (97), Table 5-5b Lower Limit FEV1 for Women (98)
Guidance: Rating is performed as described in Section 5.5, Asthma (102-104), with reference to Table 5-9, Impairment Classification for Asthma Severity (104), and Table 5-10, Impairment Rating for Asthma (104). Asthma-related impairment is calculated in two steps. First, the asthma severity score is calculated using the criteria in Table 5-9 which considers factors of the postbronchodilator FEV1, % of FEV1 Change (reversibility), the PC20 mg/ml or equivalent (degree of airway hyperresponsiveness), and medication requirements. FEV1 indicates forced expiratory volume in the first second; PC20 is the provocative concentration that causes a 20% fall in FEV1. Add the scores for postbronchodilator FEV1, reversibility of FEV1 (or PC20), and medication use to obtain a summary severity score for rating respiratory impairment. When FEV1 is greater than the lower limit of normal, PC20 should be determined and used for rating of impairment; when FEV1 is less than 70% of the predicted, the degree of reversibility should be used; and when FEV1 is between 70% of the predicted and the lower limit of normal, either reversibility or PC20 can be used. The score for minimum medication use is added to the appropriate measurement criteria outlined above. The need for minimum medication should be demonstrated by the treating physician, for example, through previous records of exacerbation when medications have been reduced. The correct use of Table 5-9 requires determining whether the individual’s FEV1 is above or below the lower limit of normal. If the individual’s prebronchodilator FEV1 is below the lower limit of normal, the degree of reversibility is assessed with inhaled bronchodilators (Table 5-9, column 2). If the individual’s prebronchodilator FEV1 is above the lower limit of normal, a methacholine challenge is used to assess airway responsiveness (Table 5-9, column 3). Thus, calculating the asthma severity score requires either a score for percentage of change in FEV1 (Table 5-9, column 2) or a score for PC20 mg/mL (Table 5-9, column 3) but not both. Second, the calculated asthma severity score is used to determine the percent impairment class and impairment of the whole individual by means of Table 5-10.
